Seismic Verification of Nuclear Testing Treaties

May 1988

NTIS order #PB88-214853 Recommended Citation: U.S. Congress, Office of Technology Assessment, eism”c Verification of Nuclear Testing Treaties, OTA-ISC-361 (Washington, DC: U.S. Government Printing Office, May 1988).

Library of Congress Catalog Card Number 88-600523

For sale by the Superintendent of Documents U.S. Government Printing Office, Washington, DC 20402-9325 (order form can be found in the back of this report) Foreword

Since the advent of the atomic bomb there has been interest from both an arms control and environmental perspective to restrict the testing of nuclear weapons. Although the debate over nuclear testing has many facets, verification is a central issue to the consideration of any treaty. At the requests of the Senate Select Committee on Intelligence, the House Committee on Foreign Affairs, and the House Permanent Select Committee on Intelligence, OTA undertook an assessment of seismic capabilities to monitor underground nuclear explosions. Like an earthquake, the force of an underground nuclear explosion creates seismic waves that travel through the Earth. A satisfactory seismic network to monitor such tests must be able to both detect and identify seismic signals in the presence of “noise,” for example, from natural earthquakes. In the case of monitoring a treaty that limits testing below a certain size explosion, the seismic network must also be able to estimate the size with acceptable accuracy. All of this must be done with an assured capability to defeat adequately any credible attempt to evade or spoof the monitoring network. This report addresses the issues of detection, identification, yield estimation, and evasion to arrive at answers to the two critical questions:

● Down to what size explosion can underground testing be seismically monitored with high confidence? ● How accurately can the yields of underground explosions be measured? In doing so, we assessed the contribution that could be made if seismic stations were located in the country whose tests are to be monitored, and other cooperative provisions that a treaty might include. A context chapter (chapter 2) has been included to illustrate how the technical answers to these questions contribute to the political debate over:

● Down to what yield can we verify Soviet compliance with a test ban treaty? ● Is the 1976 Threshold Test Ban Treaty verifiable? ● Has the Soviet Union complied with p-resent testing restrictions? In the course of this assessment, OTA drew on the experience of many organi- zations and individuals. We appreciate the assistance of the project contractors who prepared background analysis, the U.S. Government agencies and private companies who contributed valuable information, the project advisory panel and workshop participants who provided guidance and review, and the many additional reviewers who helped ensure the accuracy and objectivity of this report.

JOHN H. GIBBONS u Director

Ill Seismic Verification of Nuclear Testing Treaties Advisory Panel

Howard Wesley Johnson, Chair Honorary Chairman of the Corporation Massachusetts Institute of Technology Walter Alvarez Franklyn K. Levin Professor former Senior Research Scientist, Exxon Department of Geology and Geophysics Raymond J. McCrory University of California, Berkeley former Chief, Arms Control Intelligence Thomas C. Bache Staff Manager, Geophysics Division Central Intelligence Agency Earth Sciences Operation Karen McNally Science Applications International Corp. Director, Charles F. Richter Seismological William E. Colby Laboratory former Director of Central Intelligence University of California, Santa Cruz Agency John R. Murphy F. Anthony Dahlen Program Manager and Vice President Professor S-CUBED Department of Geological and Geophysical Wolfgang K.H. Panofsky Sciences Director Emeritus Princeton University Stanford Linear Accelerator Center H.B. Durham Stanford University Distinguished Member of Technical Staff Paul G. Richards Sandia National Laboratories Professor John R. Filson Department of Geological Sciences Chief, Office of Earthquakes, Volcanos, Lamont-Doherty Geological Observatory and Engineering Columbia University U.S. Geological Survey Jack Ruina W. J. Hannon, Jr. Director, Defense & Arms Control Studies Assistant Program Leader Program Analysis & Assessment Center for International Studies Verification Program Massachusetts Institute of Technology Lawrence Livermore National Laboratory Lynn R. Sykes University of California Higgins Professor of Geological Sciences Roland F. Herbst Lamont-Doherty Geological Observatory Member of President’s Staff Columbia University RDA Associates Thomas A. Weaver Eugene T. Herrin, Jr. Deputy Group Leader, Geophysical Group Professor Los Alamos National Laboratory Department of Geological Sciences University of California Southern Methodist University

NOTE: OTA appreciates and is grateful for the valuable assistance and thoughtful critiques provided by the advisory panel members. The panel does not, however, necessarily approve, disapprove, or endorse this report. OTA assumes full responsibility for the report and the accuracy of its contents. iv OTA Project Staff—Seismic Verification of Nuclear Test Ban Treaties

Lionel S. Johns, Assistant Director, OTA Energy, Materials, and International Security Division

Peter Sharfman, International Security and Commerce Program Manager

Gregory E. van der Vink, Project Director

Administrative Staff Jannie Home Marie C. Parker Jackie Robinson

Contractors Steven M. Day Zoltan A. Der Frederick K. Lamb Robert P. Masse´ Acknowledgments In addition to the advisory panel and contractors, OTA gratefully acknowledges the valuable contributions made by the following: Ralph W. Alewine, III, Defense Advanced Research Projects Agency Shelton S. Alexander, Pennsylvania State University Charles B. Archambeau, University of Colorado Robert R. Blandford, Defense Advanced Research Projects Agency John S. Derr, United States Geological Survey Don Eilers, Los Alamos National Laboratory Jack Evernden, United States Geological Survey Frederick E. Followill, Lawrence Livermore National Laboratory Robert A. Jeffries, Los Alamos National Laboratory James F. Lewkowicz, Air Force Geophysics Laboratory J. Bernard Minster, Scripps Institute of Oceanography Otto W. Nuttli, St. Louis University Mary Peters, Naval Research Laboratory Robert A. Phinney, Princeton University Keith Priestley, University of Nevada Alan S. Ryan, Center for Seismic Studies Robert H. Shumway, University of California Stewart Smith, Incorporated Research Institutions for Seismology Sean Solomon, Massachusetts Institute of Technology Jack Rachlin, United States Geological Survey Irvin Williams, Department of Energy Robert Zavadil, Air Force Technical Applications Center Department of Energy, Office of Classification Workshop I: Network Capabilities Stewart Smith, Chair President, Incorporated Research Institute for Seismology Charles B. Archambeau Willard J. Hannon Professor Assistant Program Leader for Analysis & Cooperative Institute for Research in Assessment Environmental Science Verification Program University of Colorado, Boulder Lawrence Livermore National Laboratory University of California Thomas C. Bache Manager, Geophysics Division Keith Nakanishi Earth Sciences Operation Assistant Program Leader for Seismic Science Applications International Corp. Research Lawrence Livermore National Laboratory John S. Derr University of California Geophysicist U.S. Geological Survey Paul G. Richards Professor Jack Evernden Department of Geological Sciences Research Geophysicist Lamont-Doherty Geological Observatory U.S. Geological Survey Columbia University Frederick E. Followill Robert J. Zavadil Seismologist Chief, Evaluation Division Lawrence Livermore National Laboratory Directorate of Geophysics University of California Air Force Technical Applications Center

Workshop II: Identification J. Bernard Minster, Chair Visiting Professor of Geophysics Scripps Institute of Oceanography University of California, San Diego Charles B. Archambeau Jack Evernden Professor Research Geophysicist Cooperative Institute for Research in U.S. Geological Survey Environmental Science Willard J. Hannon University of Colorado, Boulder Assistant Program Leader for Analysis & Thomas C. Bache Assessment Manager, Geophysics Division Verification Program Earth Sciences Operation Lawrence Livermore National Laboratory Science Applications International Corp. University of California Robert R. Blandford John R. Murphy Program Manager for the Nuclear Program Manager and Vice President Monitoring Office S-CUBED Defense Advanced Research Projects Agency

vi Keith Priestley Robert H. Shumway Professor Professor Seismological Laboratory Division of Statistics University of Nevada University of California, Davis Jack Rachlin Lynn R. Sykes Geologist Higgins Professor of Geological Sciences U.S. Geological Survey Lamont-Doherty Geological Observatory Columbia University Paul G. Richards Professor Robert J. Zavadil Department of Geological Sciences Chief, Evaluation Division Lamont-Doherty Geological Observatory Directorate of Geophysics Columbia University Air Force Technical Applications Center

Workshop III: Evasion Sean C. Solomon, Chair Professor, Department of Earth, Atmospheric & Planetary Science Massachusetts Institute of Technology Charles B. Archambeau Eugene T. Herrin, Jr. Professor Professor Cooperative Institute for Research in Department of Geological Sciences Environmental Science Southern Methodist University University of Colorado, Boulder Keith Priestley Thomas C. Bache Professor Manager, Geophysics Division Seismological Laboratory Earth Sciences Operation University of Nevada Science Applications International Corp. Jack Rachlin Robert R. Blandford Geologist Program Manager for the Nuclear U.S. Geological Survey Monitoring Office Lynn R. Sykes Defense Advanced Research Projects Higgins Professor of Geological Sciences Agency Lamont-Doherty Geological Observatory Jack Evernden Columbia University Research Geophysicist U.S. Geological Survey Willard J. Harmon Assistant Program Leader for Analysis & Assesmsent Verification Program Lawrence Livermore National Laboratory University of California

vii Workshop IV: Yield Determinations Robert A. Phinney, Chair Chairman, Department of Geological & Geophysical Sciences Princeton University Ralph W. Alewine, III Frederick K. Lamb Director of Nuclear Monitoring Office Professor Defense Advanced Research Projects Department of Physics Agency University of Illinois, Urbana Charles B. Archambeau Keith Priestley Professor Professor Cooperative Institute for Research in Seismological Laboratory Environmental Science University of Nevada University of Colorado, Boulder Paul G. Richards Thomas C. Bache Professor Manager, Geophysics Division Department of Geological Sciences Earth Sciences Operation Lamont-Doherty Geological Observatory Science Applications International Corp. Columbia University Donald Eilers Alan S. Ryall Associate Group Leader Manager of Research Los Alamos National Laboratory Center for Seismic Studies University of California Robert H. Shumway Willard J. Hannon Professor Assistant Program Leader for Analysis & Division of Statistics Assessment University of California, Davis Verification Program Lynn R. Sykes Lawrence Livermore National Laboratory Higgins Professor of Geological Sciences University of California Lamont-Doherty Geological Observatory Eugene T. Herrin, Jr. Columbia University Professor Robert J. Zavadil Department of Geological Sciences Chief, Evaluation Division Southern Methodist University Directorate of Geophysics Robert A. Jeffries Air Force Technical Applications Center Program Director for Verification & Safeguards Los Alamos National Laboratory

Vlll Contents

Page I. Executive Summary ...... 3 2. Seismic Verification in the Context of National Security ...... 23 3. The Role of Seismology ...... 41 4. Detecting Seismic Events ...... 55 5. Identifying Seismic Events ...... 77 6. Methods of Evading a Monitoring Network ...... 95 7. Estimating the Yields of Nuclear Explosions ...... 113 Appendix. Hydrodynamic Methods of Yield Estimation ...... 129 Chapter 1 Executive Summary CONTENTS

Page Introduction ...... 3 The Test Ban Debate...... 3 The Meaning of Verification ...... 5 Aspects of Monitoring Underground Nuclear Explosions ...... 6 Detecting Seismic Events ...... 6 Identifying Seismic Events as Nuclear Explosions...... 8 Evading a Seismic Monitoring Network...... 11 How Low Can We Go?...... 13 Leve 1–Above 10 kt ...... 14 Level 2–Below10 kt but Above 1-2 kt ...... 14 Level 3–Below 1-2 kit ...... 14 Level 4–Comprehensive Test Ban ...... 14 Estimating the Yield of Nuclear Explosions...... 15 Soviet Compliance...... 17 Verification of the TTBT and the PNET...... 19 A Phased Approach ...... 19

Boxes Box Page l-A. NRDC/Soviet Academy of Sciences ...... 10 1-B. CORRTEX...... 18

Figures Figure No. Page l-1. Signal From Semipalatinsk ...... 7 l-2. Explosion and Earthquake ...... 12 Chapter 1 Executive Summary

Technologies that were originally developed to study earthquakes may now enable the United States to verify a treaty with the Soviet Union to further limit the testing of nuclear weapons.

INTRODUCTION Seismology now provides a means to moni- The answers to these questions provide the tor underground nuclear explosions down to technical information that lies at the heart of low yields, even when strenuous attempts are the political debate over: made to evade the monitoring system. By doing so, seismology plays a central role in verify- 1. how low a threshold test ban treaty with ing arms control agreements that limit the test- the Soviet Union we could verify, ing of nuclear weapons. Seismology, however, 2. whether the 1976 Threshold Test Ban is like any other technology: it has both strengths Treaty is verifiable, and and limitations. If the capabilities of seismic 3. whether the Soviet Union has complied monitoring are to be fully realized, it is neces- with present testing restrictions. sary to understand both how the strengths can Seismic monitoring as discussed in this study be used and how the limitations can be avoided. is evaluated without specific references to the To a great extent, the capabilities of any particular treaty regime to which it is to ap- given seismic monitoring network are deter- ply. There will always be some limit to the ca- mined by how the monitoring task is approached pability of any given monitoring network, and and what supplementary provisions are nego- hence there will always be a threshold below tiated within the treaty. If agreements can be which a seismic network could not monitor negotiated to reduce uncertainty, then seismol- with high confidence. Consequently, should a ogy can be very effective and extremely low total test ban be enacted there will be a very yields could be monitored with high confidence. low threshold below which seismic methods cannot provide high confidence monitoring. This report addresses two key questions: Such a treaty could still be considered to be 1. down to what size explosion can under- in the national interest if, taking both seismic ground testing be seismically monitored and nonseismic verification methods into ac- with high confidence, and count, the significance of undetected violations 2. how accurately can the yields of under- (if they were to occur) would be outweighed ground explosions be measured seismically? by the benefits of such a treaty.

THE TEST BAN DEBATE Test ban treaties are a seemingly simple ap- advantages in one area must be weighed against preach to nuclear arms control, yet their im- advantages in another. Consequently, all aspects pact is complex and multi-faceted. The deci- of a new treaty must be considered together sion as to whether a given test ban treaty is and the cumulative impact evaluated in terms in our overall national interest is dependent of a balance with the Soviet Union. Finally, on many questions concerning its effects. Dis- the total net assessment of the effects of a

3 —

Photo credit: Department of Energy Signal cables and test device being lowered down test hole. treaty on our national security must be weighed of a nuclear deterrent and the extent to which against the alternative: no treaty. arms control can contribute to national security. It is perhaps because test ban treaties go One’s opinion about the effects of a test ban, to the very heart of nuclear weapons policy that and thus its desirability, is largely dependent the debate over them remains unresolved. Three on one’s philosophical position about the role decades of negotiation between the United 5

States and the U.S.S.R. have produced only ful nuclear explosions (PNEs) to yields no three limitation treaties, two of which remain greater than 150 kt, and aggregate yields unratified: in a salvo to no greater than 1500 kt. It 1. 1963 Limited Nuclear Test Ban Treaty was signed by the United States and the (LTBT). This treaty bans nuclear explo- U.S.S.R. on May 28,1976. Although PNET sions in the atmosphere, in outer space, has yet to receive the consent of the U.S. and under water. It was signed by the Senate, both nations consider themselves United States and the U.S.S.R. on August obligated to adhere to it. 5, 1963 and has been in effect since Oc- tober 10, 1963. Over 100 other countries Nuclear explosions compliant with these have also signed this treaty. treaties can only be conducted by the United 2. 1974 Threshold Test Ban Treaty (TTBT). and the U.S.S.R. underground, at specific test This treaty restricts the testing of under- sites (unless a PNE), and with yields no greater ground nuclear weapons to yields no greater than 150 kt. Although they have had impor- than 150 kilotons (kt). It was signed by tant positive environmental and arms control the United States and the U.S.S.R. on impacts, these treaties have not prevented the July 3,1974. Although the TTBT has yet development of new types of warheads and to receive the consent of the U.S. Senate, bombs. For this reason, public interest in a both nations consider themselves obligated complete test ban or a much lower threshold to adhere to it. remains strong, and each year a number of 3. 1976 Peaceful Nuclear Explosions Treaty proposals continue to be brought before the (PNET). This treaty is a complement to Congress to limit further the testing of nuclear the TTBT and restricts individual peace- weapons.

THE MEANING OF VERIFICATION For the United States, the main national tion of the benefits of a treaty compared with security benefits derived from test limitation one’s perception of its disadvantages and the treaties are a result of the Soviet Union being likelihood of violations. No treaty can be con- similarly restricted. In considering agreements sidered to be either verifiable or unverifiable that bear on such vital matters as nuclear without such a value judgment. Moreover, this weapons development, each country usually judgment is complex because it requires not assumes as a cautious working hypothesis that only an understanding of the capabilities of the other parties would cheat if they were suffi- the monitoring systems, but also an assess- ciently confident that they would not be caught. ment as to what is an acceptable level of risk, Verification—the process of confirming com- and a decision as to what should constitute sig- pliance and detecting violations if they occur— nificant noncompliance. Consequently, people is therefore central to the value of any such with differing perspectives on the role of nu- treaty. clear weapons in national security and on the motivations of Soviet leadership will differ on “To verify” means to establish truth or ac- the level of verification required.l curacy. Yet in the arena of arms control, the process of verification is political as well as technical. It is political because the degree of ‘This issue is discussed further in chapter 2, Seism”c Verifi- verification needed is based upon one’s percep- cation in the Context of National Security. 6

ASPECTS OF MONITORING UNDERGROUND NUCLEAR EXPLOSIONS Like earthquakes, the force of an underground The threat of evasion can be greatly reduced nuclear explosion creates seismic waves that by negotiating within a treaty various coop- travel through the Earth. A seismic monitor- erative monitoring arrangements and testing ing network must be able both to detect and restrictions. However, there will eventually be to identify seismic signals. Detection consists a yield below which the uncertainty of any of recognizing that a seismic event has oc- monitoring regime will increase significantly. curred and locating the source of the seismic The point at which the uncertainties of the mon- signals. Identification involves determining itoring system no longer permit adequate veri- whether the source was a nuclear explosion. fication is a political judgment of the point at In the case of a threshold treaty, the monitor- which the risks of the treaty outweigh the benefits. ing network must also be able to estimate the Determining the credibility of various eva- yield of the explosion from the seismic signal sion methods requires subjective judgments to determine if it is within the limit of the about levels of motivation and risk as well as treaty. All of this must be done with a capa- more objective technical assessments of the bility that can be demonstrated to adequately capability of the monitoring system. To sepa- defeat any credible attempt to evade the mon- rate the technical capabilities from the subjec- itoring network. tive judgments, we will first describe our ca- If the seismic signals from explosions are not pability to detect and identify seismic events deliberately obscured or reduced by special ef- and then will show how this capability is lim- forts, seismic networks would be capable of de- ited by various possible evasion methods. All tecting and identify with confidence nuclear considerations are then combined to address explosions with yields below 1 kt. What stops the summary question: How low can we go? this capability from being directly translated into a monitoring threshold is the requirement Detecting Seismic Events that the monitoring network be able to accomplish detection and identification with high The first requirement of a seismic network confidence in spite of any credible evasion sce- is that it be capable of detecting a seismic nario for concealing or reducing the seismic sig- event. If the Earth were perfectly quiet, this nal from a test explosion. would be easy. Modem seismometers are highly sophisticated and can detect remarkably Demonstrating that the monitoring capabil- small motions. Limitations are due not to the ity meets this requirement becomes complex inherent sensitivity of the instrument but rather at lower yields. As the size of the explosion to phenomena such as wind, ocean waves, and becomes smaller: even rush hour traffic. All of these processes

● cause ground vibrations that are sensed by there are more opportunities for evading seismometers and recorded as small-scale back- the monitoring network, ● ground motion, collectively referred to as there are more earthquakes and industrial “noise.” Seismic networks, consisting of groups explosions from which such small clandes- of instruments, are designed to distinguish tine explosions need to be distinguished, events like earthquakes and explosions from and ● this ever-present background noise. The extent there are more factors that can strongly to which a seismic network is capable of de- influence the seismic signal. tecting such events is dependent on many fac- The cumulative effect is that the lower the tors. Of particular importance are the types yield, the more difficult the task of monitor- of seismic stations used, the number and dis- ing against possible evasion scenarios. tribution of the stations, the amount of back- 7 ground noise at the site locations, and the effi- even smaller events in selected regions. The ciency with which seismic waves travel from existing seismic array in Norway, for exam- the source to the receiving station through the ple, has easily detected Soviet explosions with surrounding geologic area. yields of a fraction of a kt conducted 3,800 kilometers away at the Soviet test site in East- Detecting seismic events can be accomplished ern Kazakhstan (figure l-l). with high certainty down to extremely small yields. A hypothetical seismic network with Seismic stations within the Soviet Union stations only outside the Soviet Union would would further improve the detection capabil- be capable of detecting well-coupled2 explo- ity of a network. In principle, almost any sions with yields below 1 kt anywhere within desired signal detection level could be achieved the Soviet Union and would be able to detect within the Soviet Union if a sufficient number of internal stations were deployed. In this sense, the detection of seismic events does not provide a barrier for monitoring even the lowest 2A “well-coupled” explosion is one where the energy is well transmitted from the explosion to the surrounding rock. threshold treaties. From a monitoring stand-

Figure 1-1 .-Signal From Semipalatinsk

Noress array beam

I I I I I I I I I 0 10 20 30 40 50

Time (seconds)

Seismic signal from a 0.25 kt explosion at the Soviet test site near Semipalatinsk. Recorded 3,800 km away at the NORESS seismic array in Norway on July 11, 1985. The signal to noise ratio is about 30 indicating that much smaller explosions could be detected even at this great distance.

SOURCE: R.W. Alewire Ill, “Seismic Sensing of Soviet Tests,” Defense 85, December 1985, pp. 11-21. .

point, stations within the Soviet Union are same size as those of potential underground more important for improving identification nuclear explosions. Several methods can be ap- capabilities than for further reduction of the plied to differentiate earthquakes from under- already low detection threshold. ground nuclear explosions. Note, however, that no one method is completely reliable. It is the Identifying Seismic Events as set of different identification methods taken Nuclear Explosions as a whole and applied in a systematic fashion that is assessed when summaries on capabil- Once a seismic signal has been detected, the ity are given. In this sense, identification is next task is to determine whether it was cre- a “winnowing” process. ated by a nuclear explosion. Seismic signals The most basic method of identification is are generated not only by nuclear explosions, but also by natural earthquakes, rockbursts to use the location and the depth of the event. in mines, and chemical explosions conducted Over 90 percent of all seismic events in the for mining, quarry blasting, and construction. U.S.S.R. can be classified as earthquakes simply because they are either too deep or not in Every day there are many earthquakes a plausible location for a nuclear explosion. For around the globe whose seismic signals are the seismic events that are in a location and at a

Photo credit: Department of Energy

Craters formed by cavity collapse in Yucca Flat, Nevada Test Site. 9 depth that could bean explosion, other meth- tify such chemical explosions. However, the ods of discrimination based on physical differ- absence of evidence for ripple firing cannot be ences between earthquakes and explosions are taken as evidence that the event is not a chem- used. ical explosion. Because of the size consideration, industrial explosions are not an identifi- When a nuclear device explodes underground, cation problem for normal nuclear explosions it applies uniform pressure to the walls of the above 1 kt. The difficulty, as we will see later, cavity it creates. As a result, explosions are comes in distinguishing between a small decou- seen seismically as highly concentrated sources pled nuclear test and a large salvo-fired chem- of compressional waves, sent out with approx- ical explosion. This difficulty can be limited imately the same strength in all directions from through such treaty provisions as options for the point of the detonated device. An earth- inspections and constraints on chemical ex- quake, on the other hand, occurs when two plosions. 3 blocks of the Earth’s crust slip past each other along a fault. An earthquake generates shear Because a seismic signal must be clearly de- waves from all parts of the fault that rupture. tected before it can be identified, the threshold for identification will always be greater These fundamental differences between earth- than the threshold for detection. As described quakes and explosions are often exhibited in in the previous section, however, the detection their seismic signals. As a result, seismologists threshold is quite low. Correspondingly, even have been able to develop a series of methods a hypothetical network consisting of stations to differentiate the two sources based on the only outside the Soviet Union would be capa- different types of seismic waves they create. ble of identifying seismic events with magni- The combination of all methods, when applied tudes corresponding to about 1 kt if no at- in a comprehensive approach, can differenti- tempts were made to evade the monitoring ate with high confidence between explosions, system.4 Seismic stations within the Soviet down to low yields, and earthquakes. Union would further improve the identifica- As the size of the seismic events gets smaller, tion capability of a network. nuclear explosions must be distinguished not It has been argued that the use of high fre- only from earthquakes, but also from other quency seismic data will greatly improve our kinds of explosions. Industrial chemical explo- capability to detect and identify low-yield nu- sions (e.g. in a quarry operation) pose a par- clear explosions.56 Recent experiments con- ticularly difficult problem because their seismic ducted by the Natural Resources Defense signals have physical characteristics similar Council together with the Soviet Academy of to those of nuclear explosions. Consequently, Sciences are beginning to provide high fre- the seismic methods that are routinely used quency seismic data from within the Soviet to differentiate earthquakes from explosions Union that shows clear recordings of small ex- cannot distinguish between some legitimate plosions (see box l-A). There remains, however, chemical explosions for mining purposes and a lack of consensus on the extent to which the a clandestine nuclear test explosion. Fortu- use of higher frequency data will actually im- nately, industrial explosions in the range of 1 to 10 kt are rare (less than one a year). Large 3See chapter 6, Eva&-ng a Monitoring Network, for a discus- explosions are usually ripple fired so as to min- sion of treaty constraints. imize ground vibration and fracture rock more ‘See chapter 5, Identifying Seism”c Events. efficiently. Ripple firing is often accomplished ‘For example, much lower identification thresholds have been defended by J.F. Evernden, C.B. Archarnbeau, and E. Cran- with bursts spaced about 0.2 seconds apart swick, “An Evaluation of Seismic Decoupling and Underground over a duration of about a second. Recent work Nuclear Test Monitoring Using High-Frequency Seismic Data,” suggests that this ripple-firing has an identifi- Reviews of Geophysics, vol. 24, May 1986, pp. 143-215. ‘See chapter 4, De&ctingSeismz”cE vents, and chapter 5 Iden- able signature apparent in the observed seis- tifying Seism”c Events, for discussions of high frequency mon- mic signals, and therefore can be used to iden- itoring. 10

Box 1-A.—NRDC/Soviet Academy of Sciences New seismic data from the Soviet Union is becoming available through an agreement between the Natural Resources Defense Council and the Soviet Academy of Sciences. The agreement provides for the establishment of a few high-quality seismic stations within the Soviet Union around the area of the Soviet test site in Kazakhstan. The agreement also included experiments in which the Soviet Union detonated chemical explosions of known yield near the test site so that the test site could be calibrated. The seismograph below is from a 0.01 kt chemical explosion detonated near the Soviet test site and recorded 255 km away. The signal of the explosion can be clearly seen along with the coinciden- tal arrival of seismic waves caused by a large earthquake that occurred south of New Zealand. The three components of ground motion are east-west (E), north-south (N), and vertical (Z).

Start of explosion recording

Start of earthquake record 1

SOURCE: Natural Resources Defense Council. prove monitoring capabilities. The lack of con- til more extensive data can be collected at re- sensus is due to differences in opinion as to gional distances from areas throughout the how well U.S. experience and the limited ex- Soviet Union. Nevertheless, there is general perience near the Soviet test site can be ex- agreement among seismologists that good trapolated to an actual comprehensive moni- data is obtainable at higher frequencies than toring system throughout the Soviet Union. those used routinely today, and that this data Consequently, the debate over improved ca- offers advantages for nuclear monitoring that pability will probably remain unresolved un- should continue to be explored. 11

Photo credit: Sand/a National Laboratories An example of what an internal seismic station might look like.

Evading a Seismic Monitoring Network by detonating the explosion during or soon after an earthquake. If the earthquake were suffi- To monitor a treaty, nuclear tests must be ciently large and the small explosion properly detected and identified with high confidence timed, the seismic signal of the explosion would even if attempts are made to evade the moni- be hidden in the seismic signal of the earth- toring system. As mentioned earlier, it is the quake. However, it is not practical for an feasibility of various evasion scenarios that evader to wait for an earthquake that is in the sets the lower limit on the monitoring capa- immediate vicinity of the test site. Therefore, bility. The major evasion concerns are: the masking earthquake would have to be large ● that the signal of an explosion could be and at some distance. The smaller nuclear ex- hidden in the signal of a naturally occur- plosion will produce higher frequency signals ring earthquake, than the earthquake, and filtering the signals ● that an explosion could be muffled by det- will reveal the signals from the explosion (fig- onating it in a large underground cavity, ure 1-2). For this and other reasons (chapter or 6), the hide-in-earthquake scenario need no ● that a nuclear test could be disguised as longer be viewed as a credible evasion threat or masked by a large legitimate industrial for explosions above 5-10 kt. To counter this explosion. threat for explosions below 5-10 kt may require access to data from seismic stations within the The hide-in-earthquake scenario assumes Soviet Union, because the higher frequency that a small nuclear test could be conducted seismic signals from explosions below 5-10 kt 12

Figure l-2.—Explosion and Earthquake

Conventional Recording Using Low Frequencies

50 100 150 Large ½ kiloton earthquake explosion Time (seconds)

Same Recording But With High Frequency Passband

I I o 50 1( 150 200 Large ½ kiloton Time (seconds) earthquake explosion Both the upper and lower seismograms were recorded in Norway and cover the same period of time. The upper seismogram is the conventional recording of low-frequency seismic waves. Both it and the lower recording show, at the time of 30 sec- ends, the arrival of waves from a large earthquake that occurred in the eastern part of the Soviet Union. About one minute after the earthquake, at a time of 100 seconds, the Soviet Union conducted a very small (about ½ kt) underground nuclear explosion at their Kazakhstan test site. With the standard filter (upper seismogram), the signal of the explosion appears hidden by the earthquake. Using a passband filter for higher frequency seismic waves (lower seismogram), the explosion is revealed.

SOURCE: Semiannual Technical Summary for Norwegian Seismic Array for 1984, Royal Norwegian Council for Scientific and Industrial Research, Scientific Report No. 1-84/85. 13 may not always be picked up by stations out- taneously detonated under the guise of legiti- side the Soviet Union. mate industrial activity. As discussed in the section on identification, Decoupling appears to be potentially the differentiating a small nuclear explosion from most effective of all evasion methods. It in- a legitimate industrial explosion associated volves detonating a nuclear device in a large with mining and quarry blasting is difficult underground cavity so as to muffle the seis- because both produce similar seismic signals. mic signal. If the explosion occurs in a large This is not a problem for identification under cavity, the explosive stresses are reduced be- normal circumstances because industrial ex- cause they are spread over the large area of plosions in the 1-10 kt range occur less than the cavity wall. At a certain cavity size, the once a year. However, industrial explosions stresses will not exceed the elastic limit of the may create a problem when considering the rock. At such a point, the explosion is said to decoupling evasion scenario. That is, some low- be “fully decoupled” and the seismic signal be yield decoupled explosions might produce seis- comes greatly reduced. At low seismic frequen- mic signals comparable to those observed from cies, a fully decoupled explosion may have a large chemical explosions and no routine ca- signal 70 times smaller than that of a fully cou- pability has yet been developed to differentiate, pled explosion. At high frequencies the decou- with high confidence, between such signals. pling factor is probably reduced to somewhere None of the evasion scenarios poses any seri- between 10 and 7. ous problem for monitoring explosions above 10 kt. However, to provide adequate monitor- Above 10 kt, decoupling is not considered ing capability below 10 kt, efforts must be to be a credible evasion scenario because: 1) made to limit decoupling opportunities. This the clandestine construction of a cavity large would include an internal seismic network and and strong enough to decouple a 10 kt explo- provisions within the treaty (such as pre-notifi- sion is not feasible; and 2) even if such an ex- cation with the option for on-site inspection) plosion were somehow fully decoupled, the seis- to handle the large numbers of chemical ex- mic signal would stand a good chance of being plosions. At a few kt, decoupling becomes pos- detected and possibly identified. Therefore, sible when considered as part of an evasion sce decoupling could be most effective for small nario. Arranging such a test, however, would explosions up to a few kt, particularly when be both difficult and expensive; and it is not done in conjunction with a legitimate indus- clear that the evader could have confidence trial explosion. For example, a potential eva- that such a test (even if it were successful) sion method would be to secretly decouple a would go undiscovered. However, the possi- small nuclear explosion in a large underground bility remains that a few tests of small magni- cavity and mask or attribute the muffled sig- tude could be conducted and remain unidenti- nals to a large chemical explosion that is simul- fied as nuclear explosions.

HOW LOW CAN WE GO? Given all the strengths and limitations of that question depends largely on what is ne- seismic methods in detecting and identifying gotiated in the treaty. As we have seen, the Soviet nuclear explosions, combined with the challenge for a monitoring network is to dem- credibility of the various evasion scenarios, the onstrate a capability to distinguish credible ultimate question of interest for monitoring evasion attempts from the background of fre- any low-yield threshold test ban treaty is es- quent earthquakes and legitimate industrial sentially: How low can we go? The answer to explosions that occur at low yields. 14

The monitoring burden placed on the seis- resources. Consequently, the signals from ex- mic network by various evasion scenarios can plosions below 10 kt could perhaps be muffled. be greatly lessened if seismology gets some The seismic signals from these small muffled help. The sources of help are varied and numer- explosions would need not only to be detected, ous: they include not only seismic monitoring but also distinguished from legitimate chemi- but also other methods such as satellite sur- cal industrial explosions and small earthquakes. veillance, radioactive air sampling, communi- Demonstrating a capability to defeat credible cation intercepts, reports from intelligence evasion attempts would require seismic sta- agents, information leaks, interviews with tions throughout the Soviet Union (especially defectors and emigres, on-site inspections, etc. in areas of salt deposits), negotiated provisions The structure of any treaty or agreement should within the treaty to handle chemical explo- be approached through a combination of seis- sions, and stringent testing restrictions to limit mic methods, treaty constraints, and inspections decoupling opportunities. If such restrictions that will reduce the uncertainties and difficul- could be negotiated, most experts believe that ties of applying seismic monitoring methods to a high-quality, well run network of internal sta- every conceivable test situation. Yet with these tions could monitor a threshold of around 5 considerations in mind, some generalizations kt. Expert opinion about the lowest yields that can still be made about monitoring at various could reliably be monitored ranges from 1 kt levels. to 10 kt; these differences of opinion stem from differing judgments about what technical pro- Level l–Above 10 kt visions can be negotiated into the treaty, how much the use of high frequencies will improve Nuclear tests with explosive yields above 10 our capability, and what levels of monitoring kt can be readily monitored with high confi- capability are necessary to give us confidence dence.7 This can be done with external seismic that the Soviet Union would not risk testing networks and other national technical means. above the threshold. The seismic signals produced by explosions of this size are discernible and no method of evad- Level 3–Below 1-2 kt ing a seismic monitoring network is credible. However, for accurate monitoring of a 10 kt For treaty thresholds below 1 or 2 kt, the threshold treaty it would be desirable to have burden on the monitoring country would be stations within the Soviet Union for improved much greater. It would become possible to yield estimation, plus treaty restrictions for decouple illegal explosions not only in salt handling the identification of large chemical domes but also in media such as granite, allu- explosions in areas where decoupling could vium, and layered salt deposits. Although it take place. may prove possible to detect such explosions with an extensive internal network, there is no convincing evidence that such events could Level 2—Below 10 kt but be confidently identified with current technol- Above 1-2 kt ogy. That is, additional work in identification Below 10 kt and above 1-2 kt, the monitor- capability will be required before it can be de- ing network must demonstrate a capability to termined whether such small decoupled explo- defeat evasion scenarios. Constructing an un- sions could be reliably differentiated from the derground cavity of sufficient size to fully background of many small earthquakes and routine chemical explosions of comparable decouple an explosion in this range is believed magnitude. to be feasible in salt, with dedicated effort and Level 4—Comprehensive Test Ban 7The United States and the Soviet Union presently restrict their testing to explosions with yields no greater than 150 kt. The bomb dropped on Hiroshima was estimated to have a yield There will always be some threshold below of 13 kt. which seismic monitoring cannot be accom- 15 plished with high certainty. A comprehensive weigh the significance of any undetected clan- test ban treaty could, however, still be consid- destine testing (should it occur) below the mon- ered adequately verifiable if it were determined itoring threshold. that the advantages of such a treaty would out-

ESTIMATING THE YIELD OF NUCLEAR EXPLOSIONS For treaties that limit the testing of nuclear other hand, is more similar to the geology weapons below a specific threshold, the moni- found in the eastern United States. It is a geo- toring network not only must detect and iden- logically old and stable area, away from any tify a seismic event such as a nuclear explosion recent plate tectonic activity. When an explo- but also must measure the yield to determine sion occurs at the Soviet test site, the cold, whether it is below the threshold permitted by solid rock transmits the seismic energy strongly. the treaty. This is presently of great interest As a consequence, waves traveling from the with regard to our ability to verify compliance main Soviet test site in Eastern Kazakhstan with the 150-kt limit of the 1974 Threshold appear much larger than waves traveling from Test Ban Treaty. the Nevada test site. Unless that difference is taken into account, the size of Soviet explo- The yield of a nuclear explosion maybe esti- sions will be greatly overestimated. mated from the seismic signal it produces. Yield estimation is accomplished by measur- The geological difference between the test ing from the seismogram the size of an identi- sites can result in systematic error, or “bias, fied seismic wave. When corrected for distance in the way that measurements of seismic waves and local effects at the recording station, this are converted to yield estimates. Random er- measurement is referred to as the seismic mag- ror is also introduced into the estimates by the nitude. The relationship between seismic mag- measurement process. Thus, as with any meas- nitude and explosive yield has been determined urement, there is an overall uncertainty asso- using explosions of known yields. This rela- ciated with determining the size of an under- tionship is applied to estimate the size of un- ground nuclear explosion. This is true whether known explosions. The problem is that the rela- the measurement is being made using seismol- tionship was originally determined from U.S. ogy, hydrodynamic methods, or radiochemical and French testing and calibrated for the Ne- methods. It is a characteristic of the measure- vada test site. As a result, Soviet tests are ment. To represent the uncertainty, measure- measured as if they had been conducted at the ments are presented by giving the most likely Nevada test site unless a correction is made. number (the mean value of all measurements) No correction would be needed if the U.S. and and a range that represents both the random Soviet test sites were geologically identical, scatter of the measurement and an estimate but they are not. of the systematic uncertainty in the interpretation of the measurements. It is most likely The Nevada test site, in the western United that the actual value is near the central num- States, is in a geologically young and active ber and it is increasingly unlikely that the ac- area that is being deformed by the motion be- tual number would be found towards either end tween the North American and Pacific tectonic of the scatter range. When appropriate, the un- plates. This recent geologic activity has cre- certainty range can be expressed by using what ated an area of anomalously hot and possibly is called a “factor of uncertainty. ” For exam- even partially molten rock beneath the Nevada ple, a factor of 2 uncertainty means that the test site. As a result, when an explosion oc- best estimate of the yield (the “measured cen- curs at the Nevada test site, the rock deep be- tral value”) when multiplied or divided by 2, neath Nevada absorbs a large proportion of defines a range within which the true yield will the seismic energy. The Soviet test site, on the fall in 95 out of 100 cases. This is the high de- 76 gree of certainty conventionally used in seis- capability could be greatly improved by incor- mology and may or may not be appropriate porating new methods of yield estimation. in a verification context.8 Most seismologists feel that if new methods The uncertainty associated with estimates were applied, the resulting uncertainty for measuring explosive yields in the range of 150 of the systematic error can be greatly reduced kt at the Soviet test site would be closer to by negotiating provisions that restrict testing 10 a factor of 1.5 than a factor of two. Present to specific test sites and by calibrating the test methods are stated to be accurate only to a sites. If such calibration were to be an integral factor of two in part because they have not yet part of any future treaty, the concern over sys- incorporated the newer methods of yield esti- tematic errors of this kind would become min- mation that use surface waves and Lg waves. 11 imal. The majority of errors that would remain The uncertainty of this comprehensive ap- would be random. As discussed in chapter 2, proach could be further reduced if calibration a country considering cheating could not take shots were performed and testing were re- advantage of the random error, because it stricted to areas of known geologic composi- would not be possible to predict how the ran- tion. It is estimated that through such measures, dom error would act on any given evasion attempt. In other words, if a country attempted the uncertainty in seismically measuring Soviet tests could be reduced to a level comparable to to test above the threshold, it would have to the uncertainty in seismically measuring U.S. realize that with every test the chances of ap- tests. An uncertainty factor of 1.3 is the current pearing in compliance would decrease and at capability that seismic methods are able to the same time the chance that at least one of achieve for estimating yields at the Nevada test the tests would appear in unambiguous viola- site. tion would increase. For this reason, the range of uncertainty should not be considered as a As with detection and identification, yield range within which cheating could occur. estimation becomes more difficult at low yields. Most of the systematic error associated with Below about 50 kt, high-quality Lg-wave sig- estimating the yields of Soviet nuclear explo- nals can only be reliably picked up by stations sions is due to geological differences between within the Soviet Union less than 2,000 km the U.S. and Soviet test sites and in the coup- from the test site. For explosions below 10 kt, ling of the explosion to the Earth. Therefore, the uncertainty increases because small explo- the single most important thing that can be sions do not always transmit their signals effi- done to reduce the uncertainty in yield esti- ciently to the surrounding rock. For small ex- mation is to calibrate the test sites. Calibra- plosions, the uncertainty could be reduced by tion could be accomplished through an ex- restricting such tests to depths below the water change of devices of known yield, or through independent measures of the explosive yield such as can be provided by radiochemical or iOThis reduction of uncertainly derives from using More than hydrodynamic methods (See box l-B). one statistically independent method. Consider as an example the situation where there are three independent methods of cal- Our present capability to estimate seismi- culating the yield of an explosion, all of which (for the sake of this example) have a factor of two uncertainty in a log normal cally the yields of Soviet explosions is often distribution: 9 cited as a factor of two. While this may re- # of Methods Resulting Uncertainty flect present operational methods, it is not an 1 2.0 2 1.6 accurate representation of our capability. Our 3 1.5 By combining methods, the uncertainty can be reduced below the uncertainty of the individual methods. This methodol- ‘See chapter 2, Seismic Verification in the Context of National ogy, however, can only reduce random error, Systematic error, Security. such as differences between the test site, will remain and limit ‘U. S. Department of State, “Verifying Nuclear Testing Limi- the extent to which the uncertainty can be reduced unless cali- tations: Possible U.S.-Soviet Cooperation, Special Report No. bration is performed. 152, ” Aug. 14, 1986. ‘‘See chapter 7, Estimating the Yields of Nuclear Explosions. 17 table, so that their signal will be transmitted the capability of seismic methods could be in- effectively. proved to a point comparable to the accuracy of other methods, such as CORRTEX, that re- Our capability to estimate explosive yields quire a foreign presence and equipment at the would depend to a large degree on the approach that was taken and what was negotiated in fu- test site. In any case, if the objective of reduc- ture treaties. If new methods of yield determi- ing the uncertainty is to reduce the opportu- nation were incorporated into the measure- nities for cheating, small differences in random uncertainty do not matter. ments and calibration shots were performed,

SOVIET COMPLIANCE The decision as to what constitutes adequate In examining compliance with the 150-kt verification should represent a fair assessment threshold, seismic evidence is currently con- of the perceived dangers of non-compliance. sidered the most reliable basis for estimating This necessarily involves a weighing of the ad- the yields of Soviet underground nuclear ex- 14 vantages of the treaty against the feasibility, plosions. The distribution of Soviet tests in- likelihood, and significance of noncompliance. dicates that about 10 (out of over 200) Soviet Such decisions are subjective and in the past explosions since the signing of the Threshold have been influenced by the desirability of the Test Ban Treaty in 1974 could have estimated treaty and the political attractiveness of par- yields with central values above the 150 kt ticular monitoring systems.12 Specific concern threshold limit, depending on how the estimate over compliance with test ban treaties has been is made.15 These 10 tests could actually be at heightened by findings by the Reagan Admin- or below the 150 kt limit, but have higher yield istration that: estimates due to random fluctuations in the seismic signals. In fact, when the same meth- “Soviet nuclear testing activities for a number of tests constitute a likely violation of le- ods of yield estimation are applied to U.S. tests, gal obligations under the Threshold Test Ban approximately the same number of U.S. tests Treaty.”13 also appear to be above the 150 kt threshold limit. These apparent violations, however, do Although the 1974 Threshold Test Ban not mean that one, or the other, or both coun- Treaty and the 1976 Peaceful Nuclear Explo- tries have cheated; nor does it mean per se that sions Treaty have remained unratified for over seismology is an inadequate method of yield 10 years, both nations have expressed their in- estimation. It is inherent in any method of tent to abide by the yield limit. Because nei- measurement that if several tests are per- ther the United States nor the Soviet Union formed at the limit, some of these tests will has indicated an intention not to ratify the trea- have estimated central values above the yield ties, both parties are obligated under interna- limit. Because of the nature of measurements tional law (Article 18, the 1969 Vienna Con- (using any method), it is expected that about vention on the Law of Treaties) to refrain from half the Soviet tests at 150 kt would be meas- acts that would defeat their objective and ured as slightly above 150 kt and the other half purpose. would be measured as slightly below 150 kt.

“%e chapter 2, Seisnn”c Verification in the Context of Na- “Conclusion of the Defense Intelligence Agency Review Panel tional Security. as stated in a letter from Roger E. Batzel, Director of Lawrence “’’The President’s Unclassified Report on Soviet Noncom- Livermore National Laboratory to Senator Claiborne Pen on pliance with Arms Control Agreements, ” transmitted to the Feb. 23, 1987. Congress, Mar. 10, 1987. *’See chapter 7, Estimating the Yields of Nuclear Explosions. 18

I Box 1-B.–CORRTEX CORRTEX (Continuous Reflectometry for Radius versus Time Experiments) is a technique that was developed in the mid-1970s to improve yield estimation using non-seismic (hydrodynamic) methods. In the CORRTEX technique, a satellite hole is drilled parallel to the emplacement hole of the nuclear device and an electrical sensing cable is lowered down the hole (see figure). When the explosion occurs, a shock wave moves outward crushing and electrically shorting the cable. By measuring with electronic equipment at the surface the rate at which the cable is shorted out, the rate of expansion of the shock wave can be calculated. From the rate of expansion of the shock wave and the properties of the surrounding medium, the yield of the nuclear device can be estimated. A full assessment of this method is presented in the Appendix, Hydrodynamic Methods of Yield Estimation.

CORRTEX CORRTEX recorder recorder r 1 t Surface 1 t Surface

‘T- Sensing Sensing cable cable

ostic / \ \ /I \ Y point d point \ / \ Shock front / \ progression

Typical cable emplacement Moving shock wave from in satellite hole nuclear detonation crushes and shortens cable 19

All of the estimates of Soviet and U.S. tests limit consistent with compliance with the 150 are within the 90 percent confidence level that kt limit of the Threshold Test Ban Treaty.16 17 one would expect if the yields were 150 kt or ‘81bid. carry less. Extensive statistical studies have exam- ITone of the fir9t ~oupS to out such statistical studies ined the distribution of estimated yields of ex- was Lawrence Livermore National Laboratory. Their conclusion was reported in open testimony before the Senate Armed plosions at Soviet test sites. These studies have Services Committee on Feb. 26, 1987 by Dr. Milo Nordyke, concluded that the Soviets are observing a yield Leader of the Treaty Verification Program.

VERIFICATION OF THE TTBT AND THE PNET As noted above, the 1974 Threshold Test to have a factor of 1.3 uncertainty for measur- Ban Treaty and the 1976 Peaceful Nuclear Ex- ing yields greater than 50 kt at the Soviet test plosions Treaty have not been ratified. Most site.21 22 The drawbacks are that it requires pre- recently, the Senate failed to consent to ratifi- notification and cooperation of the host coun- cation at least in part because of concerns that try to the extent that foreign personnel and the size of Soviet explosions cannot be meas- their equipment must be allowed at the test ured with adequate confidence.18 As a result, site for each test. Also, CORRTEX has limited for over 10 years the United States has con- application for monitoring a low-yield treaty tinued to abide by the treaties, yet refused to and none for detecting clandestine testing, and ratify them, ostensibly because they cannot so it would not improve our ability to monitor be adequately verified. Note that if the trea- low-yield testing thresholds. ties were ratified, the protocols would come Alternatively, advanced seismic methods into force calling for an exchange of data which, could be used. The advantage of seismic meth- if validated, would then improve the verifica- 19 20 ods is that a continued presence of foreign per- tion. However, the treaty contains no pro- sonnel at the test site would not be necessary. visions for an independent verification of the Additionally, our ability to monitor all Soviet data, most importantly, the yields of the cali- testing (not just testing above 50 kt) would bration shots. Therefore, many experts ques- be improved. If the Soviet test site was tion the value of such data, unless the data can calibrated and advanced seismic methods were be validated. utilized, the uncertainty in seismic yield estima- As a solution to the problem of uncertainty tion could be reduced to a level comparable to in yield estimation, the administration has re- CORRTEX. In fact, CORRTEX could be used cently proposed the use of an on-site measure- to confirm independently the yields of the cal- ment system called CORRTEX for all tests ibration shots. The Soviet Union has already above 50 kt. The CORRTEX system is stated agreed to the use of CORRTEX for one or two such explosions at the Soviet test site.23 ‘“Threshold Test Ban Treaty and Peaceful NucJear Explosions Treaty, Hearings before the Senate Committee on Foreign Re- lations, Jan. 13 and 15, 1987, S. Hrg. 100-115. ‘The protocol of the TTBT calls for an exchange of geologi- *’U. S. Department of State, Bureau of Public Affairs, “U.S. cal data plus two calibration shots from each geophysically dis- Policy Regarding Limitations on Nuclear Testing, Special Re- tinct testing area. port No. 150, ” August 1986. ‘“Monitoring would also be improved by the data that would “see appendix, Hydrodynm”c Methods of Yield Estimation. be obtained from all PNE shots that have occurred in the So- ‘sStatement of the Secretary of State of the United States viet Union since 1976. and Minister of Foreign Affairs of the Soviet Union, Dec. 9, 1987. A PHASED APPROACH If the policy decision were made that trea- terest, then from a verification viewpoint there ties further restricting or eliminating the test- is much to be said for a phased approach to ing of nuclear weapons are in our national in- this goal. Conceptually, it would begin with 20

a limit that can be monitored with high confi- After such a network had been in operation dence using current methods, but would estab- for some time, many of the disagreements con- lish the verification network for the desired cerning hypothetical networks would be re- lowest level. The threshold would then be lo- solved. It would be known how well seismic wered as information, experience, and confi- waves travel through the geology of the So- dence increase. viet Union and what noise levels exist in various areas. Through such a process, experience For example, the United States and the So- with large-scale monitoring would be gained viet Union could begin with a treaty that pro- and there would be more accurate knowledge hibited testing above 10 kt. This is a level that of what level of monitoring effort is needed. can currently be well monitored seismically .24 After this information and experience had been The verification network negotiated within the obtained, provisions within the treaty could treaty, however, would be designed with the call for the further reduction of the threshold. goal of monitoring down to the 1-2 kt level. At that point, it would be better known down This would include a network of advanced seis- to what level monitoring could be accom- mic stations throughout the Soviet Union to plished. If it were determined that a lower detect off-site testing plus negotiated provi- threshold could be verified, the testing thresh- sions to reduce evasion possibilities.25 Principal old could be set to the new verifiable limit. By among these would be to restrict testing to be- the continuation of periodic reviews, the treaty low the water table and to a specified test site would always be able to use developments in where decoupling opportunities would be lim- seismology to maximize the restriction of nu- ited, to require some special handling (such as clear testing. pre-notification and on-site inspection) for the detonation of large chemical explosions, and This procedure, however, does not take into to institute measures to confirm a prohibition account any considerations other than seismic on decoupling. To reduce the systematic un- verification. It simply presents the maximum certainty in yield estimation, a series of cali- restrictions that could be accomplished from a seismic verification standpoint. Considera- bration shots would need to be conducted at each test site using either an independent tions other than seismic verification may re- method such as CORRTEX or the exchange sult indifferent thresholds being more desirable. Lower thresholds, or even a complete ban of devices of known yield. on testing, may be chosen if the political advantages are seen to outweigh the risk, and if the significance of minor undetected cheating is seen to be small when all monitoring 24some exWrts be~eve that decoupling is not feasible above methods are considered. Higher thresholds 5 kt and consequently, that a 5 kt threshold could be well monitored with existing methods and facilities; while others would may be chosen to permit certain types of test- place the threshold somewhere between 5 and 10. However, vir- ing or to avoid placing a threshold at a bound- tually all experts agree that tests above 10 kt can be well moni- ary where particularly significant tests could tored, even assuming the monitored country is intent on cheating. occur at yields only slightly above the 26CJ& chapter G, Eva&”ng a Mom”toring Network. threshold. Chapter 2 Seismic Verification in the Context of National Security CONTENTS

Page The Role of Verification...... , ...... 23 The Definition and Value Judgment of “Verification” ...... 24 Are Test Ban Treaties in Our National Interest?...... 25 Evaluating the Capability of a Verification Network...... 29 Uncertainty and Confidence Levels, , ...... , , . 29 The Relationship Between Uncertainty and Cheating Opportunities . . . . 33 What Constitutes Adequate Verification? ...... 35 The Question of Determining Compliance ...... 36

Boxes Box Page 2-A. First Interest in Test Ban Treaties ...... 25 2-B. Recent Interest in Nuclear Test Limitations...... , ...... 28

Figures Figure No. Page 2-1. Nuclear Testing, July 16, 1945-December 31, 1987 ...... 27 2-2. Measurements of 150 kt With Factor-of-2 Uncertainty ...... 32 2-3. Fifty-Percent Confidence Level for Measurements of 150 kt With Factor-of-2 Uncertainty ...... 32

Table Table No. Page 2-1. Confidence Intervals for an Explosion Measured at 150 kt ...... 32 Chapter 2 Seismic Verification in the Context of National Security

Seismic monitoring is central to considerations of verification, test ban treaties, and national security.

THE ROLE OF VERIFICATION For an arms control agreement to be success- and treaties have a mutually beneficial rela- ful, each participating country must feel that tionship. The United States monitors Soviet the provisions of the treaty will enhance its weapons developments whether or not a treaty own national security. This requires an evalu- exists, because the information is important ation by each country of the costs and bene- for our national security. Treaties make mon- fits to its national security of the treaty’s itoring easier and more accurate, because they restrictions. In the case of nuclear test ban include provisions explicitly intended to aid treaties, the cost is accepting restrictions on verification. Additionally, treaties create paths the ability to test nuclear weapons. In return of communication that can be used to resolve, for paying this price, each country gains the clarify, or correct ambiguous situations. In this direct benefit of similarly restricting the other sense, treaties have national security value country. In addition to the direct benefits of that extends beyond their direct purpose. the agreement, participating countries can also gain the political and non-proliferation bene- Verification of treaties is a complex process. fits of working for arms control, as well as the The question of whether a treaty is “verifiable” environmental benefits of reducing the hazards cannot be answered in any absolute or techni- of radioactive contamination of the environ- cal sense. It can only be answered relatively ment from testing. by referring to values that are influenced by a wide range of political and philosophical view- points. Consequently, verification involves not In considering agreements that bear on such only technical considerations, but also judge- vital matters as a restriction on nuclear weap- ments as to how these technical considerations on’s development, each country may assume translate into the policy world. as a cautious working hypothesis that the participants would cheat if they were sufficiently In the past several years, Congress has been confident that they would not be caught. Ver- asked to consider proposals for treaties that ification is a process that is undertaken to con- prohibit testing above various thresholds. firm treaty compliance and, therefore, the abil- Each proposal has sparked controversy within ity of the United States to monitor Soviet both the technical and policy communities. For activity is central to the value of any such any given treaty, some within both communi- treaty. ties will claim it is “verifiable,” whereas others will assert that the Soviet Union would be able Verification is most often viewed as a proc- to cheat and hence that stricter verification ess that improves confidence in a treaty. The provisions are needed to ensure our national converse, however, is also true. Establishing security. This chapter is intended to provide a treaty generally improves our ability to mon- a framework for understanding how to weigh itor Soviet activity. In this way, monitoring the risks and benefits of such treaties.

76-584 0 - 88 : QL 3 - 2 24

THE DEFINITION AND VALUE JUDGMENT OF “VERIFICATION” In the case of test ban treaties, measures are treaty compared with the risk of violation. If taken to ensure that the advantages of the the countries lack insight into each other’s treaty cannot be undermined by the other value systems and decision processes, this un- country testing clandestinely. These measures, certainty will result in the perception that a assessments of the measures’ capabilities, high degree of verification is needed. As a re- evaluations of the risks and benefits, and the sult, the degree of verification needed to satisfy political climate within which all of these judg- the concerned country may be higher than ments are made make up the process referred what is really needed to discourage cheating. to as verification. To illustrate this argument, it is useful to “To verify” means to establish truth or ac- consider the analogy of a treaty restricting curacy. Realistically, in the arena of arms con- each party to one side of a river. If the river trol, verification can never be perfect or abso- freezes over, one or both countries may con- lute: it necessarily involves uncertainty and sider crossing to the other side. If the water this is often described in probabilistic terms. is deep and there is nothing worth having on Because the process of verification involves de- the other side, then the ice does not have to termining acceptable levels of uncertainty, it be very thin to discourage a party from cross- is political as well as scientific. The degree of ing. If, on the other hand, the water is shallow verification needed is based on one’s percep- and there is something of great value to be ob- tion of the benefits of the treaty compared with tained from the other side, than the ice must one’s perception of the disadvantages and the be very thin to discourage a party from cross- likelihood of violations. Consequently, the level ing. The thinness of the ice combined with the of verification required will always be different depth of the water is the degree of deterrent for people with different perspectives. available to dissuade a party from trying to cross the ice. How thin it has to be to actually In U.S.-Soviet agreements, the concern about deter depends on each party’s perception of verification is exacerbated by societal asym- the risk and the reward. In arms control, cross- metries whereby monitoring compliance is usu- ing the ice represents cheating on a treaty. The ally achieved more easily in the United States level of verification capability needed to deter than in the Soviet Union. These asymmetries crossing (the thinness of the ice) depends on may cause the United States to insist on stric- each side’s perception of the risks and rewards ter verification procedures than the Soviets would judge are needed. This difference makes of cheating. The attraction of cheating (get- ting to the other side) would be the belief that negotiations difficult, and can create the im- it could result in some sort of advantage that pression that the United States is obstruct- ing negotiations. would lead to a significant improvement to the country’s national security. The consequence A country considering cheating would have of being caught (falling through the ice) would to evaluate the risks and costs of being caught depend on the depth of the water. This would against the benefits of succeeding. A country involve international humiliation, the possible concerned about preventing cheating has to abrogation of the treaty resulting in the loss guess the other country’s values for making of whatever advantages the treaty had pro- this decision and then evaluate them against vided, and the potential loss of all other present their own estimations of the advantages of the and future agreements. 25

ARE TEST BAN TREATIES IN OUR NATIONAL INTEREST? Test Ban treaties are a seemingly simple ap- Consequently, all aspects of a new treaty must proach to arms control, yet their impact is com- be considered together and their cumulative plex and multi-faceted. Determining the advan- impact evaluated in terms of a balance with tages of such a treaty depends upon weighing the Soviet Union. Such a net assessment is dif- such questions as: ficult because even greater uncertainty is introduced when we try to guess how a given ● Is testing necessary to develop future weapon systems? Do we want both the United States and the Soviet Union to de- Box 2-A.—First Interest in Test Ban Treaties velop new weapon systems? ● Is testing necessary to ensure a high de- Interest in restricting the testing of nuclear gree of reliability of the nuclear stockpile? weapons began with an incident that occurred Do we want the nuclear arsenals of both over 30 years ago. On February 26, 1954, an the United States and Soviet Union to be experimental thermonuclear device, named highly reliable? Bravo, was exploded on the Bikini Atoll in ● the Pacific Ocean. The explosion was the Is continued testing necessary to main- United States’ 46th nuclear explosion. It tain high levels of technical expertise in produced a yield equivalent to 15 million tons the weapons laboratories? Do we want to of TNT, which was over twice what was ex- continue high levels of expertise in both pected. The radioactive fallout covered an the United States and Soviet weapons lab- area larger than anticipated and accidently oratories, and if so, for what purposes? contaminated an unfortunate Japanese fish- ● Is testing necessary to ensure the safety ing boat named Lucky Dragon. When the of nuclear devices? boat docked at Yaizu Harbor in Japan, twenty- ● Could more conservative design practices three of the crew had radiation sickness re- reduce the need for nuclear testing? sulting from fallout. The captain of the ves- ● Would the effects of a test ban impact the sel, Aikichi Kuboyana, died of leukemia in September 1954. In another such accident, United States and the Soviet Union dif- radioactive rain caused by a Soviet hydrogen ferently? bomb test fell on Japan. These incidents fo- ● Would a decrease in confidence in nuclear cused worldwide attention on the increased weapons’ performance increase or decrease level of nuclear testing and the dangers of the likelihood of nuclear war? radioactive fallout. Soon after, the first pro- ● Would a test ban treaty discourage nu- posal for a test ban was put forth.* The 1954 clear proliferation? Could it be extended proposal presented by India’s Prime Minis- to cover other nations? ter Jawaharlal Nehru was described as: ● Would the effects of a treaty be stabiliz- . . . some sort of what maybe called “standing or destabilizing? still agreement” in respect, at least, of these ● Overall, do the advantages outweigh the actual explosions, even if the arrangements disadvantages? about the discontinuance of production and stockpiling must await more substantial Due to the immense uncertainties associated agreements among those principally con- with nuclear conflict, there are few definitive cerned. answers to these questions. One’s opinion Since that time over 1,600 nuclear explo- about the answers is largely dependent on one’s sions have occurred and at least four more philosophical position about the role of a nu- countries (United Kingdom, France, People’s clear deterrent, and the extent to which arms Republic of China, and India) have success- control can contribute to national security. fully tested nuclear devices. None of these questions, moreover, can be con- *See Bmce A. Bolt, Nuclear Explosions and Earthquakes, W.H. sidered in isolation. Disadvantages in one area Freeman and Company, 1976. must be weighed against advantages in another. 26

treaty would affect the Soviet Union. Finally, Union to yields no greater than 150 kt, the total net assessment of the effects of a and aggregate yields to no greater than treaty on our national security must be weighed 1,500 kt. This treaty was signed May 28, against the alternative: no treaty. 1976. It was submitted to the United States Senate for advice and consent to The first formal round of negotiations on a ratification on July 29, 1976 and again on comprehensive test ban treaty began on Oc- January 13, 1987. It remains unratified, tober 31,1958 when the United States, the So- but both nations consider themselves ob- viet Union, and the United Kingdom opened, ligated to adhere to it. in Geneva, the Conference on the Discontinu- ance of Nuclear Weapon Tests. Since then, in- Although these treaties have fallen far short terest in a test ban treaty has weathered three of banning nuclear testing, they have had im- decades of debate with a level of intensity that 1 portant environmental and arms control im- has fluctuated with the political climate. Dur- pacts. Since 1963, no signatory country coming this time, three partial nuclear test limita- pliant with these treaties has tested nuclear tion treaties were signed. Nuclear explosions weapons in the atmosphere, in outer space, or compliant with these restrictions are now con- under water, thus eliminating a major environ- ducted only underground, at specific test sites, mental hazard. And from an arms control per- and at yield levels no greater than 150 kilo- spective, testing of warheads over 150 kt has tons (kt). These three treaties are: been prohibited since 1974. 1.1963 Limited Nuclear Test Ban Treaty While these treaties have had important (LTBT). Bans nuclear explosions in the positive impacts, figure 2-1 illustrates that, in atmosphere, outer space, and under water. fact, they have not resulted in any decline in This treaty was signed August 5, 1963. the amount of testing. The development of new Ratification was advised and consented types of warheads and bombs has not been to by the United States Senate on Sep- limited by restricting testing, and so advocates tember 24, 1963 and the treaty has been of test ban treaties continue to push for more in effect since October 10, 1963. restrictive agreements. 2. Threshold Test Ban Treaty (TTBT). Re- stricts the testing of underground nuclear A Comprehensive Test Ban Treaty was the weapons by the United States and Soviet declared goal of the past six U.S. Administra- Union to yields no greater than 150 kt. tions, but remained elusive. The Reagan Ad- This treaty was signed July 3,1974. It was ministration, however, has viewed limitations submitted to the United States Senate for on nuclear testing as not in the national secu- advice and consent to ratification on July rity interests of the United States, both at 29, 1976 and again on January 13, 1987. present and in the foreseeable future. The It remains unratified, but both nations stated policy of the Reagan Administration is consider themselves obligated to adhere as follows: to it. 3. Peaceful Nuclear Explosions Treaty (PNE). A Comprehensive Test Ban (CTB) remains This treaty is a complement to the TTBT. a long-term objective of the United States. It restricts individual peaceful nuclear ex- As long as the United States and our friends plosions by the United States and Soviet and allies must rely upon nuclear weapons to deter aggression, however, some nuclear testing will continue to be required. We believe such a ban must be viewed in the context of ‘For an overview of the history of test ban negotiations, the a time when we do not need to depend on nu- reader is referred to G. Allen Greb, “Comprehensive Test Ban clear deterrence to ensure international secu- Negotiations 1958-1986: An Overview, ” in IVuclear Weapon Z’ests: Prohibition or Lizm”tation?, edited by Jozef Goldblat and rity and stability and when we have achieved David Cox, SIPRI, CIIPS, Oxford University Press, London, broad, deep, and verifiable arms reductions, 1987. substantially improved verification capabil- 27

Figure 2-1.— Nuclear Testing, July 16, 1945-December 31, 1987

USA LTBT TTBT 100 i 90 1 I 80 I I I 70 I I 60 I

30 20 10 0 1945 1950 1955 1960 1965 1970 1975 1980 1985 1990

USSR 50 I I I 40 I : I 30 - i !I 20 ! I 10 I 0 L — n 1945 1950 1955 1960 I 1965 1970 1975 1980 1985 1990 I I I France 2 I I i 10 I o 1945 1950 1955 1960 1970 1975 1980 1985 1990 I UK ! I I I 10 I o 1945 1950 1955 1960 I 1965 1970 1975 1980 1985 1990 I I I I i China I 10 o 1945 1950 1955 1960 1965 1970 1975 1980 1985 1990 i India I 10 I

1945 1950 1955 1960 1965 1970 1975 1980 1985 1990

Atmospheric Underground nuclear explosions nuclear explosions

SOURCE: Data from the Swedish Defense Research Institute, 28

ities, expanded confidence building measures, Recently, a series of events have heightened 2 and greater balance in conventional forces. worldwide interest in test ban treaties and a Despite this declared United States position, number of proposals have been brought before public interest in a test ban remains strong. Congress to further limit the testing of nuclear weapons (see box 2-B). To evaluate these proposals requires an understanding of the desired 2U.S. Department of State, Bureau of Public Affairs, “U.S. Policy Regarding Limitations on Nuclear Testing, ” Special Re- and available levels of verification needed to port No. 150, Washington, DC, August 1986, monitor compliance.

Box 2-B.—Recent Interest in Nuclear Test Limitations August 6, 1985 to February 26, 1987: The Soviet Union observes a 19-month unilateral test moratorium. Upon ending its moratorium, the Soviet Union declares that testing would be stopped again as soon as a U.S. testing halt was announced. February 26, 1986: House Joint Resolution 3 (with 207 cosponsors) passes the House with a vote of 268 to 148, requesting President Reagan to resume negotiations with the Soviet Union towards a comprehensive test ban treaty and to submit to the Senate for ratification the Threshold Test Ban Treaty (TTBT) and the Peaceful Nuclear Explosions Treaty (PNE). The wording of the House Resolution is nearly identical to a similar proposal which previously passed the Senate by a vote of 77 to 22 as an amendment to the 1985 Department of Defense (DoD) Authorization Bill. May 28, 1986: The Natural Resources Defense Council, a private environmental group, signs an agreement with the Soviet Academy of Sciences for the establishment of three independent seismic monitoring stations near the principal nuclear test sites of each country. The agreement specifies that the stations are to be jointly manned by American and Soviet scientists and the data is to be made openly available. Summer 1986: UN Conference on Disarmament agrees to a global exchange by satellite of sophisticated seismic data. August 7, 1986: The Five Continent Peace Initiative formed by the leaders of six nonaligned countries (India, Sweden, Argentina, Greece, Mexico, and Tanzania) urges a fully verifiable suspension of nuclear testing and offers assistance in monitoring the ban. August 8, 1986: House of Representatives votes 234 to 155 in favor of an amendment to the Defense Authorization Bill that would delete funding in calendar year 1987 for all nuclear tests with yields larger than 1 kt, provided that the Soviet Union does not test above 1 kt and that the Soviet Union accepts a U.S. monitoring program. The House Amendment is dropped prior to the Reykjavik sum- mit when the Administration agrees to submit the TTBT and PNET to the Senate for advice and consent. May 19, 1987: House of Representatives votes 234 to 187 in favor of an amendment to the fiscal year 1988 DoD Authorization Bill to delete funding for all nuclear tests with yields larger than 1 kt during fiscal year 1988 provided that the Soviet Union does the same and, if reciprocal (in- country) monitoring programs are agreed on and implemented. July 1987: At the expert talks on nuclear testing, Soviets propose calibration of test sites to reduce the uncertainty in yield estimates. The proposal invites U.S. scientists to the Soviet nuclear testing site to measure Soviet test yields using both the CORRTEX system and seismic methods. In return, Soviet scientists would measure a U.S. test at the Nevada test site using both methods. November 9, 1987: Formal opening of negotiations in Geneva on nuclear test limitations. January 1988: Teams of U.S. and Soviet scientists visit each other’s test site to prepare for joint calibration experiments to reduce uncertainty in yield estimation. 29

EVALUATING THE CAPABILITY OF A VERIFICATION NETWORK Making a decision on whether verification old? The answers to these questions can be ob- is adequate requires an understanding of the scured by the manner in which they are por- capability of the verification system, the sig- trayed. In particular: nificance of the potential violation, and a decision as to what is an acceptable level of risk. • differences between verification systems Developing the basis for the decision is diffi- can be made to look superficially either cult because it involves two different commu- large or small, ● opportunities for Soviet cheating can be nities: those who can assess the system’s ca- misrepresented, and pabilities (a technical question) generally do not form the same community as those who ● the decision of what defines adequate ver- are officially responsible for assessing the over- ification can be made through an arbitrary all risks and benefits (a policy question). process. The next three sections illustrate the issues For policymakers to weigh the benefits of that arise in assessing a verification capability the treaty against the risks posed by the pos- —and the misrepresentations that are possi- sibility of unilateral noncompliance, a clear un- ble–by considering a question that aroused derstanding of the capabilities of the monitor- much Congressional interest in early 1987: ing system is necessary. In the frozen river What is our ability to measure seismically the example, this understanding would result from yields of Soviet explosions near the 150 kiloton measurements of how thin the ice is and a tech- limit of the Threshold Test Ban Treaty? The first nical interpretation of how much weight the section presents the statistical representations ice can bear. The decision as to what consti- that are used to describe yield estimation. This tutes an acceptable level of risk is a policy de- includes the meaning of uncertainty and con- cision because it is based on an assessment of fidence levels, along with a comparative dis- the overall benefits of the treaty weighed cussion that enables the reader to understand against the risk. In the frozen river example, what changes in the uncertainty represent. The this assessment would represent the decision next section examines how these uncertainties as to how thin the ice would need to be to de- translate into opportunities for Soviet cheat- ter crossing, how deep the river is, and how ing. And finally, the third section illustrates significant a crossing would be. how the policy decision of what constitutes adequate yield estimation capability has changed The burden on the policy-making commu- in apparent response to variations in the at- nity, therefore, is to understand technical tractiveness of particular monitoring systems. descriptions of the verification system’s capability and incorporate this knowledge into their risk-benefit decision. As we shall see, the dif- Uncertainty and Confidence Levels ficulty is that monitoring capabilities are not certain, but rather they can only be described In determining the verifiability of the 1974 in probabilistic terms. For example: What are Threshold Test Ban Treaty, policymakers the chances that a clandestine nuclear test wanted to know the capabilities of a seismic above a certain yield could go undetected? monitoring system for estimating whether So- What are the chances that a detected seismic viet tests are within the treaty’s limits. The event of a certain magnitude could have been description of such capabilities is accomplished a nuclear explosion rather than an earthquake? through the use of statistics. While the statis- If an underground nuclear explosion is retical calculations are relatively straightfor- corded, how certain can we be that the yield ward, difficulties arise in correctly appreciat- of the explosion was below a specific thresh- ing what the numbers mean. To illustrate how 30

such presentations can be misleading, we will The point here is that the interpretation of first use an example from a common and com- numbers is tricky and statistical presentations paratively well-understood event:3 can often be misleading. The real challenge is At the end of the 1971 baseball season, the not in calculating the numbers, but in correctly San Francisco Giants were playing the Los interpreting what different numbers mean. In Angeles Dodgers in a televised game. In the an area with as many technical considerations first inning, Willie Mays, approaching the end and political influences as arms control verifi- of his illustrious career, hit a home run. Now, cation, one has to be particularly careful that one expects that hitting a home run in the different numbers represent truly significant first inning should be a rather unusual occur- differences and not just arbitrary distinctions. rence because the pitcher is at his strongest and the batter has not had time to get used As with every real-world measurement, esti- to the pitcher. In any case, Willie Mays hit mating the size of a nuclear explosion results a home run and it triggered what every base- in variation, or scatter, among the estimates. ball fan would recognize as a typical baseball The use of different instruments at different statistician’s response. The calculations were locations, interpretations of the measurements made and it was discovered that, of the 646 by different people in slightly different ways, home runs Mays had hit, 122 of them had and unknown variations in signals being ob- been hit in the first inning: 19 percent! In the most unlikely one-ninth of the innings, Wil- served result in slightly different estimates. lie Mays had hit nearly one-fifth of his home Similarly, if one were to measure the daily tem- runs. This realization captured the interest perature outside using a number of thermom- of the reporting community and was dis- eters located in several areas, there would be cussed extensively in the media. In response slight differences in the temperature depend- to the publicity, the Giants’ publicity direc- ing on the particular thermometer, its location tor explained it by saying that”. . . Willie was (surrounded by buildings and streets, or in a always surprising pitchers in the first inning park), how each scale was read, etc. by going for the long ball before they were really warmed up. ” The power of statistical In seismology, errors come from the instru- analysis was able to draw out the hidden mentation, from the interpretation of the data, truths about Willie Mays’ performance. from our incomplete knowledge about how well Although the data and calculations were cor- an explosion transmits its energy into seismic rect, the interpretation could not have been waves (the coupling), and from our limited understanding of how efficiently seismic waves more wrong. Throughout Mays’ career, he had travel along specific paths (the path bias). almost always batted third in the Giants’ Some of these errors are random-they vary lineup (occasionally, he batted fourth). That unpredictably from one measurement to the meant he almost always batted in the first inning. Because he averaged about four at-bats next. Other errors are not random, but are systematic and are the same from one measure- per game, approximately one-quarter of his at- ment to the next. bats came in the first inning. Therefore, he only managed to hit 19 percent of his home runs In our example of measuring temperature, during the first inning which comprised 25 per- a systematic error would be introduced if each cent of the time that he was at bat. Of the mil- reading were made using an improper zero on lions of people who must have heard and read the thermometer. With such a systematic er- about the item, not one pointed out the misin- ror, the measurements would continue to be terpretation of the statistic. This included not distributed randomly, but the distribution just casual observers, but also experienced would be shifted by the difference between the professionals who spend their careers interpret- true and incorrect zero. The distance from the ing just that kind of information. incorrect value to the actual value would represent the size of the systematic error. In seis- Whis example is paraphrased from David L. Goodstein, States mology, an example of a systematic error re- of Matter (Englewood Cliffs, NJ: PrenticeHall) 1975. 31 suits from the failure to allow correctly for the with roughly equal scatter distributed to ei- difference in seismic transmission between the ther side. It would be most likely that the best United States and Soviet test sites. The bias actual value for the daily temperature would term added to the calculation corrects for this be near the central number and it would be in- effect, although there still remains an uncer- creasingly less likely that the actual value tainty associated with the bias. would be off towards either end of the scatter distribution. The distinction between random and systematic errors, however, is not a clear boundary. In seismology, that central number, meas- In many cases, random errors turn out to be ured using several techniques from many seis- systematic errors once the reason for the er- mometers located in different locations, is re- ror is understood. However, if the systematic ferred to as the “central value yield. ” No errors are not understood, or if there are lots matter how accurately we could measure the of systematic errors all operating in different yields of explosions known to be 150 kt, we ways, then the systematic errors are often ap- would still expect—due to the normal random proximated as random error. In such cases, the scatter of measurements and presuming there random uncertainties are inflated to encom- were no systematic error—that roughly half pass the uncertainties in estimating the sys- the explosions would be recorded as being be- tematic effect. In monitoring the yields of low 150 kt and roughly half would be recorded Soviet testing near 150 kt, most of the uncer- as being above 150 kt. The width of the distri- tainty is associated with estimation of system- bution above and below depends on the capa- atic error because the test site has not been bility of the measuring system and can be de- calibrated. In describing the capability to scribed using a “factor of uncertainty. ” measure Soviet tests, the estimate of the ran- Unless otherwise stated, the factor of uncer- dom error has been inflated to account for the tainty for a given measurement is defined as uncertainties in the systematic error. that number which, when multiplied by or We will see in the next section that while sys- divided into an observed yield, bounds the tematic errors might be exploited if they hap- range which has a 95 percent chance of includ- pen to be to one country’s advantage, random ing the actual (but unknown) value of the yield. errors do not provide opportunities for cheat- There is only a 5 percent chance that the meas- ing. Furthermore, the uncertainty in the esti- urement would be off by more than this fac- mates of the systematic errors can often be sig- tor. For example, a “factor of 2“ uncertainty nificantly reduced by negotiating into the would mean that the measured central value treaty such provisions as the calibration of yield, when multiplied and divided by two, each test site with explosions of known yield. would define a range within which the true For this reason, calibration is important and yield exists 95 percent of the time. should certainly be part of any future agree- Naturally, the more confident one wants to ment. But, before we discuss how random and be that the true value lies within a given range, systematic errors affect monitoring, the method the larger that range will have to be. The 95 of statistically describing the capabilities needs percent range is used by convention, but there to be explained. For this, it is useful to return is no real reason why this should be the confi- to our temperature example. dence level of choice for comparing monitoring systems. Using a different confidence level While collecting measurements of the daily than 95 percent to define the factor of uncer- temperature by using many thermometers in tainty would cause the factor to have a differ- many locations, we would find that some of ent value. the measurements were high and some were low, but most of them were somewhere in the As an example, imagine that all Soviet ex- middle. If all of the measurements were plot- plosions are detonated with an actual yield of ted, they would cluster around one number 150 kt and that these explosions are measured 32

Figure 2-2.–Measurements of 150 kt With Factor-of-2 Figure 2-3.—Fifty-Percent Confidence Level for Uncertainty Measurements of 150 kt With Factor-of-2 Uncertainty

Measurements of 150 kt Measurements of 150 kt w/ factor-of-2 uncertainty W/ factor-of-2 uncertainty - True yield = 150 kt 0.010 ./ 0.005 “ 0 75 150 225 300 375 Yield estimate, in kilotons 0 75 150 225 300 375 Yield estimate, in kilotons Same distribution as figure 2-2 with 50 percent of the area marked. Over 50 percent of the measurements would be ex- Measurements of a 150 kt explosion using a factor-of-2 un- pected to fail within 118 and 190 kt. certainty would be expected to have this distribution. The probability that the actual yield lies in a particular range is given by the area under the curve. Ninety-five percent of the area lies between 75 and 300 kt. ments fall, the range would have to be extended to 60 to 372 kt. In real yield estimation, however, there is no meaningful distinction be- by a monitoring system described as having tween 95 percent confidence and 99 percent a factor of 2 uncertainty. It would then follow confidence. The normal distribution is used as that 95 percent of the yield estimates will be a convenience that roughly represents reality between 75 and 300 kt. Therefore, if the Soviets near the center of the distribution. The tails conduct 100 tests all at 150 kt, we would ex- of the distribution are almost certainly not pect that about 5 of them would be measured close approximations of reality. The general with central yields either above 300 kt or be- point can still be made: namely that, it takes low 75 kt. This is graphically illustrated in fig- ever greater increases in range for slight im- ure 2-2, where the probability that the actual provements in the confidence level. Table 2-1 yield lies in a particular range is given by the illustrates this for the case of a normal distri- area under the curve over that range. bution by showing how the yield range varies Although the range from 75 to 300 contains for given factors of uncertainty at various con- 95 percent of the measurements, we can see fidence levels.4 that for the distribution assumed (the normal From the table, it is clear that a quoted range distribution) most of the measurements are, of values is highly dependent not only on the in fact, much closer to the actual yield. For ex- factor of uncertainty of the monitoring system, ample, figure 2-3 showing the 50 percent con- but also on the chosen confidence level. For fidence level for the same distribution illus- example, the same quoted uncertainty range trates that over half of the measurements will fall between 118 and 190 kt. 4Table 2-1 can be read as follows: “If an explosion is meas- On the other hand, if one wanted to specify ured at 150 kt using a system with a factor of [1.5] uncertainty, a range in which 99 percent of the measure- the [70 percent] percent confidence ranges from [121 to 186]kt. ”

Table 2.1.–Confidence Internals for an Explosion Measured at 150 kt

CONFIDENCE –––––––– UNCERTAINTY –––––––––1 LEVEL Factor of 2 Factor of 1.5 Factor of 1.3 99% 61-372 88-255 106-211 95% 75-300 100-225 115-195 90% 84-269 106-212 120-187 80% 96-236 116-195 126-179 70% 104-216 121-186 130-173 50% 118-191 130-173 138-164 SOURCE: Office of Technology Assessment, 1988. 33 that can be achieved using a factor of 1.3 mon- ● as seen from table 1-1, differences can be itoring system at the 95 percent confidence made to look small or large depending on level can be achieved using a factor of 1.5 mon- which particular confidence level is cho- itoring system at the 80 percent confidence sen; and level, or even using a factor of 2 monitoring ● although very high confidence levels have system at the 50 percent confidence level. large ranges of uncertainty, it is with de- In selecting an appropriate confidence level creasing likelihood that the actual value to use for quoting uncertainty, the purpose of will be at the extremes of those ranges of the comparison must also be recognized. In the uncertainty. case of monitoring compliance with a thresh- These considerations are important to ensure old test ban treaty, one is concerned that in- that common mistakes are not made, namely tentional cheating could be missed or that un- that: acceptable false alarms could occur. It must ● be remembered, therefore, that the uncertainty the range of uncertainty is not equated describes the likelihood that the actual value with a range in which cheating can occur, will not fall either above or below the range. and For example, the 95 percent confidence level ● the range of uncertainty chosen for com- means that given repeated trials there is only parative reasons does not evolve into the a 5 percent chance the true yield was either range of uncertainty that is used to de- above or below the range of the measured yield. termine what constitutes adequate veri- In other words, there is a 2.5 percent chance fication. that it could have been above the range and a 2.5 percent chance that it could have been The Relationship Between Uncertainty below the range. The 2.5 percent below the and Cheating Opportunities range, however, is not a concern if it is below the threshold. The concern is only the 2.5 per- The main reason for designing monitoring cent chance that it had a yield above the thresh- systems with low uncertainty is to reduce the old. For the purposes of monitoring a thresh- opportunities for cheating. The relationship be- old, this would only be the half of the 5 percent tween uncertainty and opportunities for cheat- above the threshold, or in other words 2.5 per- ing, however, is not always straightforward. cent. Consequently, the 95 percent confidence Even if the uncertainty of a particular moni- level really corresponds to a 97.5 percent con- toring system is large, this does not mean that fidence level for monitoring violations of a the opportunities for cheating are correspond- threshold. Following the same argument, the ingly large. 50 percent confidence level really corresponds As mentioned before, the uncertainty is cre- to a threshold monitoring at the 75 percent con- ated by two types of error: random and sys- fidence level, and so on. tematic. Although we try to estimate the sys- From the previous discussions, it is obvious tematic error (such as path bias) as accurately that while it maybe convenient to chose a par- as possible, there is a chance that our estimates could be slightly too high or too low. For ex- ticular confidence level to compare monitor- 5 ing systems (such as the 95 percent confidence ample, if our estimate of the bias is too low, level), it should only be done with great cau- we would overestimate the yields of Soviet ex- tion. In particular, it should be kept in mind plosions, and Soviet testing near 150 kt would that: appear as a series of tests distributed around a yield value that was above 150 kt. If our esti- ● the choice of any particular confidence mate of the bias were too high, Soviet testing level is arbitrary, in the sense of being only a convenience to allow comparison of the accuracies of different yield estimation 5% chapter 7, “Estirnating the Yields of Nuclear Explosions, ” methods; for an explanation of bias. 34

near 150 kt would appear as a series of tests The seismic methods that we currently distributed around a yield below 150 kt. The must rely onto estimate yields of Soviet nu- case where we systematically underestimate clear detonations are assessed to have about Soviet yields and they presume this underes- a factor-of-two uncertainty for nuclear tests, timation is occurring is the only case that pro- and an even greater uncertainty level for So- vides opportunity for unrecognized cheating. viet peaceful nuclear explosions. If this were happening, it could happen only This uncertainty was then explained as to the extent that the systematic effect has follows: been underestimated. This uncertainty factor means, for exam- As chapter 7 discusses, the systematic part ple, that a Soviet test for which we estimate of the uncertainty can be significantly reduced a yield of 150 kilotons may have, with 95 per- by restricting testing to specific calibrated test cent probability, an actual yield as high as 300 kilotons-twice the legal limit-or as low sites. If such calibration were an integral part 7 of any future treaty, the concern over system- as 75 kilotons. atic errors of this kind should be minimal. The These statements are misleading in that they majority of the error that would remain is ran- create the impression that there is a high prob- dom. A country considering violating a treaty ability, in fact, almost a certainty, that the could not take advantage of the random error Soviets could test at twice the treaty’s limit because it would be unable to predict how the but we would measure the explosions as be- random error would act. ing within the 150 kt limit. They imply that In early 1987, both the Senate Foreign Re- a factor of 2 uncertainty means that there is lations Committee and the Senate Armed Serv- a high probability that an explosion measured ices Committee held hearings on verification at 150 kt could, in actuality, have been 300 kt. capabilities in their consideration of advice and Yet as we have seen in the discussion of un- consent to ratification of the 1974 Threshold certainty, given a factor of two uncertainty, Test Ban Treaty and the 1976 Peaceful Nu- the likelihood of an explosion with a yield of clear Explosions Treaty. Members of these 300 kt actually being measured (with 95 per- Committees wanted to know whether seismic cent probability) as 150 kt or below is less than methods could adequately measure the size of 1 chance in 40. Soviet underground nuclear tests or whether The chances decrease even further if more more intrusive methods were required. The tes- than a single explosion is attempted. For ex- timony was often confusing due to the vari- ample, the chance of two explosions at 300 kt ous means of representing statistical uncer- both being recorded as 150 kt or less is about tainties. For the time being, we will analyze 1 in 1,600; and the chance of three explosions only the use of statistics and take as given the at 300 kt or greater being recorded as 150 kt underlying information. However, that accept- or less would be roughly 1 in 64,000. Thus, it ance is also controversial and is discussed is highly unlikely that explosions could be separately in the chapter on yield estimation repeatedly conducted at 300 kt and systemat- (chapter 7). ically recorded as being 150 kt or less. The Department of Defense presented the So far we have been looking only at the likeli- capabilities of seismic monitoring to the Sen- hood that a test will appear as 150 kt or less. ate Committee on Foreign Relations on Janu- ary 13, 1987 and the Senate Committee on Armed Services on February 26, 1987 in the ‘This statement is nearly identical to the wording in the U.S. following manner:6 Department of State, “Veri~g Nuclear Test Limitations: Pos- sible U.S.-Soviet Cooperation, ” Special Report No. 152, Aug. 14, 1986, which states “A factor of two uncertainty means, for ‘Testimony of Hon. Robert B. Barker, Assistant to the Sec- example, that a Soviet test for which we derive a ‘central yield’ retary of Defense (Atomic Energy) and leader of formal nego- value of 150 kt may have, with a 95 percent probability, a yield tiations on Nuclear Test Limitations. as high as 300 kt or as low as 75 kt. ” 35

From a practical point of view, it must be rec- considerations would severely diminish the ap- ognized that the test would not need to look peal of any such opportunity. like 150 kt or less; it would only have to ap- In conclusion, it can be seen that although pear as though it were within the error of a the statistical descriptions of the capabilities 150 kt measurement in order to avoid credible of various methods of yield estimation have assertions of non-compliance. Some could mis- been debated extensively, the differences they interpret this as meaning that a test well above represent are often insignificant. There is both the threshold might have enough uncertainty systematic and random uncertainty in the associated with it so that its estimate might measurements of Soviet yields. The system- appear to be within the expected uncertainty atic error would provide only a limited oppor- of a test at 150 kt. They might then conclude tunity for cheating, and then only if it was in that the opportunity to test well above the the advantage of the cheater. Even in such a threshold cannot be denied to the Soviets. case, only the portion of the error that is sys- Such a one-sided assessment of the uncer- tematic can be exploited for cheating. Further- tainty is extremely misleading because it as- more, much of the systematic error would be sumes all of the errors are systematic and can removed through such treaty provisions as be manipulated to the evader’s advantage. A calibrating the test site. Once the systematic country considering violating the threshold error had been nearly eliminated, the remain- would also have to consider that even if part ing uncertainty would be random. The random of the systematic uncertainty could be con- uncertainty does not provide opportunity for trolled by the evader, the random part of the cheating. In fact, if a country were consider- uncertainty could just as likely work to its dis- ing undertaking a testing series above the advantage. This point was briefly recognized threshold, it would have to realize that the ran- in the following exchange during a Senate Com- dom uncertainty would work against it. With mittee on Foreign Relations hearing: each additional test, there would be a lesser “ chance that it would be recorded within the . . . knowing these probabilities, if you limit and a greater chance that at least one of really started to cheat, as a matter of fact you the tests would appear to be unambiguously would take the risk of being out at the far tail. outside the limit. That would really show up fast. If you set out to do a 300 kt, you could show up on our seis- mographs as 450, right?” What Constitutes Adequate Senator Daniel P. Moynihan Verification? “The problem is, if you fired an explosion After the accuracies and uncertainties of at 300 or 350 . . . it could very well look 450 various verification systems have been under- or 500 and the evader has to take that into stood, a decision must be made as to what con- consideration in his judgment. ” stitutes an acceptable level of uncertainty. In Dr. Milo Nordyke, Director of Verification 1974, when the Threshold Test Ban Treaty was Lawrence Livermore first negotiated, a factor of 2 uncertainty was National Laboratory considered to be the capability of seismic methods. At that time, a factor of 2 uncertainty was Also, this analysis assumes that only one also determined to constitute adequate verifi- method of yield estimation will be used. Other 8 cation. Presently, the level of accuracy claimed methods of yield estimation are also available and their errors have been shown to be only 80riginally, the factor of 2 uncertainty was established for partially correlated. The evader would have to the 90 percent confidence level, whereas today it refers to the take into account that even if the uncertainty 95 percent confidence level. It should be noted that a factor is known and can be manipulated for one of 2 at the 90 percent confidence level corresponds to about a factor of 2.5 at the 95 percent confidence level. Thus the ac- method of yield estimation, other methods cepted level of uncertainty in 1974 was really about a factor might not behave in the same manner. Such of 2.5 using the present confidence level. The insistence on a (continued on next page) 36

for the on-site CORRTEX method is a factor tainly influences our decision as to whether a of 1.3.9 This level of 1.3 has subsequently been treaty is worthwhile, but it should not influ- defined as the new acceptable level of uncer- ence the standards we set to make that deci- tainty, although many believe it was defined sion. Also, the capability of a monitoring sys- as such only because it corresponds to the ca- tem is just one aspect to be considered, along pabilities of this newly proposed system. with other important issues such as negotia- bility and intrusiveness. It appears that the determination of adequate compliance is a subjective process that What constitutes adequate verification may has been influenced by the capabilities of spe- also vary for different treaty threshold levels. cific monitoring systems. A decision as to what For example, a factor of 2 uncertainty for mon- constitutes adequate verification should not itoring a 100 kt threshold would mean that 95 be determined by the political attractiveness percent of the measurements at the threshold ● of any particular monitoring system, but rather limit would be expected to fall within 50 and it should represent a fair assessment of the pro- 200 kt (a total range of 150 kt), while a factor tection required against non-compliance. In the of 2 uncertainty for monitoring a 1 kt thresh- frozen river analogy, this would be a fair assess- old would mean that 95 percent of the meas- ment of how thin the ice must be to deter some- urements at the threshold limit would be ex- one from crossing. Monitoring capability cer- pected to fall within% and 2 kt (a total range of 1.5 kt). A range of uncertainty of 1.5 kt may not provide the same opportunities or incen- {continued from previous page) tives for cheating as a range of uncertainty of higher confidence level occurred simply because it was more 150 kt. Consequently, at lower treaty thresholds, convenient to use the 95 percent confidence level which corresponds to 2 standard deviations. the significance of a given yield uncertainty ‘See appendix, Hydrodynamic Methods of Yield Estimation. will almost certainly diminish.

THE QUESTION OF DETERMINING COMPLIANCE In addition to understanding the accuracy where we come by and see marks on the ice. and uncertainty of the verification system, and We must then determine whether the marks deciding on an acceptable level of uncertainty, indicate that someone successfully crossed the a decision will also have to be made as to what ice. This could be misleading because all that would constitute compliance and non-compli- we are doing is looking in isolation at the prob- ance. Violations of the treaty must be distin- ability that a certain mark could have been . guished from errors in the measurements (both made by someone crossing the ice. Thus the systematic and random) and errors in the test. likelihood that a mark was made by a person This is of particular concern in light of find- becomes the likelihood that someone crossed ings by the administration that: the ice. This, however, is only part of the issue. If we knew for example that the ice were Soviet nuclear testing activities for a num- so thin that there was only a 1 in 10 chance ber of tests constitute a likely violation of le- it could have been successfully crossed, that gal obligations under the Threshold Test Ban Treaty .’” the water was deep, and that there was no reason to get to the other side, these factors might To examine the context in which this must weigh in our determination of whether a mark be viewed, we can once again return to the fro- was mane-made or not. (Why would a person zen river analogy and imagine a situation have taken such risks to gain no value?) On the other hand, if we knew the ice were thick and could be crossed with high confidence, that ‘“’’ The President’s Unclassified Report on Soviet Noncom- pliance with Arms Control Agreements,” transmitted to the the water was shallow, and that real value was Congress Mar. 10, 1987. to be obtained by crossing, then we might 37 make a different judgment as to whether the breaches per year would not be considered a marks were man-made because there would violation of the Treaty; (2) such breaches really be understandable motivation. Thus the would be a cause for concern, however, and, question of compliance is also dependent on at the request of either Party, would be sub- a judgment reflecting one’s perception of the ject for consultations. advantages that could be obtained through a Technical violations are violations of the violation. treaty that do not result in any sort of strate- In the case of test ban treaties, there are also gic advantage. An example would be a techni- “gray areas” due to the associated error of the cal violation of the 1963 Limited Test Ban measurements. For example, it must be as- Treaty (LTBT) which prohibits any explosion sumed that a country will test up to the limit that: of the treaty, and therefore, some of the esti- . . . causes radioactive debris to be present mates would be expected to fall above 150 kt outside the territorial limits of the State un- simply due to random error.11 der whose jurisdiction or control such explosion is conducted.12 Assuming that the errors are known and that apparent violations of the treaty due to such This prohibition includes the venting of errors are recognized, there may also be other radioactive debris from underground explo- violations that cause concern but do not ne- sions. Both the United States and the Soviet gate the benefits of the treaty. These include Union have accused each other of releasing accidental violations, technical violations, and radioactive material across borders and of vio- violations of the “spirit” of the treaty. lations of the 1963 LTBT. These violations are “technical” if the treaty is viewed as an arms Accidental violations are violations of the control measure. However, they are material treaty that may occur unintentionally due to violations if the treaty is viewed as an envi- the inexact nature of a nuclear explosion. It ronmental protection measure. is possible that the explosion of a device with a yield that was intended to be within the limit Violations of the “spirit” of the treaty are of the treaty would produce an unexpectedly also of concern. These include, for example, ac- higher yield instead. This possibility was rections which are contrary to the treaty’s pream- ognized during negotiations of the TTBT. The ble. The treaty’s preamble declares the inten- transmittal documents which accompanied the tions and provides a context for the treaty. TTBT and the PNE Treaty when they were Such declarations, however, are nonbinding. submitted to the Senate for advice and con- Another area concerns treaties that have sent to ratification on July 29, 1976 included been signed but never ratified. Both the 1974 the following understanding recognized by TTBT and the 1976 PNE Treaty remain un- both the United States and Soviet Union: ratified, although they were signed over 10 Both Parties will make every effort to com- years ago. Because neither the United States ply fully with all the provisions of the TTB nor the Soviet Union have indicated an inten- Treaty. However, there are technical uncer- tion not to ratify the treaties, both parties are tainties associated with predicting the pre- obligated under international law (Article 18, cise yields of nuclear weapon tests. These un- the 1969 Vienna Convention on the Law of certainties may result in slight, unintended Treaties) to refrain from acts which would de- breaches of the 150 kt threshold. Therefore, feat their objectives and purposes. the two sides have discussed the problem and agreed that: (1) One or two slight, unintended All of these types of violations contribute to the gray area of compliance versus noncompliance and illustrate why determining com- “For example, if the Soviet Union tested 20 devices at 150 kt and we estimated the yields using a system that was described as having a factor of 2 uncertainty, the probability of measuring at least one of them as being 225 kt or greater is 92 percent. ‘2 Article I,b. 38

pliance is a political as well as a technical deci- as likely as it might produce a yield within the sion. In the case of monitoring underground expected error range of a treaty compliant test. nuclear tests, the actual measured yield that It must be recognized, however, that the cal- would constitute clear evidence of a violation culated yield for declaring a treaty violation would always be higher than the yield limit will always be higher than the limit of the of the treaty. Perhaps an analogy for uncer- treaty. Consequently, one or two small breaches tainties in yield estimates and Soviet compli- of the treaty could occur within the expected ance under the TTBT is in monitoring a speed uncertainty of the measurements. A country limit of 55 mph. Under the present 150 kt limit, intent on cheating might try to take advan- an observed yield of 160 kt is like comparing tage of this by risking one or two tests within 58.7 mph to 55 mph. The police do not give the limits of the uncertainty range. Even if de- tickets when their radar shows a speed of 58.7 tected, a rare violation slightly above the per- mph because most speedometers are not that mitted threshold could be explained away as accurate or well calibrated, and because curves an accidental violation due to an incorrect and other factors can lead to small uncertain- prediction of the precise yield of the nuclear ties in radar estimates of speed. Similarly, al- test. This should be kept in mind when choos- though a 160 kt measurement maybe regarded ing a threshold so that small violations of the by some as a legal lack of compliance, such a limit (whether apparent or real) do not fall in number can well arise from uncertainties in a range that is perceived to be particularly sen- seismic estimates. At radar measurements sitive. over 65 mph the police do not question that the 55 mph limit has been exceeded, and the What is done about violations is an addi- speeder gets a ticket. With this standard, it tional problem. In domestic law there are vari- would take a calculated yield of about 180 to ous kinds of violations. Traffic tickets, mis- 190 kt to conclude that a violation had likely demeanors, felonies, capital crime–all are taken place. As mentioned before, however, different levels. Similarly, in monitoring com- this argument does not mean that the Soviets pliance, there are some things that amount to could test up to 180 to 190 with confidence, traffic tickets and some that amount to felo- because the uncertainty could just as likely nies. We must decide in which cases violations work against them. A 180 to 190 kt test might or noncompliance are at the heart of a treaty produce an observed yield well over 200 kt just and in which cases they area marginal problem. Chapter 3 The Role of Seismology CONTENTS

Page Introduction...... 41 The Creation of Seismic Waves, ...... 41 Types of Seismic Waves ...... 41 Teleseismic Waves ...... 42 Body Waves...... 42 Surface Waves ...... 44 Regional Waves...... 45 Recording Seismic Waves ...... 48 Seismic Arrays ...... 50

Figures Figure No. Page 3-1. Seismic Waves Propagating Through the Earth ...... 42 3-2. Body Waves ...... 43 3-3. Seismic Radiation Patterns ...... 44 3-4. Reflected and Refracted Waves ...... 44 3-5. Surface Waves ...... 45 3-6. Seismograph Recording of P, S, and Surface Waves From a Distant Earthquake ...... 46 3-7. Regional and Teleseismic Signals ...... 47

3-8. Pn Wave ...... 47

3-9. Pg Wave ...... 47

3-1o. Lg Wave ...... 47 3-11. Seismic Instrumentation...... 49 3-12. Distribution of RSTN Stations ...... 50 3-13. Beamforming ...... 50 Chapter 3 The Role of Seismology

Seismology provides a technical means for monitoring underground nuclear testing

INTRODUCTION Verifying a ban on nuclear testing requires efficiently. Underwater explosions would be global monitoring systems capable of detect- detected by the acoustic sensors already in ing explosions in the atmosphere, underwater, place as part of anti-submarine warfare sys- and below ground. Tests in the atmosphere and tems. The most uncertain part of the global under water can be readily detected with high verification system is the monitoring of under- degrees of confidence. The atmosphere is mon- ground nuclear explosions that might be con- itored effectively with satellites containing sen- ducted within the Soviet Union. The main tech- sors that can detect the visible and near- nical tools for monitoring underground nuclear infrared light emitted by a nuclear explosion. explosions come from the field of seismology, The oceans can also be monitored very effec- which is the study of earthquakes and related tively because water transmits acoustic waves phenomena.

THE CREATION OF SEISMIC WAVES A nuclear explosion releases its energy in less ing medium. Eventually, the shock wave than 1/1,000,000 of a second (a microsecond). weakens to the point where the rock is no The explosion initially produces a gas bubble longer crushed, but is merely compressed and containing vaporized rock and explosive ma- then returns (after a few oscillations) to its terial. Within a microsecond, the temperature original state. These cycles of compression and within the bubble reaches about 1 million relaxation become seismic waves that travel degrees and the pressure increases to several through the Earth and are recorded by seis- million atmospheres. A spherical shock wave mometers. By looking at seismic records (seis- expands outward from the bubble into the sur- mograms) and knowing the general properties rounding rock, crushing the rock as it travels. of the travel paths of various waves, seismol- As the hot gases expand, rock is vaporized near ogists are able to calculate the distance to the the point of the explosion and a cavity is seismic event and what type of motion caused created. the wave. While the cavity is forming, the shockwave continues to travel outward into the surround-

TYPES OF SEISMIC WAVES As with earthquakes, seismic waves result- eling along the Earth’s surface (surface waves). ing from an explosion radiate outward in all The body and surface waves that can be directions (figure 3-l). These waves travel long recorded at a considerable distance (over 2,000 distances either by passing deep through the km) from the earthquake or explosion that cre body of the Earth (body waves), or else by trav- ated them are referred to by seismologists as

41 42

Figure 3-1 .—Seismic Waves Propagating Through Teleseismic waves contrast with what seismologists call regional waves. Regional waves are seismic waves that travel at relatively high frequencies within the Earth’s crust or outer layers and typically are observed only at dis- Source= . tances of less than 2,000 km. Regional waves are therefore recorded at seismic stations that, for application to monitoring Soviet nuclear explosions, would have to be located within the territory of the Soviet Union. Such stations are called in-country or internal stations.

When seismic waves reach a seismic station, the motion of the ground that they cause is An earthquake or underground explosion generates seismic recorded by seismometers. Plots of the wave waves that propagate through the Earth. forms are called seismograms. Because the

SOURCE: Office of Technology Assessment, 1988. different waves travel at different speeds and along different routes, they arrive at seismic teleseismic waves. Teleseismic body and sur- stations at different times. The farther away face waves are important to monitoring be- the seismic station is from the source of the cause to be recorded they do not usually re- waves, in general, the more dispersed in time quire a network of seismometers near the the different arrivals will be. By studying seis- source. This allows for seismometers to be lo- mograms, seismologists are able to recognize cated outside the region that is being mon- the various types of waves produced by events itored. such as earthquakes and nuclear explosions.

TELESEISMIC WAVES The various types of teleseismic waves are P waves travel in a manner similar to sound differentiated by both the paths along which waves, that is, by molecules “bumping” into they travel and the type of motion that ena- each other resulting in compression and dila- bles them to propagate. The two main subdi- tion of the material in which they propagate. visions (body waves and surface waves) are dis- A cycle of compression moves through the tinguished by the areas through which they Earth followed by expansion. If one imagined travel. a particle within the wave, its motion would be one of shaking back and forth in the direc- Body Waves tion of propagation in response to the alternating compressions and expansions as the The waves traveling through the body of the wave trains move through. The particle mo- Earth are of two main types: compressional tion is in the direction of travel and the wave waves (also called P waves) and shear waves can propagate through both solids and liquids. (also called S waves). The designations P and A sudden push or pull in the direction of wave S are abbreviations originally referring to propagation will create P waves (figure 3-2a). primary and secondary arrivals; compressional waves travel faster than shear waves and, con- In contrast, S waves propagate by molecules sequently, arrive first at a recording station. trying to “slide” past each other much as hap- Apart from their different speeds, P waves and pens when one shakes a rope from side to side S waves also have different characteristics. and the disturbance passes down the rope. The 43

Figure 3-2.—Body Waves

P wave I Compressions -1 Undisturbed medium

– Dilatations

S wave

I Amplitude —------I 1 Wavelength

Waves traveling through the body of the Earth are of two main types: compressional waves (P waves) and shear waves (S waves). SOURCE B A Bolt, Nuclear Explosions and Earthquakes, W H Freeman & Co , 1976 wave motion is at right angles to the direction explosions should be a source of nearly pure of travel. Any particle affected by the wave P waves. In practice, however, some S waves would experience a shearing motion as the are also observed. These waves are usually due wave passed through. Because liquids have lit- to asymmetries of the cavity or the pressure tle resistance to shearing motion, S waves can within the cavity created by the explosion, only pass through solid materials. (Liquids can structural inconsistencies in the surrounding be compressed, which is why P waves can rock, or the presence of pre-existing stresses travel through both solids and liquids.) A sud- in the host rock that are released by the explo- den shearing motion of a solid will result in sion. An earthquake, on the other hand, is S waves that will travel at right angles to the generally thought of in terms of blocks of the direction of propagation (figure 3-2 b). Earth’s crust breaking or slipping past one another along a fault region. Because of the In a P wave, particle motion is longitudinal shearing motion, an earthquake creates mostly in the direction of propagation, thus there can S waves. P waves are also generated from an be no polarization of a P wave. S waves, earthquake, but only in a four-lobed pattern however, being transverse, are polarized and that reflects the opposite areas of compression can be distinguished as either horizontal or ver- and expansion caused by the shearing motion tical. Particle motion in an SH wave is horizontal. In an SV wave, particles oscillate in a ver- (figure 3-3). As discussed in chapter 5, this difference in source geometry can be used as tical plane perpendicular to the path. a means of distinguishing the seismic signals An underground explosion creates a uniform caused by explosions from the seismic signals pressure outward in all directions. Therefore, generated by earthquakes. 44

Figure 3.3. —Seismic Radiation Patterns Figure 3-4.– Reflected and Refracted Waves

Explosion Earthquake Reflected Incident s s Reflected

Medium 1

Medium2

Refracted S

The pattern of seismic body waves generated When a wave hits a boundary within the Earth both reflected by a given source becomes complicated when and refracted waves can be generated. the waves interact with the structures of the Earth. In general, when a wave hits a bound- leigh wave is retrograde to the path of wave ary within the Earth, such as where two differ- propagation (figure 3-5a). The energy of the ent rock types meet, both reflected and trans- Rayleigh wave is trapped near the ground sur- mitted P and S waves are generated at several face, so as depth increases, the rock particle different angles. (figure 3-4) displacements decrease until, ultimately, no Because of all these possibilities, P waves motion occurs. and S waves break into many types as they The second type of surface wave which travel through the Earth. At a depth of 30 to travels around the Earth is called a Love wave. 60 kilometers in the Earth, the velocity with Love waves have a horizontal motion that is which sound passes through the Earth in- shear, or transverse, to the direction of propa- creases markedly. This discontinuity, called gation (figure 3-5b). It is built up of trapped the Mohorovicic Discontinuity (Moho), serves SH waves and has no vertical motion. as a wave guide to trap seismic energy in the upper crust of the Earth. While Rayleigh waves can be detected by seismometers sensitive either to vertical or Surface Waves horizontal motion of the Earth’s surface, Love waves are only detected by seismometers sens- At the Earth’s surface, two additional seis- ing horizontal motions. mic wave types are found. These surface waves, called Rayleigh waves and Love waves,1 are Surface waves of either type, with a period produced by constructive interference of body of 10 seconds, travel with a velocity of about wave energy in the upper layers of the crust. 3 kilometers per second. This corresponds to a wavelength of about 60 kilometers. Because The particle motion of Rayleigh waves is of this long wavelength, surface waves are somewhat analogous to that of ripples spread- much less likely to be affected by small-scale ing over the surface of a lake. The analogy is variations in Earth structure than short period not exact, however, because unlike a water body waves. This makes the use of surface wave, the orbital motion of a particle in a Ray- waves attractive for measuring yields of So- viet tests. Body waves (P waves and S waves) are faster than surface waves and therefore ar- ‘These waves are named after the mathematicians who first developed the theory to describe their motion (Lord Rayleigh rive first at a recording station. This is fortu- and A.E.H. Love). nate because the surface waves have larger am- 45

Figure 3-5. -Surface Waves

Rayleigh wave

Love wave

Waves traveling along the surface of the Earth are of two main types: Rayleigh waves and Love waves. SOURCE: B.A Bolt, Nuclear Explosions and Earthquakes, W.H. Freeman & Co., 1976. plitudes than the body waves. If they all unless the records were filtered to take advan- arrived simultaneously, the smaller amplitude tage of the different frequency content of the P waves would be hidden by the surface waves two types of waves (figure 3-6).

REGIONAL WAVES

In contrast with the teleseismic waves de- that goes down through the crust of the Earth, scribed above, regional waves are usually ob- then travels mostly horizontally near the top served only at distances of less than 2,000 km. of the upper mantle (in contrast to the usual In general they have larger amplitudes and body, which goes deeply within the mantle), higher frequency content then waves from the and finally travels upward through the crust, same source recorded at teleseismic distances. where the receiver is located. This path is Depending on their propagation characteris- shown in the upper part of figure 3-8. In the tics, such regional waves are denoted by Pn, lower part are shown some of the multiple

Pg, Sn, and Lg. A comparison of teleseismic bounces that also contribute to the Pn wave. and regional seismic waves is illustrated in fig- The Moho marks the boundary between the ure 3-7. Note the different time scales. The am- crust and the upper mantle. plitude scales are also different. Pg is a wave that comes in later than Pn, and

At regional distances, Pn is usually the first travels the path between source and receiver wave to arrive at any given station. It is a wave wholly within the Earth’s crust. As shown in 46

Figure 3-6.—Seismograph Recording of P, S, and Surface Waves From a Distant Earthquake

Surface P s waves

Horizontal ground motion

SOURCE: Modified from F. Press and R. Seiver, Earth, W.H. Freeman & Co., San Francisco, 1974.

figure 3-9, Pg is thought to be guided by a observation of Lg out to teleseismic distances, boundary layer within the crust. It does not however, can only occur across continents. Be- propagate to as great a distance as Pn. neath an ocean, where the crust is much thin- ner, L fails to propagate even short dis- S is a shear wave that travels a path very g n tances. similar to that of Pn, but arrives later. Sn arrives later because it is composed of shear Figure 3-10 illustrates how the L wave waves, which propagate slower than the P g waves that make up P . propagates. At the top is shown a seismic n source within the Earth’s crust. Energy de- For purposes of explosion monitoring, L parting downward is partially reflected in the can be the most important of the regional crust and partially transmitted down into the waves because it is typically the largest wave mantle. However, for waves that travel in the observed on a seismogram at regional dis- more horizontal direction, as shown in the mid- tances. Lg is a type of guided shear wave that dle part of the figure, energy cannot get into has most of its energy trapped in the Earth’s the mantle and is wholly reflected back into crust. In fact, this wave can be so strong that the crust. The type of reflection occurring here (contrary to what is implied by its inclusion is the total internal reflection that is similar in the class of “regional waves”) L for a large to that occurring within the prisms of a pair explosion can be observed even at teleseismic of binoculars. For a fixed source and receiver, distances, i.e. well in excess of 2,000 km. The there may be many reflection paths, all totally 47

Figure 3-7.—Regional and Teleseismic Signals

Regional signal S P n P g n

1 1 I o 50 100 Time (seconds)

I I o 200 400 Time (seconds)

Upper seismogram is of a “regional distance” from the source. Signals here are associated with waves that propagate in the crust and upper most mantle. Lower seismogram is of a “teleseismic distance. ” This wave has propagated through the deep interior of the Earth. SOURCE” Modified from Defense Advanced Research Projects Agency.

Figure 3-8.—Pn Wave Figure 3-10.—Lg Wave

Source Receiver Source Crust (slow; low attenuation) Moho

Mantle (faster; higher attenuation) \

Pn - wave

Total reflection

Figure 3-9.-Pg Wave Wave guide. Many “trapped” rays in continental crust. Receiver Source Receiver Moho All totally reflected Lower crust Moho Lg - wave Pg - wave 48 reflected and thus trapped within the crust. For most of the last 30 years, regional waves This is illustrated in the bottom of the figure. have been thought of mainly in the context of

The Lg wave is composed of the entire family monitoring small explosions which may not be of trapped waves, with the crest acting as a detected teleseismically. In recent years, how- wave guide. For regions in which the crust is ever, Lg has come to be recognized as useful composed of material that transmits seismic for estimating yields for large explosions, in- waves efficiently, Lg may be recorded at dis- dependent of the more conventional body wave tances of thousands of kilometers. and surface wave measurements (see ch. 7).

RECORDING SEISMIC WAVES Seismic waves are measured at observato- by seismometers and recorded. Although seis- ries around the world by recording the ground mic instruments are sensitive enough to de- motion. Most observatories have “triaxial” tect seismic waves generated by even the seismometers, meaning that they record smallest explosion, it will be seen in the fol- ground motion in three directions at right an- lowing chapters that naturally occurring back- gles to each other. Typically they are oriented ground noise levels are the limiting factor in north-south, east-west, and vertical. By hav- detecting small earthquakes and explosions. ing all three components, seismologists can reconstruct the complete three-dimensional To reduce the background noise caused by ground motion from the seismograms. wind and other surface effects, seismometers have been designed to fit into torpedo-shaped The principle by which the seismometers casings and placed in narrow boreholes at a work can bethought of as a heavy mass freely depth of about 100 meters. These types of sta- supported by a spring from a frame fixed to tions are currently being used as part of the the Earth. When an earthquake or explosion Regional Seismic Test Network (RSTN). The occurs, seismic waves traveling through the RSTN is a prototype system designed to evalu- Earth reach the seismometer. The frame is ate the utility of in-country stations that could shaken in response to the motion of the wave. be placed within the Soviet Union to monitor Although the frame is displaced by the ground a low-yield or comprehensive test ban treaty. motion, the heavy mass tends to remain sta- A typical high quality installation contains tionary because of its inertia. The displacement three primary seismometers mounted in mutu- of the grounded frame relative to the station- ally perpendicular orientations along with ary mass is therefore a measure of the ground three back-up seismometers (figure 3-11). motion. This movement is then electronically magnified so that displacements as small as The seismometer installation is protected 0.00000001 centimeters (the same order as against tampering both electronically and atomic spacings) can be detected. through the inherent ability of the instrument If the Earth were perfectly still, recording to sense disturbances in the ground. The bore- small earthquakes and underground explo- hole package sends data to a surface station sions would be easy. However, processes such that contains transmitting and receiving as the winds, ocean waves, tides, man’s activ- equipment and in some cases to an antenna ity, and seismic waves generated by earth- for satellite communications. Also contained quakes in other regions continually cause mo- in surface stations that might be used for mon- tions of the Earth. All of this motion is sensed itoring are power sources, environmental con- 49

Figure 3-11 .—Seismic Instrumentation

Three seismometers mounted in mutually perpendicular orientations

Photo credits: Sandia National Laboratories Seismic station downhole package containing seismometers, authentication circuits, and processing electronics Seismometer 50

Figure 3-12.— Distribution of RSTN Stations

Photo credit: Sand/a National Laboratories An example of what an internal seismic station might look like. trol apparatus, and tamper-protection equipment. The distribution of six of these stations Six Regional Seismic Test Network (RSTN) stations are dis- in North America is designed to simulate the tributed throughout North American to simulate the distri- distribution of 10 internal stations within the bution of 10 internal stations within the Soviet Union. Soviet Union (figure 3-12). SOURCE: Modified from Sandia National Laboratories.

SEISMIC ARRAYS

The use of seismic arrays has been proposed Figure 3-13.—Beamforming as an alternative and/or supplement to single three-component seismic stations. A seismic array is a cluster of seismometers distributed over a fairly small area, usually on the order of a few kilometers. The signals from the various instruments are then combined to improve the detection and identification of any small or weak signals that maybe recorded. Seismic arrays can function like phased array radar receivers, sensitive to waves from a particular direction while excluding waves from other directions. By doing so, arrays can pull small signals out from the surrounding background noise. To test the application of arrays for moni- Beamforming is a process of shifting and adding recorded toring regional seismic events, the United waveforms to emphasize signals and reduce noise. The origi- States and Norway installed the Norwegian nal waveforms (a) can be shifted to remove propagation de- Regional Seismic Array (NORESS) in 1984. lays (b). The signal waveforms are now aligned but the noise is not. The average of the shifted waveforms in (b) would be NORESS is located north of Oslo and consists the beam. of 25 individual sensors arranged in 4 concen- SOURCE: Energy & Technology Review, Lawrence Livermore National Labora- tric rings with a maximum diameter of 3 km. tory, August 1986, p. 13. 51

To imagine how an array works, consider an to detect. This process of shifting and adding example where a seismic wave is coming from the recorded waveforms is called beamform- a known location and with a known speed. The ing. The combination of the shifted waveforms time it takes for the wave to travel to each sen- is called the beam. sor in the array can be predicted from the Arrays do, however, have some drawbacks known direction and speed. In contrast, the compared to three-component single stations: seismic background noise will vary randomly from sensor to sensor. The recorded signals 1. they are more expensive because they re- from each sensor in the array can then be quire more hardware and higher instal- shifted in time to allow for propagation across lation costs; the array and combined (figure 3-13). The sen- 2. they require access to a larger area; and sors are close enough so that the signal from 3. they have more sensors that, in turn, gen- the coherent source is nearly the same from erate more data that must be trans- each seismometer and remains unchanged. The mitted, stored and processed. background noise varies more rapidly and can- cels out when the records are added together. A comparison of the advantages and dis- Thus, it is possible to suppress the noise rela- advantages of arrays versus three-component tive to the signal and make the signal easier single stations is presented in chapter 4. Chapter 4 Detecting Seismic Events CONTENTS

Page Introduction...... 55 The Meaning of ’’Detection” ...... 55 Locating Seismic Events ...... 57 Defining Monitoring Capability ...... 58 Seismic Magnitude ...... 58 Converting Magnitude to Explosive Yield...... 60 Seismic Monitoring in Probabilistic Terms ...... 60 Limitations to Seismic Monitoring Capability ...... 61 Seismic Noise...... 61 Reduction of Signals at Source or Sensor ...... 61 Seismic Instrumentation...... 62 Seismic Magnitude Estimation Problems ...... 63 Seismic Networks ...... 64 Existing Networks and Arrays ...... 64 Planned Networks ...... 67 Hypothetical Networks ...... 67 Seismic Monitoring Capability ...... 68 Calculating Seismic Monitoring Capability ...... 68 Global Detection Capability ...... 68 Detection Capability Within the U.S.S.R. Using No Internal Stations . . 69 Detection Capability Within the U.S.S.R. Using Internal Stations . . . . . 69 Considerations in Choosing Monitoring Thresholds ...... 70 Use of High-Frequency Data ...... 70 Arrays v. Three-Component Stations ...... 74

Box Box Page 4-A. Locating A Seismic Event...... 58

Figures Figure No. Page 4-1. Seismic Signals ...... 56 4-2. Seismic Noise ...... 62 4-3. Effect of Noise on Event Magnitude Computation...... , . . . 63 4-4. Contributing Seismograph Stations...... 65 4-5. Worldwide Standardized Network Station Distribution ...... 66 4-6. Detection Capability of a Seismic Station ...... 68 4-7. Example of Calculated Detection Capability ...... 71 Chapter 4 Detecting Seismic Events

The first requirement for a seismic monitoring network is to detect and locate seismic events that could have been caused by an underground nuclear explosion.

INTRODUCTION The first requirement for a seismic monitor- signed to detect events like earthquakes and ing network is that it be capable of detecting distinguish them from normal background seismic events. If the Earth were perfectly noise. The extent to which a seismic network quiet, this would be easy. Modern seismome- is capable of detecting events, referred to as ters are highly sophisticated and can detect the network’s detection threshold, is depen- remarkably small motions. However, processes dent on many factors. Of particular importance such as the winds, ocean waves, and even rush are the types of seismic stations used, the num- hour traffic continually shake the Earth. All ber and distribution of the stations, and the of this ground movement is sensed by seismo- amount of background noise at the station loca- meters and creates a background from which tions. This chapter reviews these factors and signals must be recognized. Seismic networks, discusses the capability of networks to detect consisting of groups of instruments, are de- seismic events within the Soviet Union.

THE MEANING OF “DETECTION” In practical terms, detecting a seismic event signals would vary according to the path they means more than observing a signal above the took through the Earth. This fact has an im- noise level at one station. There must be enough portant impact on the results of the calcula- observations (generally from more than one tions used to determine magnitude and location. station) to estimate the location of the event The results will never have perfect precision. that created the detected signal. Measure- They will be based on averages and will have ments of the seismic waves’ amplitudes and associated with them statements of what the arrival times must be combined according to possible errors in the most probable solution standard analytical techniques to give the loca- might be. Origin times and locations of seis- tion and origin time of the event that gener- mic events, the parameters that make up a “de- ated the signal. The event could be any of sev- tection, ” are always based on some averaging eral natural or man-made phenomena, such as of the data from individual stations. To deter- earthquakes, nuclear or chemical explosions, mine the origin time, and location of a seismic meteorite impacts, volcanic eruptions, or rock event, the data from several stations must be bursts. brought together at a single place to carry out the required analysis. Seismic stations that The seismic signals from these events travel routinely send their data to a central location from their source to individual seismic record- for analysis are said to form a network. ing stations along different pathways through anon-uniform Earth. Consequently, no two sig- The energy radiating from an underground nals recorded at separate stations will be iden- nuclear explosion expands outward. While tical. Even if they came from the same source, most of the explosive energy is dissipated by the amplitudes, shape, and transit times of the crushing and melting the surrounding rock, a

76-584 0 - 88 : QL 3 - 3 56 small fraction is transformed into seismic generation of seismic instrumentation), filters waves. These seismic waves propagate out- are applied in various frequency bands before ward and, in so doing, encounter various bound- detection processing. aries and rock layers both at the surface and Relatively sophisticated techniques can now deep within the Earth. The boundaries cause be used to detect a seismic signal in the pres- a separation of the seismic waves into a vari- ence of noise. In those cases where three per- ety of wave types, some of which travel deep pendicular components of ground motion are through the interior of the Earth (body waves) recorded using a vertical and two horizontal and some of which travel along the surface (sur- component seismometers, use can also be made face waves). See chapter 3 for a full discussion of the known particle motion of P waves to of wave types and travel paths. differentiate a P wave signal from background Seismic signals are detected when they are noise. Another technique which can enhance sufficiently above the background noise in the probability of detecting a signal requires some frequency band. Figure 4-1 shows seis- a number of closely spaced seismic sensors mograms with standard filters designed to en- known as an array. The data recorded by these hance the detection of distant events. In this sensors can be summed together in a manner case, the signal can be seen because it is larger which takes account of the expected signal than the noise at high frequency. When data propagation time across the array. The array are in digital form (as they are in the latest enhances signals from great distance that prop-

Figure 4-1.—Seismic Signals

Signal

Small seismic signals in the presence of seismic noise at four different stations.

SOURCE: U.S. Geological Survey. 57 agate vertically through the array and can re- teleseismic to the Soviet test sites. The possi- ject noise that travels horizontally. Therefore, bility of establishing U.S. seismic stations the array summation process tends to enhance within the U. S. S. R., close to Soviet nuclear the signal and to reduce the noise. testing areas, was discussed seriously as part of negotiations for a Comprehensive Test Ban For many years, most seismic verification Treaty. Because techniques for monitoring ex- efforts in the United States concentrated on plosions with regional signals were not as well the use of teleseismic signals. Teleseismic sig- understood as techniques for monitoring with nals are seismic waves which travel to dis- teleseismic data, research efforts to improve tances greater than 2,000 km and go deep regional monitoring greatly increased during through the interior of the Earth. Teleseismic the late 1970s. Regional distances are defined waves are used because all seismometers mon- to be less than 2,000 kilometers and waves itoring Soviet testing are located outside the propagating to this distance travel almost en- U.S.S.R. and generally at distances which are tirely through the Earth’s outer crustal layers.

LOCATING SEISMIC EVENTS The procedure for estimating the location of station in the process of determining the loca- a seismic event, using seismic data, involves tion of the seismic event. four numbers: the latitude and lon- determining All seismically determined event locations gitude of the event location, the event depth, and have some error associated with them. The er- the event origin time. Determination of these ror results from differences between the ex- four numbers requires at least four separate measurements from the observed seismic sig- pected travel time and the actual travel time nals. These values are usually taken as the ar- of the waves being measured and from impre- cision in the actual measurements. Generally, rival time of the P wave at four or more differ- these errors are smallest for the P wave (the ent seismic stations. In some cases, however, determination of the numbers can be accom- first signal to arrive), and this is why the ca- plished with only two stations by using the pability to detect P waves is emphasized. Loca- arrival times of two separate seismic waves tion errors are computed as part of the loca- at each station and by using a measure of the tion estimation process, and they are usually direction of the arriving signal at each station. represented as an ellipse within which the A relatively poor estimate of location can also event is expected to be. Generally, this com- be obtained using data from only a single array. putation is made in such a way that there is a 95 percent probability that the seismic event The event location process is an iterative one occurred within the area of the error ellipse. in which one compares calculated arrival times Thus, location capability estimates are usually (based on empirical travel-time curves) with given by specifying the size of this error ellipse. the observed arrival times. The differences be- Similarly, there is always an uncertainty asso- tween calculated and observed arrival times ciated with the measured depth of the event are minimized to the extent possible for each beneath the Earth’s surface. 58

Box 4-A.—Locating A Seismic Event Estimating the location of a seismic event can be compared to deducing the distance to a light- ning bolt by timing the interval between the arrival of the flash and the arrival of the sound of the thunder. As an example, consider the use of the P wave and S wave, where the flash is the first- arriving P wave and the thunder is the slower traveling S wave. The time interval between the arrival of the P wave and the arrival of the S wave (which travels at about half the speed of the P wave), increases with distance. By measuring the time between these two arrivals and knowing the different speeds the two waves travel, the distance from the event to the seismometer could be determined (figure A). Knowing the distance from several stations allows the location to be pin- pointed (figure B).

2000 4000 6000 6000 10,000 Distance from earthquake (km)

The time required for P, S, and surface waves to travel a given dis-

tance can be represented by curves on a graph of travel time against Knowing the distance, say XA , of an earthquake from a given distance over the surface. To locate an earthquake the time interval station, as by the method Figure A, one can say only that the observed at a given station is matched against the travel-time curves earthquake lies on a circle of radius A, centered on the station. If, for P and S waves until the distance is found at which the separation however, one also knows the distances from two additional between the curves agrees with the observed P-S time difference. stations B and C, the three circles centered on the three stations,

Knowing the distance from the three stations, A, B, and C, one can with radii XA, XB, Xc, intersect uniquely at the point Q, the locate the epicenter as in figure B. location of the seismic event.

SOURCE: This example is taken from Frank Press and Raymond Seiver, Earth (San Francisco, CA: W. H. Freeman and Company, San Fran- cisco, CA, 1974).

DEFINING MONITORING CAPABILITY Seismic Magnitude of these tasks is generally described as a function of a measure-called the seismic magnitude A seismic monitoring system must be able of an event. to detect the occurrence of an explosion, to locate the explosion, to identify the explosion, Seismic magnitude was first developed as and to estimate the yield of the explosion. The a means for describing the strength of an earth- capability of a seismic network to perform each quake by measuring the motion recorded on 59

a seismometer. To make sure that the meas- tion of wave amplitude occurs for a number urement was uniform, a standard method was of reasons including: needed. The original calculation procedure was developed in the 1930s by Charles F. Richter, 1. the spreading of the wave front over a who defined local magnitude, ML, as the log- greater area, thereby reducing the energy arithm (to the base 10) of the maximum ampli- at any one point on the wavefront; tude (in micrometers) of seismic waves observed 2. the dissipation of energy through natu- on a Wood-Andersen torsion seismograph at ral absorption processes; and a distance of 100 kilometers (60 miles) from 3. energy redirection through diffraction, the earthquake. refraction, reflection and scattering of the wave at various boundaries and layers Subsequently, the definition of seismic mag- within the Earth. nitude has been extended, so that the measurement can be made using different types of As a consequence, a correction term is needed seismic waves and at any distance. For body to obtain the same magnitude measurement waves, the equation for seismic magnitude mb for a given seismic event from data taken at is: any seismic station. The correction term increases the amplitude measurement to com- m = log (A/T) + B(d,h), b pensate exactly for the amplitude decrease where A is the maximurn vertical displacement caused by the different attenuation factors. of the ground during the first few seconds of the P wave, and T is the period of the P wave. Therefore, if the amplitude of the seismic The B term is a correction term used to com- wave is to be used to estimate the size of the pensate for variations in the distance (d) be- seismic event (whether it is an explosion or a tween the seismic event and the recording sta- naturally occurring earthquake), a good under- tion and the depth (h) of the seismic event. For standing of how amplitude decreases with dis- a seismic event at the surface of the Earth, tance is needed for both body and surface h=0. The B correction term has been deter- waves. The distance-dependent numbers in the mined as a function of d and h by observing body and surface wave equations represent seismic signals from a large number of earth- average corrections which have been developed quakes. from many observations. In general, these corrections will not be exact for any one particu-

For surface waves, seismic magnitude Ms lar path from a particular seismic event to a can be calculated by a similar equation: particular sensor. It is important, therefore, either to calibrate the site to receiver path or C, M s = log (A/T) + b log d + to compute the magnitude of the event using seismic signals recorded at a number of well- where b and c are numbers determined from calibrated stations. If multiple stations are experience. A number of formulas, involving used, the event magnitude is calculated by slightly different values of b and c for M , S averaging the individual station magnitudes have been proposed. for an event. From this procedure, an average The terms in both the body wave and sur- body wave magnitude (rob) and an average face wave magnitude equations that are used surface wave magnitude (Ms are determined to compensate for distance reflect an impor- for the event. tant physical phenomenon associated with seismic wave propagate. This phenomenon, re- Obviously, the distance the wave has trav- ferred to as attenuation, can be simply stated eled must be known to determine the attenua- as follows: the greater the distance any par- tion in amplitude and correct for it. Therefore ticular seismic wave travels, the smaller the the seismic event must be located before its wave amplitude generally becomes. Attenua- magnitude can be determined. 60

Converting Magnitude that produced a magnitude mb of 4.0 when to Explosive Yield “well-coupled” might be muffled down to a

seismic signal of around mb 2.() at low fre- The detection and identification capabilities quencies. Lesser reductions can be accom- of seismic networks are described most con- plished by detonating the explosion in dry po- veniently in terms of seismic magnitudes, typi- rous material.

cally mb. This measure is used because mb is directly related to seismic signal strength. Because the yield that corresponds to a spe- When interpreting capabilities in terms of ex- cific mb depends so much on the scenario that plosive yield, however, an additional step is is being discussed, seismologists generally use seismic magnitude to describe monitoring ca- required to translate mb to kilotons. The same magnitude value can correspond to yields that pabilities. In translating seismic magnitudes range over a factor of about 10. Variations in to yields, the reader must consider the context the magnitude-yield relationship are caused by in which the comparison is made. In particular, variations in the structure of the Earth in the it should be considered whether the explosion vicinity of the test site (low signal attenuation is being recorded in an area of good transmis- versus high signal attenuation areas), the ma- sion and whether the explosion is well-coupled. terial in which the explosion is emplaced (hard, Unless specifically stated, this report trans- water-saturated rock versus dry, porous ma- lates seismic magnitudes to yields correspond- terials), and the way in which the explosion is ing to “tamped” conditions, that is, a well- emplaced (tamped versus detonated in a large coupled explosion in hard rock. Situations cavity designed to muffle the signal). where decoupling is feasible, and the effects of such decoupling, are discussed in chapter 6. For example, if an explosion is “well-coupled,” that is, if the energy is well transmitted from Seismic Monitoring in Probabilistic the explosion to the surrounding rock, an mb of 4.0 corresponds to an explosion of about 1 Terms kt. This relationship is true only for explosions Whether seismic measurements are made by in hard rock and may vary considerably de- hand or by computer, some error is involved. pending on how well the seismic waves are Even greater additional errors arise from the transmitted through the area’s geology. In imperfect estimates of how well seismic waves areas that are geologically old and stable, seis- travel through different parts of the Earth and mic waves are transmitted more efficiently. An how well seismic energy is coupled to the Earth m of 4.0 produced by a well-coupled explo- b during the explosion. All of these errors result sion in an area of good transmission might cor- in some uncertainty in the final deter respond to an explosion much smaller than a mined parameters. This is true whether these parame- kt. In areas that are geologically young and ters are event magnitude, location, identifica- active, seismic waves are not transmitted as tion characteristics, or yield. In all cases, efficiently and an m of 4.0 may correspond b however, it is possible to estimate a confidence to a well-coupled explosion larger than 1 kt. factor in probability terms for the determined Even greater changes in the relationship be- parameter. It is important to realize, therefore, tween mb and yield can occur if the explosion that while the numbers are not presented with is intentionally “de-coupled” from the sur- 100 percent certainty, estimates of the uncer- rounding rock in a deliberate attempt to muf- tainty are known. In general, this uncertainty fle the seismic signal. As we will see in chap- is greatest for the small events and decreases ter 6, decoupling can be accomplished at low for the larger events. A discussion of the un- yields under some situations by detonating the certainty and what it means in terms of na- blast in a large underground cavity. Through tional security concerns is presented in chap- such evasion methods, the same 1 kt explosion ter 2. 61

LIMITATIONS TO SEISMIC MONITORING CAPABILITY The strength of a seismic signal diminishes Seismic signals and noise are concentrated with distance. In general, the closer the seis- in various frequency bands. Only the noise mic station is to the source, the stronger the within the frequency bands in which seismic signal will be. Hence, a principal element of signals are observed is a problem. Even strong monitoring strategy is to get close to areas of noise can be eliminated by filtering as long as concern. It follows that the more high quality the noise is outside the detection bands of in- stations distributed throughout a given area, terest. the greater the capability will be to detect small events. The possibility also exists that a seismic signal from one event can be masked by the seis- Seismic Noise mic signal from another event. While this does indeed happen, it is only a problem for moni- As noted previously, if the Earth were per- toring at yields around 1 kt or less without in- fectly still, detecting even the smallest seis- ternal stations. For events of interest in the

mic event would be easy. However, the Earth’s U.S.S.R. above a magnitude of about mb 4.0, surface is in constant motion. This motion is there are a sufficient number of stations de- the result of many different energy sources. tecting the event so that masking of the sig- Major storms over ocean areas and the result- nal at a couple of stations generally poses no

ant wave action on continental shores cause serious problem. For events much below mb significant noise in the 2- to 8-second band. 4.0, a number of stations at regional distances Wind noise and noise from atmospheric pres- (distances less than about 2,000 km) would sure fronts are particularly prominent on have to be used to avoid the masking problem. horizontal-component seismic recordings. These Such stations would be available if the United more or less continuous motions of the Earth States obtains access to data from seismic sta- are referred to as seismic noise or microseisms tions placed within the U.S.S.R. as part of a (figure 4-2). For purposes of siting a seismic negotiated agreement between the United station, it is highly desirable to find an area States and the Soviet Union. that has a low background level of seismic noise. Generally, the lower the background Reduction of Signals at seismic noise at any station, the smaller will Source or Sensor be the seismic signal which can be detected at that station. Poor coupling to the geologic media, either at the explosion source or at the seismic sen- Cultural activities can also generate seismic sor, will act to reduce the amplitude of the seis- noise that appears in the frequency range used mic signal received. If the explosion is in dense to monitor nuclear explosions. Generally, this hard rock or in water saturated rock, the source man-made noise has frequencies higher than coupling will be good. If the explosion is in al- 1 Hz. Heavy machinery, motors, pumping sta- luvium, dry porous rock, or within a cavity, tions, and mills can all generate observable the coupling will not be as good. Decoupling seismic noise. However, careful siting of seis- an explosion by detonation within a cavity is mic stations can minimize the problem of most an important evasion scenario which will be man-made seismic noise. From a monitoring discussed in chapter 6. point of view, it is important that noise surveys be made prior to the final selection of sites At the seismic sensor, signal reduction can for seismic stations. If such sites are negoti- occur if the sensor is not placed on hard rock. ated within other countries, provisions should In particular, if there is a layer of soil upon be made for relocating the sites should seis- which the sensor sits, the signal-to-noise ratio mic noise conditions change. at the sensor can be far less than if the sensor 62

Figure 4-2. —Seismic Noise

Background seismic noise at three different stations.

SOURCE: U.S. Geological Survey. were placed on or within hard rock. Sensors ing seismic signals did not have the capability placed in boreholes in hard rock provide su- to record all the signal frequencies of interest perior coupling to the Earth and also provide with sufficient range. Further, the mechani- a more stable environment for the instrument cal and electronic components comprising seis- packages, with a concomitant reduction in mic recording systems generate internal noise, noise. which is recorded along with true ground noise and seismic event signals, and this internal Seismic Instrumentation noise was the limiting factor in recording seismic signals in the high frequency range. Spe- Until recently, the instrumentation that was cifically, the internal noise of the older designs available for detecting, digitizing, and record- of high frequency seismic detectors was higher 63 than ground noise at frequencies above about ronments in order to simulate the requirements 5 Hz. Thus, while ground noise is now known of in-country monitoring. to decrease with increasing frequency, the system noise remained constant or increased with increasing frequency in the older systems. Seismic Magnitude Estimation Therefore, trade-offs were made, and the en- Problems tire frequency range of the signal was gener- As discussed earlier, the estimate of an ally not recorded. Specifically, in the high fre- event’s seismic magnitude is made by combin- quency range data was generally not recorded ing the estimates obtained from many single above 10 Hz and even then was highly con- stations in an averaging procedure to reduce taminated in the 5-10 Hz range by internal sys- random errors. This procedure works well for tem noise. Consequently, small seismic events, an event which is neither too small nor too particularly small explosions, with expected large. maximum signal energy in the high frequency range above 5 Hz were not detected because For a small event, however, the averaging their high frequency signals were below the in- procedure can result in a network magnitude ternal system noise levels. Most existing seis- value which is biased high. This follows from mic stations are of this type and so are limited the fact that for a small event, the signals will for nuclear test monitoring. be small. At those stations where signals fall below the noise, the small signal amplitudes Today, broadband systems capable of re- will not be seen. Consequently, only higher am- cording the entire frequency range with a large plitude values from other stations are avail- dynamic range and with low internal noise are able for use in computing the network aver- available. However, the best high performance age. With the low values missing, this network systems are not widely distributed. To estab- average is biased high unless a statistical cor- lish confident detection-identification capabil- rection is made. ities using high frequency seismic signals at low event magnitudes, it will be necessary to Figure 4-3 illustrates this effect. All six sta- expand the number of high performance stations (A through F) record the same magni- tions and to place them in diverse geologic envi- tude 4 event. Stations A, B, and C record the

Figure 4-3.—Effect of Noise on Event Magnitude Computation

4.8

4.5 4.2 I

seismic 3.2 noise level

STA-A STA-B STA-C

STA-D STA-E

Average event magnitude = 4.0 (6 stations) STA-F

Computed average magnitude = 4.5 (3 stations)

SOURCE: Modified from Air Force Technical Applications Center. .

64 event below average. Stations D, E, and F rec- onstrated that the small events being detected ord the event above average. Normally, the sta- were 0.2 to 0.4 magnitude units smaller than tions would all average out to a magnitude 4.0. previously thought. In terms of yield, this For a small event, however, the stations that means that the networks were actually capa- record low (A, B, &C) do not record the signal ble of detecting events down to half as large because it is below the noise level. Only the as previously thought possible. Within the last stations that record higher (D, E, & F) show few years, analysis procedures have been em- the event. Without the values from the low sta- ployed to correct for most of this bias using tions, the calculation of the average is made a procedure called maximum likelihood esti- using only the stations that record high. The mation. resulting calculation biases the average to the values of the higher stations, giving a false average magnitude value of (in this example) For large events, a similar bias problem used 4.5 for a 4.0 seismic event. to exist occasionally, but for a different reason. Old seismometers could not record very In the past, computed magnitudes of small large signals without clipping the signal. Larger events were systematically biased high in this amplitude signals were either not available or manner. As a result, for most of the last 25 were under estimated. The resulting bias of years, the U.S. capability to detect seismic large events, however, did not affect estimates events within the U.S.S.R. has, in fact, been of detection capability and only became a prob- significantly better than the estimates of this lem in the determination of the size of very capability. Not until the late 1970s was it dem- large events.

SEISMIC NETWORKS Existing Networks and Arrays velopment and deployment of the Worldwide Standardized Seismograph Network (WWSSN) Although many thousands of seismic sta- in the early 1960s (figure 4-5). The WWSSN tions exist around the world, the actual num- is maintained by the United States Geologi- ber of stations which routinely report data to cal Survey (USGS). The quality and perform- national and international data centers is a few ance of the WWSSN is generally very good, thousand. For example, in figure 4-4 the 3,500 but the recording system is limited in dynamic stations are shown that routinely report data range and resolution because of the use of what to the National Earthquake Information Cen- is now obsolete analog equipment and also be- ter (NEIC), a center in Colorado operated by cause of high internal noise in the amplifying the United States Geological Survey. Some of equipment. (For example, the data are cur- these stations report much more often than rently recorded only on photographic paper do others. The instrumentation at these sta- records.) tions is diverse and the quality of the data var- Beginning in the early 1970s, digital record- ied. While these stations are very useful for ing seismic stations were developed by the seismic signal detection, they are less useful United States and other countries. The data for purposes of magnitude estimation and for from these stations can be easily processed by research requiring stations evenly distributed digital computers to enhance the signal-to- around the world. noise for signal detection and to analyze the data for seismic source determination and for For purposes of treaty monitoring opera- research purposes. These stations are included tions and research, a well distributed network in such networks as: with a common set of instrumentation at all stations is most useful. To obtain such a stand- . The Regional Seismic Test Network ard network, the United States funded the de- (RSTN). These are high quality stations ●

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designed by the Department of Energy such stations distributed across the Soviet and now operated by the USGS. They were Union and for several more to be similarly dis- intended to be prototypes of in-country tributed across the United States. stations. There are five RSTN stations distributed over North America at inter- Planned Networks station distances that represent monitoring in the Soviet Union with 10 internal There are a number of planned new networks stations. that will provide increased capability to detect, ● The NORESS seismic array in Norway. locate, and characterize seismic events around This seismic array is funded by the De- the world. These networks are being developed fense Advanced Research Projects Agency by the United States and other countries. (DARPA) and the Department of Energy (DOE). The prototype array is located in Hypothetical Networks southern Norway, an area thought to be geologically similar to the western part Existing unclassified networks external to of the Soviet Union. the U.S.S.R. have an excellent capability for ● The recently installed China Digital Seis- monitoring events with seismic magnitudes mic Network (CDSN), which is a coopera- greater than 4.0 within the U.S.S.R. However, tive program between the People’s Repub- for explosions less than a few kt, the possibil- lic of China and the USGS. ity exists that the seismic signals from such ● The Atomic Energy Detection System explosions could be reduced through an eva- (AEDS) seismic network. This network is sion method. To demonstrate a capability to operated by the Air Force Technical Ap- defeat credible evasion scenarios that could be plications Center (AFTAC). The purpose applied to explosions with yields less than a of the AEDS network is to monitor treaty few kt, seismic stations within the Soviet compliance of the Soviet testing program. Union would be necessary. Consequently, the stations are located so as to provide coverage primarily of the Obviously, there are a number of requirements for such internal stations and their data. U.S.S.R. The capabilities of the AEDS network are described in a classified annex Among these are the following: the data must to this report. The USGS and the AEDS be provided in an uninterrupted manner; the stations are the main sources of routine data must be of high quality; the seismic noise information on Soviet testing. at the stations should be low; the operating parameters of the stations and the characteristics of the data should be completely known In addition to these networks, there is also at all times; the data should be available to a jointly operated NRDC-Soviet Academy test the United States within a reasonable time site monitoring network in the United States frame; the stations should be located for ef- and the Soviet Union. The network consists fective monitoring of the U. S. S. R.; and any in- of three stations in each country around the terruption or tampering with the operation of Kazakh and Nevada test sites at distances of the station should be detectable by the United about zOO kilometers from the boundaries of States. Obviously, these requirements can each test site. These stations are supplying most easily be achieved by deploying U.S. de- high-quality seismic signal data, in the high signed and built seismic stations within the and intermediate frequency range from 0.1 Hz U.S.S.R. at sites chosen by the United States. to about 80 Hz. The stations are designed to be modern prototypes of the in-country seis- The number of stations required within the mic stations required for monitoring a low U.S.S.R. is a function of a number of factors threshold test ban treaty and they are not including: the threshold level down to which limited by system noise in the high frequency monitoring is desired, the seismic noise at the range. Plans call for the addition of five more stations, the signal-to-noise enhancement ca- 68 pability of the stations, the signal propagation posed distributions of internal stations capa- characteristics within the U. S. S. R., and the ble of detecting seismic events down to vari- possibilities for various evasion scenarios ous thresholds. The number of internal stations thought to be effective within different areas proposed for the various distributions ranges of the U.S.S.R. Many seismologists have pro- between 10 and 50.

SEISMIC MONITORING CAPABILITY Calculating Seismic Monitoring point where the station fails to detect all Capability events. The 90-percent detection threshold (or any other threshold) can be determined from Calculating the detection capability of ex- this plot. If the event magnitude rather than isting seismic stations is straightforward. For the observed amplitude is used, it is important hypothetical stations, however, the detection that all magnitudes used in such plots be cor- capability must be estimated by adopting a rected for low-magnitude bias as previously number of assumptions. Because there exists discussed. By examining all stations of a net- a range of possible assumptions which can be work in this manner, the station detection pa- argued to have validity, there are also a range rameters can be determined and used to com- of possible capabilities for a network of hypo- pute overall network performance for the given thetical internal stations. region. For existing stations, the average detection For hypothetical internal stations, no detec- capability is easily determined by observing tion statistics are generally available for com- the number of seismic events detected as a puting the cumulative detection curve as a function of log amplitude (figure 4-6). The sta- function of magnitude. Therefore, the detection can be expected to detect all events within tion capability must be estimated by assum- a given region down to some magnitude level. ing the following factors: the seismic noise at A cumulative plot of the detections, such as the station, the propagation and attenuation illustrated in figure 4-6, will show that a straight characteristics of the region through which the line can fit these values down to this magni- signals will travel, the efficiency of the seis- tude threshold level. This threshold marks the mic source, and the signal-to-noise ratio required for the signal to be detected. All the above factors must be evaluated in the fre- Figure 4-6.—Detection Capability of a Seismic Station quency range assumed to be best for detecting a signal. The result of all these factors, when considered together, is to provide an estimate of the probability that a signal from a 1,000 source of a given size and at a given location will be detected at a certain station. Individ- ual station detection probabilities are then combined to determine the overall probability that four or more stations will detect the event. Translating this capability to situations where evasion might take place requires additional considerations (see ch. 6).

o 3 Global Detection Capability Log amplitude There are many seismic stations that exist

SOURCE: Modified from Air Force Technical Applications Center around the world from which data can be ob- 69 tained. While no attempt has ever been made example, the capability of a hypothetical net- to determine the global detection capability work consisting of a dozen or so seismic ar- of all these stations, a rough estimate could rays that are all outside the borders of the be made by reviewing the various reporting Soviet Union can be calculated. A cautious esti- bulletins and lists. However, the current global mate is that if such a network were operated detection capability does not really matter be- as a high-quality system, it would have 90 per- cause it will soon change as various planned cent probability of detecting at four or more networks become installed. Consequently, an stations all seismic events within the Soviet accurate assessment of global capability is best Union with a magnitude at least as low as 3.5. addressed by discussing planned networks. This corresponds to an explosion having a yield below 1 kt unless the explosion is decoupled. For regions external to the U. S. S. R., and particularly for regions of the Southern Hemi- The hypothetical detection threshold of mb sphere, the greatest detection and location ca- 3.5 is considered cautious because it is known pability will reside not with the AEDS, but that a greater detection capability might ex- with a number of existing and planned seis- ist at least for parts of the U.S.S.R. For exam- mic networks which are unclassified. This is ple, the single large NORSAR array in Nor- a logical consequence of the AEDS being tar- way has the potential to achieve detection geted primarily at events within the U.S.S.R. thresholds equivalent to an event of mb 2.5 or In particular, national networks such as those lower (corresponding to a well-coupled explo- of Australia, China, the United States, Italy, sion between 0.1 and 0.01 kt) overlarge regions and Canada will provide significant global ca- of the Soviet Union.1 pability. Also, fewer stations (fewer than the four Given all the national and global data sources, needed above) may detect much smaller events, a cautious estimate of the global detection ca- and this can provide useful information. How- pability by the year 1991 (assuming 90 per- ever, detection by one or a few stations may cent confidence of four or more stations detect- not be adequate to, with high confidence, locate ing an event using only open unclassified or identify events. Also, reductions of the de- stations) is mb 4.2. For many regions, such as tection threshold must be accompanied by a the Northern Hemisphere, the detection capa- comparable capability to locate the events (for bility will be, of course, much better. There- focusing other intelligence resources) and to fore, by 1991, any explosion with a magnitude separate nuclear explosions from earthquakes corresponding to 1-2 kt well-coupled that is det- and legitimate industrial explosions. onated anywhere on Earth will have a high probability of being detected and located by Detection Capability Within the networks external to the U.S.S.R. Opportuni- U.S.S.R. Using Internal Stations ties to evade the seismic network outside the Soviet Union are limited. Because evasion Seismic stations located internalto a coun- scenarios require large amounts of clandestine try for the purpose of monitoring have a num- work, they are most feasible within the borders ber of important advantages: improved detec- of a closed country such as the Soviet Union. tion capability, improved location capability, Consequently, monitoring networks are de- and improved identification capability. signed to target principally the Soviet Union. The potential instantaneous detection thresholds for the large NORSAR array (42 seismometers spread over an area of about Detection Capability Within the 3,000 km’), as described in “Teleseismic Detection at High Fre- U.S.S.R. Using No Internal Stations quencies Using NORSAR Data” by F. Ringdal in IVORSAR Semiannual ?’ecfmical Summary, Apr. I-Sept. 30, 1984, are:

West of Ural Mountains-mb 2.0-2.5 (possibly better) Given that a large range of possible networks Caspian Area–rob 2.0-2.5 exists, a few type examples are useful to con- Semipalatinsk-mb 2.5-3.0 2.5-3.5 vey a sense of what can be accomplished. For Siberia–mb 70

Although much debate is associated with the Another consideration is that all detection predicted detection capabilities of internal sta- estimates used in this report are based on a tions, improved detection capability alone is 90-percent probability of four or more stations probably not of the greatest significance at this detecting an event. While this maybe a pru- time because the current detection capability dent estimation procedure from the monitor- is already very good. The improvement that ing point of view, an evader who did not wish internal stations will provide to identification to be caught might adopt a considerably more capability (differentiating explosions from nat- cautious point of view. (See chapter 2 for a dis- ural events) is by far the most important rea- cussion of the relationship between uncertainty son for requiring internal stations and should and cheating opportunities.) Such concerns be considered the basic requirement for inter- might be increased by the realization that for nal stations. The problem of detecting and many seismic events, there will beat least one identifying seismic events in the face of vari- station that will receive the signal from the ous evasion scenarios will be discussed in the event with a large signal-to-noise ratio. The sig- next two chapters. nal will be so large with respect to the noise at this station that the validity of the signal Based on cautious assumptions for a net- will be obvious and will cause a search for other work of 30 internal arrays or about 50 three- associated signals from neighboring seismic component internal stations, it appears likely stations. The possible occurrence of such a sit- that a detection threshold of mb 2.5 (90 per- uation would be of concern to a country con- cent probability of detection at four or more templating a clandestine test. stations) could be reached. This corresponds to a well-coupled explosion of 0.1-0.01 kt, or Throughout all of this discussion it must be a fully decoupled nuclear explosion with a yield kept in mind that an improvement in detec- of about 1 kt. Based on more optimistic as- tion capability does not necessarily correspond sumptions about the conditions to be encoun- directly to an improvement in our monitoring tered and prospective improvements in data capability. Although a reduced detection thresh- processing capability, this same network could old must be accompanied by a reduced identification threshold; occurrences such as indus- have a detection capability as low as mb 2.0. Detection capability contours for one such pro- trial explosions might ultimately limit the posed 30-array internal network are shown in identification threshold. For example, the esti- figure 4-7. mate of detection capability for internal stations which is given above, mb 2.0 to 2.5, corresponds to a decoupled explosion of about 1 kt. A decoupled 1 kt explosion produces the Considerations in Choosing same mb signal as a 1/70 kt (15) ton chemical Monitoring Thresholds explosion. At such small magnitude levels, there are hundreds of chemical explosions det- Depending on the number of internal sta- onated in any given month in the U.S.S.R. and tions, detection capabilities could either in- in the United States. crease or decrease. The point, however, is that very low detection thresholds, down to magnitude 2.0, can be achieved. In fact, almost any Use of High-Frequency Data desired signal detection level can theoretically be obtained by deploying a sufficient number It has long been known to seismologists that of internal stations; although there maybe dis- seismic signals of moderate to high frequen- agreements over the number and types of sta- cies can indeed be detected at large distances tions needed to achieve a given threshold. The from the source under favorable geological con- disagreements could be resolved as part of a ditions. Such is the case in the eastern United learning process if the internal network is built States, and it is generally thought that this up in stages. is also true of most of the Soviet Union. In con- 71 72 trast, tectonic regions such as the western high-frequency (30 -40 Hz) seismic data.3The United States and the southern fringe of the major points of the argument are: U.S.S.R. are generally characterized by stronger ● that natural seismic ground noise levels attenuation of high frequencies, which are therefore lost beyond relatively short distances. are very low at high frequencies, and that large seasonal fluctuations are not antic- The design and development of a nuclear ipated; monitoring strategy based on high-frequency ● that careful station selection could make seismic signals calls for: it possible to emplace seismic sensors in particularly quiet sites; and ● A determination of Earth structure within ● and around the region to be monitored, that present seismic recording technology and an evaluation of its signal transmis- allows high-fidelity recording; by suppres- sion characteristics at high frequencies. sion of system-generated noise to levels below ground noise even at high frequen- ● A methodology for identifying and selecting sites with low ground noise, and equip- cies and at quiet sites. ping such sites with high performance sen- Advocates of high-frequency monitoring ex- sors and recording systems, so as to achieve plain the efficiency of high-frequency wave the largest possible signal-to-noise ratio. propagation observed in the North American ● A reliable understanding of the high-fre- shield in terms of a simple model for attenua- quency radiation of natural (earthquakes) tion of seismic waves in stable continental re- and man-made (explosions) seismic sources. gions and argue that the model applies as well Although empirical evidence based on di- to stable continental Eurasia. Finally, they ar- rect observations is sufficient in principle, a predictive capability, based on theory, retical models of earthquakes and underground is required to assess properly new and un- explosions provide adequate predictive esti- tested monitoring conditions. mates of the relative production of high-frequency energy by various seismic sources and Such requirements parallel in every respect justify their choice by comparison with limited the usual constraints placed on standard mon- observations. itoring systems. However, direct experimen- tation pertinent to high-frequency monitoring Based on these arguments and a systematic has been rather limited so far, and the relevant modeling procedure, some seismologists rea- data available today are neither abundant nor son that the most favorable signal-to-noise ra- diverse. Consequently, an assessment of whether tio for detection of low yield (i.e. small magni- these requirements can be met relies of neces- tude) events in stable continental areas will be sity on some degree of extrapolation from our found at moderate to high frequencies (about present experience, based on theoretical argu- 30 Hz). They further infer that a well-designed ments and models. This situation leaves room network of 25 internal and 15 external high- for debate and even controversy.2 quality stations using high frequencies would yield multi-station, high signal-to-noise detec- Recently, it has been argued that the capa- tion of fully decoupled 1-kt explosions from any bility to detect and identify low-yield nuclear of the potential decoupling sites within the explosions could be greatly improved by using U. S. S. R., and result in a monitoring capability at the l-kt level. On the other hand, the inference that such ‘High quality seismic data is now becoming available from significant benefits would necessarily accrue the NRDC-Soviet Academy of Sciences stations in the Soviet Union and the United States, with more widely distributed sta- ‘For example, J. F. Evemden, C. B. Archarnbeau, and E. Cran- tions to be added in 1988 in both countries. This data may help swick, “An Evaluation of seismic Decoupling and Underground reduce the necessity for extrapolation and decrease the uncer- Nuclear Test Monitoring Using High-Frequency Seismic Data, ” tainties that foster the debate. Reviews of Geophysics, vol. 24, No. 2, 1986, pp. 143-215. 73 by relying on high-frequency recordings has mance systems with low system noise be- been strongly questioned in the seismological come more wide-spread, this concern will community. Indeed many scientists feel that disappear. the case is currently unproven. Major points ● The concern that observations which can of disagreement include: be employed to test directly the validity of the proposed use of high frequencies are ● The concern that the theoretical seismic as yet quite scant, and their interpreta- source models used so far in the analysis tion is not free of ambiguities. For exam- described above are too simple. Studies ple, the characterization of source spec- aimed at constructing more realistic models tra and the propagation and attenuation indicate that the high-frequency waves of high-frequency waves remain issues generated by seismic sources are strongly which are not resolved unequivocally by affected by complexities of source be- observations, and yet are critical to the havior that the simple models do not take formulation of a high-frequency monitor- into account. On the other hand, advo- ing strategy. Similarly, available data cates of high frequency seismic monitor- often exhibit an optimal signal-to-noise ra- ing believe that the models they have used tio at frequencies near 10 Hz, in apparent have successfully predicted a number of disagreement with the arguments enun- characteristics of seismic sources that ciated earlier. In response to these con- were subsequently verified and that none cerns, proponents argue that the NRDC- of the many well-documented observa- Soviet observations of signal-to-noise ra- tions of seismic wave characteristics from tios greater than 1 and out to frequencies large events are in conflict with their theo- above 20 Hz provides evidence for the retical model predictions. Thus they ar- potential of high frequency monitoring. gue that the model predictions, for some- While proponents of high frequency mon- what smaller events at somewhat higher itoring agree that the observations of the frequencies than are ordinarily studied, largest signal to noise ratios for signals are reasonable extrapolations. from seismic events often occur near 10 ● The concern that it may be difficult to Hz, this is not in disagreement with the identify candidate station sites where the predictions or technical arguments ad- high-frequency noise is sufficiently low to vanced for high-frequency monitoring. permit actual realization of the desired They argue that these observations are ob- benefits. Experience to date is limited, and tained from seismic receivers with system one does not really know whether a given noise that is greater than ground noise at site is suitable until it has been occupied frequencies above 10 Hz, and that many and studied for at least a year. On the of the observations are from industrial ex- other hand, advocates of high frequency plosions that are ripple-fired and so are monitoring feel that suitable low-noise expected to have lower high-frequency sites are not at all rare and can be rather content than a small nuclear test. easily found in most, if not all, geologic ● The concern stated earlier that the moti- environments within the continents. They vation for the proposed high-frequency argue that stations selected so far have monitoring approach uses contested theo- had adequately low high-frequency noise retical arguments and models to extrapo- characteristics and the selection process late from our present experience and thus was neither difficult nor time consuming. attempt to guide further steps towards a They conjecture that doubts are based on significantly improved capability. In the misidentification of high-frequency inter- present case, models are used to extrapo- nal seismic recording system noise as late both toward high frequencies and ground noise, and that once high-perfor- toward small yields, and just how far one 74

may extrapolate safely remains a matter observations of this nature has been collected of debate. The proponents of high fre- can we expect to achieve a broad consensus quency monitoring agree that the data re- about the performance of high-frequency mon- lating to high frequency monitoring is itoring systems. limited with respect to the geologic regions to which it pertains. Furthermore, it is clear that experience in the system- Arrays v. Three-Component Stations atic detection and identification of very Both small-aperture, vertical-component ar- small seismic events using high-frequency rays such as NORESS, and three-component, data is absent and that, as a consequence, it has been necessary to extrapolate from single-site stations such as the RSTN station have been considered for use as internal sta- experience with larger seismic events tions. In choosing which to use, it should be where lower frequency data is used. Propo- realized that many combinations of arrays and nents believe, however, that what limited single stations will provide the same capabil- data is available does support the most ity. For example, for any array network, there critically important predictions; these be- is a single station network with comparable ing the apparent availability of low-noise capability; but the network of single stations sites and the efficiency of high-frequency probably requires about twice as many sites. wave propagation to large distances. Although a single array has advantages over These controversial aspects notwithstand- a single three-component station (see chapter ing, there is general agreement among seismol- 3), for monitoring purposes it is preferable to ogists that good signal-to-noise ratios persist have a large number of station sites with three- to higher frequencies than those used routinely component stations rather than to have a small today for nuclear monitoring. In particular, number of sites with arrays. This is true be- data in the 10-20 Hz band show clear signals cause it permits better accommodation to de- which are undoubtedly not used optimally. tails of regional geology, and better protection Given the fact that recording of even higher against noise sources temporarily reducing ca- frequencies is demonstrably feasible in some pability of the network as a whole. However, situations, and given the potential advantages if the number of sites is limited by negotiated for low-yield monitoring, the augmentation of agreement, but the instrumentation can in- our experience with such data, the concomi- clude either arrays or three-component sta- tant continued development of appropriate tions, then arrays are preferable. This is true analysis techniques to deal with them, and the both because of the inherent redundancy of ar- validation of the models used in their interpre- rays and their somewhat better signal-to-noise tation are goals to be pursued aggressively. enhancement capability over single three-com- Not until a sufficient body of well-documented ponent stations. Chapter 5 Identifying Seismic Events CONTENTS

Introduction...... 77 Basis for Seismic Identification ...... 78 Earthquakes ...... 78 Chemical Explosions ...... 84 Rockbursts ...... 85 Methods of Identification ...... 85 Location ...... 85 Depth ...... 85

Ms:mb ...... 86 Other Simple Methods ...... 88 Spectral Methods...... 88

High-Frequency Signals ...... !! ...... 89

Identification Capability ...... 90 Identification Capability Within the U.S.S.R. Using No Internal Stations ...... 91 Identification Capability .Within the U.S.S.R Using Internal Stations:: 91

Boxes Box 5-A. Theory and Observation...... 82 5-B. Progress in Seismic Monitoring ...... 92

Figures Figure No. 5-i. Seismicity of the World, 1971 -1986 ...... 79 5-2. Seismicity of the U.S.S.R. and the Surrounding Areas, 1971-1986 . . . . 80 5-3. Cross-Sectional View of Seismicity Along the Kurile-Kamchatka Coast ...... 81 5-4. Earthquakes v. Explosions ...... 83

5-5. Ms v. mb for a Suite of Earthquakes and Explosions...... 86

5-6. Msv. mb ...... 86

5-7. The mb:MsDiscriminant for Populations of NTS Explosions and Western U.S. Earthquakes ...... 87 5-8. The VFM Method ...... 89

Table No. Page 5-l. Approximate Numbers of Earthquakes Each Year, Above Different Magnitude Levels ...... 78 Chapter 5 Identifying Seismic Events

Once a seismic event has been detected, the next step is to determine whether it was created by an underground nuclear explosion.

INTRODUCTION Once the signals from a seismic event have tempts may be made to muffle or hide the been detected, and the event located, the next seismic signal. step in seismic monitoring is that of identification. Was the event definitely, or possibly, Each of these difficulties becomes more severe an underground nuclear explosion, or can the at lower yields. signals be identified unambiguously as having another cause? As discussed in the previ- The last difficulty listed above brings up the ous chapter, seismic signals are generated by subject of evasion which is discussed in the natural earthquakes and by natural rockbursts next chapter. For the purposes of understand- in mines, as well as by chemical and nuclear ing this chapter, however, the reader should explosions. The identification problem in seis- recognize that identification capabilities must mic monitoring, called the discrimination prob- always be considered against the feasibility of lem, is to distinguish underground nuclear ex- various evasion scenarios. The successful use plosions from other seismic sources. of a muffling or decoupling evasion technique could cause a 1-10 kt decoupled nuclear explo- In the case of a located seismic event for sion to produce seismic signals comparable to which signals are large, that is, larger than either a chemical explosion of 10-100 tons re- could be ascribed to a chemical explosion or spectively, or an earthquake ranging from a rockburst, the only candidates for the source magnitude 2-3 respectively. The similarity in of the signals are an earthquake or a nuclear size and, in some cases, the properties of the explosion. Physical differences between earth- signals for these three types of events pose the quakes and nuclear explosions cause their seis- most serious monitoring challenges. The mon- mic signals to differ, and these differences can itoring system must be able to demonstrate be used to identify the events. a capability to identify with high confidence Identification becomes more complicated, seismic events whose signals might be inten- however, for small events. When identifying tionally reduced or hidden through credible small events (comparable in size to an explo- evasion techniques. It is the need to demon- sion of less than 10 kilotons), the identifica- strate such a capability, as signals become tion procedures encounter four types of diffi- smaller, that ultimately sets the threshold for culty that do not arise for larger events: seismic identification. 1. the quality of available signals is typically The present chapter describes the different lower; kinds of seismic sources and the basis for solv- 2. the number of natural events that must ing the identification problem. It then describes be discriminated against is larger; capabilities of current seismic networks, and 3. the possibilities now include chemical ex- how these capabilities might be improved by plosions and rockbursts; the addition of new seismic stations, includ- 4. the event, if nuclear, is of a size where ating stations within the U.S.S.R.

77 78

BASIS FOR SEISMIC IDENTIFICATION The seismic signals from a nuclear explosion As table 1 illustrates, there are about 7,500 must be distinguished from seismic signals cre- earthquakes that occur with mb 4.0 or above ated by other events, particularly earthquakes, each year, and about 7 percent of these occur chemical explosions, and rockbursts. Most in the Soviet Union. However, approximately seismic signals from located events are caused two-thirds of the Soviet seismicity occurs in by earthquakes, although chemical explosions oceanic areas, mainly off the Kurile-Kamchatka are also present in large numbers. Significant coast. Seismicity in oceanic areas does not raise rockbursts and other sources of seismic sig- an identification problem because acoustic sen- nals are rare. A monitoring network, however, sors provide excellent identification capabil- will encounter seismic signals created by all ity for nuclear explosions in water. Conse- of these sources and must be able to identify quently, it is earthquakes on Soviet land areas them. Consequently, the burden on any moni- from which nuclear explosions need to be dif- toring network will be to demonstrate a capa- ferentiated. For example, at the magnitude bility to detect and identify with high confi- level of mb 4.0 and above, where there are dence a clandestine nuclear test against the 7,500 earthquakes each year, approximately background of large numbers of earthquakes 183 occur on Soviet land areas and would need and industrial explosions and infrequent rock- to be identified. The number of such earth- bursts. This section reviews basic properties quakes that occur each year above various of these other seismic sources and discusses magnitudes is shown in the third column of the physical basis for discriminating them from table 1. The smaller the magnitude, the more nuclear explosions. earthquakes there are. Earthquakes and their associated seismo- Earthquakes grams have been studied on a quantitative basis for about 100 years. Since 1959, seismolo- Earthquake activity is a global phenomenon, gists have engaged in a substantial research though most of the larger earthquakes are con- effort to discriminate between earthquakes and centrated inactive tectonic regions along edges explosions by analyzing seismic signals. Thou- of the Earth’s continental and oceanic plates sands of studies have been conducted and (figure 5-l). In the U. S. S. R., most large earth- reports written. As will be discussed in the fol- quakes are located in a few active regions along lowing section, this identification problem is the southern and eastern borders of the coun- now considered to be solved for events above try (figure 5-2). For the many earthquakes a certain magnitude. occurring along the Kurile Islands region on the Pacific border of the Soviet Union, the shal- Table 5-1.—Approximate Numbers of Earthquakes low earthquakes generally occur on the ocean Each Year, Above Different Magnitude Levels side of the islands, while the deeper earth- m a b c quakes occur beneath the islands and toward b Globally Soviet Union Soviet land areas the U.S.S.R. landmass (figure 5-3). Elsewhere, 5.0 950 70 23 4.5 2,700 200 67 earthquakes can occur as deep as 700 kilome- 4.0 7,500 550 183 ters in the Earth’s mantle. 21,000 1,500 500 3.0 59,000 4,200 1,400 In figure 5-2, large areas of the U.S.S.R. are 2.5 170,000 12,000 4,000 shown for which there is no significant earth- aBased On F. Ringdal’s global study of a 10-year period, for which he finds the statistical fit log N = 7.47-O.9m . F.Ringdal, “Studyof Magnitudes, Seismicity, quake activity. Given presently available in- and Earthquake DetectabilityJ sing a Global Network,” in The Vela Program, Ann U. Kerr, (cd.), Defense Advanced Research Projects Agency, 1985. formation, such areas are referred to as aseis- bBa9ed on 7 percent of global earthquakes with numbers rounded uP slightly mic; although some activity may occur in these cBa9ed on removing two-thirds of Soviet Union earthquakes. Two-thirds of the earthquakes in the Soviet Union occur in oceanic areas, e.g., off the Kurile- Kamchatka coast and, therefore, do not present an identification problem. regions at magnitudes below mb 3.0. 79

●

● 80

● ● 81

Figure 5-3.—Cross-Sectional View of Seismicity Along the Kurile-Kamchatka Coast

Kurile Islands Kurile-Kamchatka Trench / Pacific Ocean

Two-thirds of Soviet seismicity at magnitudes greater than mb 3.5 is off the Kurile-Kamchatka coast. This sectional view shows that almost all these events are deep, below land or shallow, below ocean. Neither case is a candidate explosion location.

SOURCE: L.R. Sykes, J.F. Evernden, I.L. Cifuentes, American Institute of Physics, Conference Proceed/r@ 704, pp. 85-133, 1983.

From these studies, a number of identifica- 1 km. Almost no holes, for any purpose, have tion methods have evolved. These methods all been drilled to more than 10 km deep. The ex- have as their basis a few fundamental differ- ceptions are few, well known, and of a scale ences between nuclear explosions and earth- that would be difficult to hide. Because nuclear quakes. These differences are in: explosions are restricted to shallow depths (less than 2.5 km), discrimination between many ● location, ● earthquakes and explosions on the basis of geometrical differences between the point depth is possible in principle. In practice, the source of an explosion and the much larger uncertainty in depth determination makes the rupture surface of an earthquake, and ● division less clear. To compensate for the pos- the relative efficiencies of seismic wave sibility of very deep emplacement of an explo- generation at different wavelengths. sion, the depth must be determined to be be- With regard to the first difference, many low 15 km with high confidence before the earthquakes occur deeper than 10 km, whereas event is to be identified unequivocality as an the deepest underground nuclear explosions earthquake. Events with shallower depths that to date have been around 2.5 km and routine show large uncertainties in depth determina- weapons tests under the 150 kt threshold ap- tion might be considered as unidentified on the pear all to be conducted at depths less than basis of depth. 82

With regard to source geometry, the funda- a net shearing motion. It is common here to mental differences are due to how the seismic think in terms of two blocks of material (within signals are generated (see box 5-A). As dis- the crust, for shallow events) on opposite sides cussed in chapter 3, an underground nuclear of a fault. The stress release of an earthquake explosion is a highly concentrated source of is expressed in part by the two blocks moving compressional seismic waves (P waves), sent rapidly (on a time scale of seconds) with respect out with approximately the same strength in to each other, sliding in frictional contact over all directions. This type of signal occurs be- the plane of the fault as a result of the spon- cause the explosion forces apply a fairly uni- taneous process of stress release of rock— form pressure to the walls of the cavity cre- stress that has accumulated over time through ated by the explosion. The simple model of a geologic processes. Because of this shearing nuclear explosion is a spherically symmetric motion, an earthquake radiates predominantly source of P waves only. This contrasts with transverse motions–i.e., S waves, from all an earthquake, which is generated as a result parts of the fault that rupture. Though P waves of massive rock failure that typically produces from an earthquake are generated at about 20 percent of the S-wave level, they have a four- lobed radiation pattern of alternating compres- Box 5-A.—Theory and Observation sions and rarefactions in the radiated first mo- One of the points on which early efforts at tions, rather than the relatively uniform pat- discrimination stumbled was the claim that tern of P-wave compressions radiated in all explosions, because they are concentrated directions from an explosion source. These pressure sources, would generate insignifi- idealized radiation patterns,. P waves from an cant SH waves and Love waves. (These waves earthquake and an explosion, are shown sche- entail types of shearing motion, in which the matically at the top of figure 5-4. These two ground moves in a horizontal direction.) But types of source geometries result in the follow- it was soon found, in the early 1960s, that ing differences: some explosions generated quite strong SH and Love waves, so this discriminant came 1. Energy partitioning. The energy of a seis- to be seen as unreliable. mic event is partitioned into compres- A salutary further effect was that discri- sional, shear, and surface waves. Earth- minants based purely on theoretical predic- quakes tend to emit more energy in the tions came to carry little weight until they form of shear waves and surface waves had passed stringent tests with actual data than explosions with comparable compres- from explosion and earthquake sources. Oc- sional waves (figure 5-4). At lower magni- casionally, some useful discriminants are discovered empirically and a theoretical under- tudes, however, differences of this type standing is not immediately available. A may be difficult to distinguish if the sur- recent example of this is the observation that face waves from either source become too small to detect. regional phases Pn, Pg, and Lg for small earthquakes in the western U.S. typically ex- 2. Dimensions of the source. Earthquakes hibit more high-frequency energy (at 6-8 Hz) tend to have larger source dimensions than do small NTS nuclear explosions of com- than explosions because they involve a parable seismic magnitude.* However, in the larger volume of rock. The bigger the symbiotic relation between theory and obser- source volume, the longer the wavelength vation, there is generally a framework of un- of seismic waves setup by the source. As derstanding in which progress in one field a result, earthquakes usually emit seismic guides workers to new results in the other. waves with longer wavelengths than ex- *J-R. Murphy and T.J. “A Discrimination Analysis of plosions. Instead of describing the signal Short-Period Regional Seismic Data Recorded at Tonto Forest Observatory, ” BuLletin of the Seismological Society of Amer- in terms of wavelength, an equivalent ica, vol. 72, No. 4, August 1982, pp. 1351-1366. description can be given in terms of seismic wave frequency. Explosions, which 83

Figure 5-4.—Earthquakes v. Explosions

Earthquake

Short period I Long period (P waves) (surface waves) I I Earthquake

I I I I Explosion —. I

+ I 5 seconds I 5 minutes I

(Upper) Different radiation patterns for earthquakes and explosions. Earthquakes involve shear motion along a fault plane. Explosions are compressional sources of energy and radiate P waves in all directions. (Lower) Recorded signals (i.e., seismograms) of earthquakes and explosions generally have different characteristic features. Note the much stronger P-wave: surface-wave ratio, for explosions as compared to the earthquake. SOURCE: F. Ringdal, “Seismological Verification of Comprehensive Test Ban Treaty,” paper presented in “Workshop on Seismological Verification of a Comprehensive Test Ban Treaty,” June 4-7, 1985, Oslo, Norway.

are small, intense sources, send out stronger been observed and the distinction may signals at high frequencies; and earth- diminish for small events. The difference quakes generally have more low-frequency in frequency content of the respective sig- (long-wavelength) energy than explosions nals, particularly at high frequencies, is (figure 5-4). However, exceptions have a topic of active research. 84

These basic differences in seismic signal are to muffle a larger nuclear explosion (say, around understood on theoretical grounds; and this 5 kt), so that its seismic signals resemble those theoretical framework guides the search for of a much smaller (say, around 0.1 kt) chemi- new empirical methods of identification. As cal explosion. new types of seismic data become available (for The problem of identifying industrial explo- example, data from within the Soviet Union), sions can be partially constrained in three it maybe expected that methods of identification will be found that work at smaller magni- ways: tude levels. At low magnitudes, however, we 1. In the United States, almost all chemical have not only the problem of distinguishing explosions detonated with yields 0.1 kt or nuclear explosions from earthquakes, but also greater for industrial purposes are in fact of distinguishing nuclear explosions from the a series of more than a hundred small ex- large chemical explosions that are commonly plosions spaced a few meters apart and used for industrial purposes such as mining. fired at shallow depth with smalltime de- This is a much more difficult problem because lays between individual detonations. One in theory the two source types should produce effect of such “ripple-firing” is to gener- similar signals. ate seismic signals rather like that of a small earthquake, in that both these types of sources (ripple-fired chemical explo- Chemical Explosions sions, and small earthquakes) occur over a large area and thus lose some of the char- Chemical explosions are used routinely in the acteristics of a highly concentrated source mining and construction industries, and they such as a nuclear explosion. If individual occur also in military programs and on nuclear salvos are large enough, however, they test sites. In general, from the seismic moni- might be able to mask the signals from toring perspective, a chemical explosion is a a sub-kiloton nuclear explosion. Recent re- small spherical source of energy very similar search on this problem has shown the im- to a nuclear explosion. The magnitudes caused portance of acquiring seismic data at high by chemical explosions are generally below m b frequencies (30-50 Hz); and indeed, such 4.0, although events with magnitudes up to data suggest the existence of a distinctive mb 4.5 occasionally occur. The fact that large 1 23 numbers of chemical explosions in the yield signature for ripple-fired explosions. 2. Many of the rare chemical explosions range from 0.001 to 0.01 kt are detected and above about 0.5 kt that are not ripple fired located seismically is a testament to the capa- can be expected to result in substantial bility of local and regional seismic networks ground deformation (e.g., cratering).4 Such to work with signals below m 3.0. b surface effects, together with absence of There is little summary information avail- a radiochemical signal, would indicate a able on the number and location of chemical chemical rather than a nuclear explosion explosions in the U. S. S. R., though it appears source. The basis for this discriminant is that useful summaries could be prepared from currently available seismic data. In the United States, chemical explosions around 0.2 kt are IA.T. Smith and R.D. Grose, “High-Frequency Observations of Signals and Noise Near RSON: Implications for Discrimina- common at about 20 mines. At each of these tion of Ripple-fired Mining Blasts,” LLNL UCID-20945, 1987. special locations, tens of such explosions may 2A.T. Smith, “Seismic Site Selection at High Frequencies: A occur each year. Presuming that similar oper- Case Study, ” LLNL UCID-21047, 1987. 9D.R. Baumgardt, “Spectral Evidence for Source Differences ations are mounted in the U. S. S. R., this activ- between Earthquakes and Mining Explosions in Norway,” Seis- ity is clearly a challenge when monitoring nu- mologi”cal Researcb Letters, vol. 38, January-March 1987, p. 17. clear explosions with yields below about 1 kt. 41t is also relevant that chemical explosions in the 0.1 -0.5 kt range are commonly observed to be quite efficient in gener- It is also a challenge when considering the pos- ating seismic waves. For example, 0.5 kt chemical explosions sibility that a nuclear testing nation may seek are known to have m~ around 4.5 for shield regions. 85

that chemical explosions are typically may occasionally rupture suddenly into the very shallow. tunnel. This is referred to as a rockburst and 3. To the extent that remote methods of results from the difference between the low monitoring chemical explosions are deemed pressure existing within the tunnel and the inadequate (for example, in a low yield great pressure that exists within the surround- threshold or comprehensive test ban re- ing rock. To prevent such mine rockbursts, gime), solutions could besought by requir- bracing structures are used in the deeper ing such constraints as prior announcement tunnels. of certain types of chemical explosions, in- In terms of magnitude, rockbursts are all spections, shot-time monitoring, and in- small. They occur over very restricted regions country radiochemical monitoring. Note of the Earth and generally have a seismic mag- that in the United States (and so perhaps nitude of less than 4.0 m . also in the U. S. S. R.) chemical explosions b above 0.1 kt occur routinely at only a The source mechanisms of rockbursts are limited number of sites. very similar to those of small earthquakes. In particular, the direction of the first seismic motion from a rockburst will have a pattern simi- Rockbursts lar to that for earthquakes. Therefore, for the seismic identification problem, rockbursts can In underground mining involving tunneling be considered small earthquakes which occur activities, the rock face in the deeper tunnels at very shallow depths.

METHODS OF IDENTIFICATION Over the years, a number of identification sis of location alone, e.g. if the site is clearly methods have been shown to be fairly robust. not suitable for nuclear explosions (such as Some of these methods perform the identifi- near population centers) or if there is no evi- cation process by identifying certain earth- dence of human activity in the area. quakes as being earthquakes (but not identifying explosions as being explosions). Other Depth identification methods identify certain earth- With the exception of epicenter locations, quakes as being earthquakes and certain ex- seismic source depth is the most useful dis- plosions as being explosions. The identification criminant for identifying large numbers of process is therefore a winnowing process. earthquakes. A seismic event can be identified with high confidence as an earthquake if its Location depth is determined to be below 15 kilometers. The principal identification method is based The procedure for determining source depth on the location of a detected seismic source. is part of finding the event location using the If the epicenter (the point on the Earth’s sur- arrival time of four or more P-wave signals. face above the location) is determined to be in Also, certain seismic signals, caused by energy an oceanic area, but no hydroacoustic signals that has traveled upward from the source, were recorded, then the event is identified as reflected off the Earth’s surface above the an earthquake. Large numbers of seismic events source region, and then traveled down into the can routinely be identified in this way, because Earth, are similar to the P wave out to great so much of the Earth’s seismic activity occurs distances. A depth estimate can be obtained beneath the ocean. If the location is determined by measuring the time-difference between the to be land, then in certain cases the event can first arriving P-wave energy and the arrival still be identified as an earthquake on the ba- time of these reflected signals. The analysis 86

of broadband data through wave-form model- Figure 5-5.—Ms v. mb for a Suite of ing is particularly useful for detecting these Earthquakes and Explosions reflected signals. Empirical methods can also 7.0 be used, based on comparison with previously interpreted seismic events in the same general region as the event under study. 6.0 In principle, an advantage of depth as an identification method is that it is not dependent on magnitude: it will work for small events as well as large ones, provided the basic data are of adequate signal quality and the signals are detected at a sufficient number of stations. It will not alone, however, distinguish between chemical and nuclear explosions unless the nu- 4.0 clear explosion is relatively large and deep; or between underground nuclear explosions and earthquakes unless the earthquakes are suffi- 2.0 4.0 5.0 6.0 M ciently deep. s SOURCE: P.D. Marshall and P.W. Basham, Geophysical Journal of the Royal As. M s:mb tronomical Society, vol. 28, pp. 431-458, 1972. Underground nuclear explosions generate Figure 5-6.—MS v. mb signals which tend to have surface wave mag- 9 nitude (Ms) and body wave magnitudes (rob) o = Earthquakes that differ from those of earthquake signals. This is basically a result of explosions emit- 8 ting more energy in the form of body waves o (high-frequency seismic radiation), and earth- 7 0 000 quakes emitting more energy in the form of .0 . o 0 0 surface waves (low frequency seismic radia- 6 o tion). The phenomenon is often apparent in the A original seismograms (figure 5-4), and examples are shown in terms of Ms:mb diagrams in A figures 5-5, 5-6, and 5-7. These diagrams can be thought of as separating the population of explosions from that of earthquakes. For any event which is clearly in one population or another, the event is identified. For any event I A which is between the two populations (as occasionally happens for explosions at the Ne- 5 6 7 8 vada Test Site), this method does not provide mb SOURCE: L.R. Sykes, J.F. Evernden, I.L. Cifuentes, American Institute of Phys- reliable identification. The method, therefore, ics, Conference Proceeding 104, pp. 85-133, 1983. (All of the events has the potential of identifying certain earth- (both earthquakes and explosions) were corrected for bias.] quakes as being earthquakes and certain ex- using external stations alone.5 This difficulty plosions as being explosions. is present particularly for explosions, with the

To use this identification method, both mb $rn ~Pi~ studies of redon~ data that evaluate the Ms:mb and MS values are required for the event. This discriminant, it has been found that between 10 percent and is no problem for the larger events, but for 20 percent of the surface waves of small events are masked by signals caused by other events. This, however, has little net smaller events (below mb 4.5) it can be very effect on the discrimin ation of events for which more than one difficult to detect low-frequency surface waves station is close enough to receive surface waves. 87

Figure 5-7.—The mb:Ms Discriminant for Populations of NTS Explosions and Western U.S. Earthquakes

x “x

o 2

- + .

1 1 I I I 1 3 4 5 6 7 8

M b X Explosion

SOURCE: Modified from S.R. Taylor, M.D. Denny, and E.S. Vergino, Regional mb:Ms Discrimination of NTS Explosions and Western United States Earthquakes A Progress Report, Lawrence Livermore National Laboratory -January- 1986. “ - result that the Ms:mb method works intrinsi- diagram tailored for the general region of the cally better for identifying small earthquakes Earth in which that event occurred. than it does for identifying small explosions. The M :m method has proven to be the Internal stations would provide important ad- s b most robust identification technique available ditional capability for obtaining Ms values for for shallow events. For events below mb 4.0, events down to mb 3.0 and perhaps below. the separation between the two Ms:mb populations has been found to decrease in some

It is known that Ms:mb diagrams, with their studies. In the opinion of many seismologists, separate populations as shown in figures 5-5, however, a useful separation (at low magni- 5-6, and 5-7, tend in detail to exhibit somewhat tudes) may be possible if good quality data for different properties for sources occurring in such small events can be found. Again, in the different geophysical provinces. That is, the context of monitoring for small underground best identification capabilities are obtained, explosions in the U. S. S. R., obtaining such data for a particular event of interest, if there is al- would require in-country stations and empiri- ready a data base and an associated Ms:mb cal confirmation.

76-584 0 - 88 : QL 3 - 4 88

Other Simple Methods Therefore, the observation of significant shear wave energy is indicative that the Several other methods, with applications in event is probably an earthquake. particular circumstances, are used to provide 3. Complexity. Many explosion-generated identification. Several are still in the research body wave signals tend to be relatively stage, and are having an impact on the design simple, consisting of just a few cycles. of new instruments and the development of Many earthquake-generated body wave new procedures in data analysis. Some of these signals tend to be relatively complex, con- methods, based on the whole spectrum of fre- sisting of a long series (known as the coda) quencies contained in a seismic signal, are de- following the initial few P-wave cycles. scribed in the following section. Other meth- The concept of complexity was developed ods have been used for decades, and are still in an attempt to quantify this difference occasionally important for use in identifying in signal duration. Complexity is the com- certain problem events. In order of decreas- parison of amplitudes of the initial part ing importance, the remaining three simple of the short-period signal with those of the methods can be listed as follows: succeeding coda. There are cases, however, 1. The use of “first motion.” By this term is where an explosion signal is complex at meant an identification method based on some stations and an earthquake signal differences in the direction of the initial is simple. Therefore, the complexity motion of P waves. As illustrated in fig- method is regarded as not as reliable as ure 5-4, an explosion is expected to create Ms:mb. In practice, whenever the com- initial compressive motions in all direc- plexity method works, other identification tions away from the source, whereas an methods also work very well. earthquake typically creates initial compressive motions in some directions and Spectral Methods rarefactional motions in others. In a com- The basis of the success of Ms:mb diagrams pressive wave, the ground first moves is, in part, the fact that the ratio of low fre- away from the source. In a rarefaction, the ground first moves toward the source. quency waves to high frequency waves is typi- From a good seismometer recording, it is cally different for earthquakes and explosions. Thus, M is a measure of signal strength at often possible to observe the direction of s around a frequency of 0.05 Hz, and mb is a this initial motion (for example, from ob- measure of signal strength at around a fre- servation of whether the ground first moves up or down at the seismometer). quency of 1 Hz. As a way of exploiting such differences, M :m diagrams are quite crude This identification method can be power- s b compared to methods that use a more complete ful if the signal-to-noise ratio is large. But for small events in the presence of seis- characterization of the frequency content of mic noise (as discussed in chapter 4) it can seismic signals. The analysis of earthquake and be difficult or impossible to determine the explosion signals across their entire spectrum direction of the first motion. Because it of frequencies is an important component of is never clear that rarefactional motions the current research and development effort in seismic monitoring. may not exist in directions for which net-

work coverage is poor, the method at best Because Ms:mb diagrams work well when identifies an earthquake as an earthquake, surface waves are large enough to be measured, but cannot unequivocally identify an ex- the main contribution required of more sophis- plosion as an explosion. ticated analysis is to make better use of the 2. The observation of S waves. Because of the information contained in the P-wave signals compressive nature of explosion sources, and other large amplitude, high frequency sig- explosions typically generate less shear nals. This offers the prospect of improved iden- wave energy than do most earthquakes. tification capabilities just where they are most 89 needed, namely for smaller events. Thus, in- eral stations provide VFM data independently stead of boiling down the P-wave signals at identifying an event, as an explosion for ex- all stations simply to a network mb value, one ample, then assigning a numerical probability can measure the amplitude at a variety of fre- that the event observed was indeed an explo- quencies and seek a discriminant which works sion can be accomplished with more reliabil- on systematic differences in the way earth- ity and confidence.) VFM is more sensitive to quake and explosion signals vary with fre- noise and regional attenuation differences than quency. One such procedure is the variable Ms:mb, but the main point is that simultane- frequency-magnitude (VFM) method, in which ous use of both methods should allow improved the short-period body wave signal strength is discrimination of small events. measured from seismograms filtered to pass 6 High-Frequency Signals energy at different frequencies, say, fl and f2. Discrimination based on a comparison of m (f1) b The use of high-frequency signals in seismic and m (f ) in many cases shows a clear sepa- b 2 monitoring is strongly linked to the question ration of earthquake and explosion populations 7 8 of what can be learned from seismic stations (figure 5-8). 9 in the U.S.S.R. whose data is made available An analogy for the VFM discriminant would to the United States. Such “in-country” or “in- be a person’s ability to tell by ear the differ- ternal” stations were considered in CTBT ne- ence between a choir with loud sopranos and gotiations in the late 1950s and early 1960s, weak contraltos, and a choir with loud con- and the technical community concerned with traltos and weak sopranos. Ms:mb is like com- monitoring in those years was well aware that paring basses and contraltos. For small events, seismic signals up to several tens of Hz could

VFM is complementary to Ms:mb in several propagate from explosions out to distances of ways: VFM is improved by interference of the main P wave with waves reflected from the sur- Figure 5-8.—The VFM Method face above the explosion, while Ms:mb is degraded; VFM is more effective for hard rock 8.0 I I I explosions, while Ms:mb will preferentially dis- Earthquakes criminate explosions in low-velocity materials; Presumed explosions o and VFM is insensitive to fault orientation. 7.0 Further, the Ms:mb method requires averag- 00 80 ing over observations from a network of sta- t tions surrounding the event, while the VFM method may be applied at a single station at any distance and direction from the source. It is found in practice, however, that the VFM method is most reliable when several high performance stations are used. (Obviously if sev-

‘W.B. Archambeau, D.G. Harkrider and D.V. Hehnberger, U.S. Arms Control Agency Report, “Study of Multiple Seismic Events,” California Institute of Technology, ACDA/ST-220, 1974. 7J.F. Evernden, “Spectral Characteristics of the P-codas of Eurasian Earthquakes and Explosions,” Bulletin of the Seis- m [2.25 Hz] mological Society of America, vol. 67, 1977, pp. 1153-1171. b ‘J.M. Savino, C.B. Archambeau, and J.F. Masse, Discri~”- nation Results From a Ten Station Network, DARPA Report The body-wave magnitude is determined at two different fre- SSS-CR-79-4566, S-CUBED, La Jolla, CA, 1980. quencies. Earthquakes and explosions fall into separate ‘J.L. Stevens and S.M. Day, “The Physical Basis for m~:M, populations because explosions are relatively efficient at and Variable Frequency Magnitude Methods for Earth- generating higher frequencies. quake/Explosion Discrimination, ’ Journal of Geophysical Re- SOURCE: J.L. Stevens and S.M. Day, Journal of Geophysical Research, voI 90, search, vol. 90, March 1985, pp. 3009-3020. pp. 3009-3020, 1985. 90

several hundred kilometers. The seismic waves from internal stations should lead to signifi- most prominent at these so-called “regional cant improvements in detection capability.10 distances” (by convention, this means dis- The daily recording of many very small seis- tances less than about 2,000 km) are known mic events, together with the possibilities for

as Pg,P n,S n, and L (see ch. 3 for a discus - evasion at low-yields, focuses attention on the

sion of these regional waves). Pg propagates discrimination problem for events in the mag- wholly within the crust; Pn and Sn travel at nitude range mb 2.0 to mb 4.0. the top of the mantle just below the base of the crust; and L , guided largely by the crus- For an explosion that is well-coupled to the g ground, an m = 2.0 can be caused by only tal layer, is often the strongest signal and is b sometimes observed across continents even at a few thousandths of a kiloton, that is, a few distances of several thousand kilometers. tons of TNT equivalent. At this level, a monitoring program would confront perhaps 1,000 However, this early interest in regional to 10,000 chemical explosions per year. Given waves lessened once the Limited Test Ban the difficulty of discriminating between chem- Treaty of 1963 was signed and it was recog- ical and nuclear explosions, it is clear that be- nized that subsequent programs of large un- low some level events will still be detected but derground nuclear explosions could be moni- identification will not be possible with high con- tored teleseismically (i.e., at distances beyond fidence. However, recognizing that relatively 2,000 km), so that internal stations were not few chemical explosions occur at the upper end needed. At teleseismic distances, seismic body of this mb range (2.0 - 4.0), where many chem- wave signals are usually simpler and have in- ical explosions can be identified by character- deed proved adequate for most purposes of cur- istics of their signals over a broad frequency rent seismic monitoring, even under treaty re- band (see the earlier discussion of ripple-fired strictions on underground nuclear testing that chemical explosions), high frequency data have developed since 1963. Thus, it is in the recorded within the U.S.S.R. can clearly con- context of considering further restraints on tribute substantially to an improved monitor- testing, such as a comprehensive or low-yield ing capability. It is difficult to reach precise test ban, that the seismic monitoring commu- but general conclusions on what future yield nity needs to evaluate high-frequency regional levels could be monitored, because much will seismic signals. The basis for the role of high- depend on the degree of effort put into new frequency monitoring, in such a hypothetical seismic networks and data analysis. Conclu- new testing regime, is that the greatest chal- sions would depend too on judgments about lenge will come from decoupled nuclear explo- the level of effort that might be put into clan- sions of a few kilotons, and for them the most destine nuclear testing. The discussion of eva- favorable signal-to-noise ratios may beat high sion scenarios is taken up in the next chapter, frequency. Internal stations, of high quality and high-frequency seismic data are clearly and at quiet sites, will be essential to address useful for defeating some attempts to hide the monitoring issues associated with such small nuclear explosions. events.

It is clear that a dedicated program to make IOFor a more complete discussion of the use of high-frequency optimal use of high-frequency seismic data data for detection, see ch. 4, “Detecting Seismic Events. ”

IDENTIFICATION CAPABILITY By applying one or more of the methods dis- set of different identification methods, taken cussed above, many seismic events can be iden- as a whole and applied in a systematic fash- tified with high confidence. Note, however, that ion, that must be assessed when giving sum- no one method is completely reliable. It is the maries on capability. 91

Identification Capability Within the Identification Capability Within the U.S.S.R. Using No Internal Stations U.S.S.R. Using Internal Stations The identification capability of any network With a number of internal stations, it is of seismic stations will always be poorer than clearly possible to attain a much improved de- the detection capability for that network. In tection and location capability within the So- general, a rough estimate of the 90 percent viet Union. From estimates described in the identification threshold will be about 0.5 mb previous chapter, there is consensus that de- above the 90 percent detection threshold for tection capability (90 percent probability of de- magnitudes above mb 3.5. Below a magnitude tection at four or more stations) of mb 2.() -2.5 of about 3.5 mb, this difference may be larer can be realized, although the debate is not yet than 0.5 mb. resolved on the number of internal stations In the previous chapter, a network consist- that would be required, nor on whether the value is closer to m 2.0 or m 2.5. ing of a dozen or so arrays all outside the So- b b viet Union was discussed as a type example A detection capability down to somewhere to convey a sense of what capabilities can be in the range mb 2.0-2.5, however, cannot be achieved. A cautious calculation of the detec- easily translated into a statement about iden- tion capability for such a network was that it tification capability. Estimation of the capa- would have a 90 percent probability of detect- bility of hypothetical networks, using regional ing at four or more stations all seismic events seismic waves, is difficult because data on noise within the Soviet Union with a magnitude levels and signal propagation efficiency are not (rob) of 3.5 or greater.” Therefore, for seismic usually available and assumptions must be events in the U. S. S. R., a cautious estimate of made that turn out to have a strong influence the identification capability of such a network on conclusions. Although there is now general external to the Soviet Union is that the 90 iden- agreement on the detection capability of tification threshold will be mb 4.0. From ta- hypothetical networks, there are some signif- ble 5-1, it can be seen that approximately 180 icant differences in opinion on the identifica- earthquakes occur at magnitude 4.0 or above tion capability such networks provide. each year on Soviet land areas. This would Discussion of identification capability for mean that approximately 18 of these earth- small events (mb in the range 2.5- 4.0), using quakes would not be identified with high con- internal stations, is one of the main areas of fidence by routine seismic means alone. This, technical debate in seismic monitoring. Inter- however, does not translate into an opportu- nal stations will significantly improve the ca- nity to cheat, because from the cheater’s per- pability to identify many small earthquakes spective there is only a one-in-ten chance that as earthquakes, because such stations extend any given event above this magnitude will not the discriminants related to the MS:mb be identified through seismic means. method down to lower magnitudes. (Internal Events larger than this will be identified seis- stations will also permit more accurate deter- mically with even higher confidence. A larger mination of epicenters and source depths, thus percentage of the events that are smaller will supplying for small events the most basic in- not be identified, and the number of events will formation upon which most sources are iden- also increase with decreasing seismic mag- tified.) But certain types of shallow earth- nitude. quakes, below some observable magnitude level, are recognized as difficult if not impossible to identify. Also, preliminary use of high frequencies in the United States, to discriminate the explosion signals of small nuclear tests in Nevada from signals of small earth- “See ch. 4, “Detecting Seismic Events. ” quakes in the western United States, has had 92

some discouraging results. While some say that this is indicative merely of difficulties Box 5-B.—Progress in Seismic Monitoring associated with high frequency seismic wave Progress in seismic monitoring has been propagation in the western United States (and characterized by research results that, when that high frequencies propagate with higher first offered, seemed optimistic but which in signal-to-noise ratios in the U. S. S. R.) or that several key areas have withstood detailed the instrumentation used in the study pre- subsequent study and thus have become ac- vented data of sufficiently high quality from cepted. Occasionally there have been setbacks, being brought to bear on the problem, others as noted elsewhere in describing explosion- claim that these preliminary results are valid induced S-waves, and signal complexity. and are indicative of a fundamental lack of ca- The current situation is still one of active pability in the seismic method, at very small research, in that spectral discriminants have magnitudes. been proposed that (if corroborated by em- In the absence of access to extensive Soviet pirical data, which is now lacking) would permit monitoring down to fractions of a kilo- data (particularly, explosion data), some guid- ton. A key problem in estimating what future ance in what might be possible is given by capability is possible is that current research detailed studies of experience with U.S. explo- suggestions often entail data analysis signif- sions. Here, it is recognized that use of more icantly more sophisticated than that required than one discriminant results in improvement for conventional discriminants. Database over use of the single best discriminant (which management would also have to be improved.

is usually Ms:mb). In one multi-discriminant The requirement, for operational purposes, experiment for U.S. explosions and earthquakes that a discriminant be simple to apply, is thus which drew heavily on data such as that pre- in conflict with the requirement that the max- sented in the last diagram of figure 5-5 (in imum amount of information be extracted from seismic signals. which mb for some earthquakes is less than 3.0), seismic discrimination was achieved for 12 13 all events that had both mb and MS values. that detect down tomb 2.0-2.5, identification The discriminants tested were Ms:mb, combined with a list that included relative excita- can be accomplished in the U.S.S.R. down to tion of short-period SH waves; relative signal at least as low as mb 3.5. This cautious identification threshold is currently set by the un- amplitudes for Pn, Pg, Lg, and the largest part of the P wave; generation of higher mode sur- certainty associated with identifying routine face waves; long-period surface wave energy chemical explosions that occur below this level. density; relative amplitudes of crustal Love Many experts claim that this identification and Rayleigh waves; excitation of S ; spectral threshold is too cautious and that with an in- n ternal network, identification could be done ratios of Pn, Pg and L ; various other spectral methods; and a dept discriminant (see box with high confidence down to mb 3.0. At 5-B). Not included, was what in practice in the present, however, this has not been accepted U.S.S.R. would be a key but not definitive dis- as a consensus view, partly because some ex- criminant, namely an interpretation of the epi- perts are on principle unwilling to extrapolate center location. from experience with limited U.S. data to a From this body of experience, there appears hypothetical situation that relies on internal to be agreement that, with internal stations stations in the U.S.S.R. On the other hand, advocates of high-frequency monitoring maintain ‘*M.D. Denny, S.R. Taylor, and E.S. Vergino, “Investigations that identification can be routine at thresholds of m~ and MS Formulas for the Western U.S. and Their Impact on the m~:M, Discriminant, ” UCRL-95103 LLNL, August well below mb 3.0. The acceptance of an iden-

1986. tification capability below mb 3.0, however, 19R.E. Glaser, S.R. Taylor, M.D. Denny, et.al., “Regional Dis- would probably require practical experience crirninants of NTS Explosions and Western U.S. Earthquakes; Multivariate Discriminants, ” UCID-20930 LLNL, November with data from a monitoring network through- 1986. out the Soviet Union. Chapter 6 Methods of Evading a Monitoring Network CONTENTS

Page Introduction. , ...... 95 Evasion Schemes ...... 95 Testing Behind the Sun or in Deep Space ...... , ...... 96 Simulating an Earthquake ...... 96 Testing During an Earthquake ...... 96 Testing in a Large Underground Cavity-Decoupling ...... 97 Testing in a Nonspherical Cavity ...... 97 Testing in Low-Coupling Materials...... 97 Masking a Test With a Large Chemical Explosion...... 97 Physics of Cavity Decoupling ...... , ...... 98 Efficiency of Cavity Decoupling ...... 98 Limits on Cavity Construction in Salt Deposits ...... 98 Limits on Cavity Construction in Hard Rock ...... 99 Cavity Size Requirement for Decoupling ...... , ...... ,100 Constraints on Decoupling ...... 101 Decoupling Factors ...... 101 Low Frequencies ...... 101 High Frequencies...... 102 Decoupling Opportunities in the Soviet Union ...... 103 Partial Decoupling ...... , ..., ...105 Monitoring Capabilities Considering Evasion ...... 106 Monitoring Capability Within The U.S.S.R. Using No Internal Stations...... 107 Monitoring Capabilities Within The U.S.S.R. With Internal Stations ...108

Figures Figure No. Page 6-l. Minimum Cavity Size Required To Decouple a 5 kt Nuclear Explosion ...... 100 6-2. Decoupling Factor as a Function of Frequency...... 102 6-3. Salt Deposits in the Soviet Union ...... , ...... 104 6-4. Partial Decoupling Effect ...... 105

Table Table No. Page 6-1. Numbers of Decoupled Explosions of Yield Greater Than 1 ktThat May Be Possible in Cavities Created by Contained U.S.S.R. Underground Explosions, 1961-86 ...... 103 Chapter 6 Methods of Evading a Monitoring Network

Seismic monitoring when combined with treaty constraints and other monitoring methods must demonstrate a capability to defeat any plausible scenario for evading the monitoring network INTRODUCTION The previous chapters have discussed the ca- As we will see in this chapter, certain eva- pability of various networks to detect and iden- sion scenarios could create serious problems tify seismic events. From this discussion it is for a seismic monitoring system under certain clear that well-coupled nuclear explosions within conditions. The need to demonstrate that these the Soviet Union could be detected and iden- evasion scenarios can be defeated (i.e., the ex- tified with high confidence down to yields well- plosions in question identified) with high con- below 1 kiloton using a high-quality seismic fidence is what limits our monitoring capabil- network. Yet, in deciding what limits on un- ity. The problem of evasion must be dealt with derground nuclear testing could be verified, by a combination of seismic methods, treaty further considerations are necessary. A coun- constraints, and other monitoring methods try attempting to conduct a clandestine test that reduce the difficulties and uncertainties would presumably use every practical means of applying seismic monitoring methods to to evade the monitoring network by reducing, every conceivable test situation. In short, seis- masking, and disguising the seismic signal cre- mic monitoring needs some help and the obvi- ated by the explosion. Consequently, detection ous approach is to require the structuring of and identification thresholds cannot be directly any treaty or agreement to create a testing translated into monitoring capabilities with- environment that makes it much more likely out considering the various possibilities for that a combination of prohibitions, inspections, evasion. and seismic methods will provide the desired high levels of verification capability.

EVASION SCHEMES Over the past three decades, researchers of extreme difficulty of execution or even in- have conceived a number of theoretical sce- feasibility. To determine which are which, narios by which a low-threshold test ban treaty standards of credibility need to be applied. might be evaded. These include: testing behind the sun, testing in deep space, detonating a For an evasion scenario to be credible it must series of explosions to simulate an earthquake, be technically feasible and it must create a testing during or soon after an earthquake, worthwhile advantage for the country consid- testing in large underground cavities, testing ering cheating. As discussed in chapter 2, a in nonspherical cavities, testing in low-coupling country considering cheating would have to material such as deposits of dry alluvium, and evaluate the risks and costs of being caught masking a test with a large, legitimate indus- against the benefits if not caught. The coun- trial explosion. While some of these scenarios try concerned about preventing cheating has warrant genuine attention from a monitoring to guess the other country’s values for mak- perspective, others can be dismissed because ing such decisions. While a slight probability

95 of detection might be sufficient to deter cheat- could be addressed politically by negotiating ing, a much more stringent standard is usu- an agreement to conduct simple inspections ally needed to achieve high confidence that any of the rare vehicles that go into deep space. cheating would be detected. Thus, the degree of confidence needed to satisfy the concerned Simulating an Earthquake country is often higher than what is needed in practice to prevent cheating. It has been suggested that a series of nuclear explosions could be sequentially deto- Although the majority of proposed evasion nated over the period of a few seconds to mimic scenarios have been shown to be readily defeated the seismic signal created by a naturally occur- by a good seismic monitoring network, a few ring earthquake. The purpose of such a sequen- concepts have evoked serious concern and anal- tial detonation would be to create a P-wave ysis on the part of seismologists and other sci- amplitude that would indicate an earthquake entists. The remainder of this first section pro- 1 when using the Ms:mb discrhninant. This eva- tides a brief listing of the various evasion sion method has been dismissed, however, be- scenarios. The following three sections discuss cause it only works if the P-wave amplitude in detail evasion scenarios involving cavity is measured over just one cycle. If the P-wave decoupling and how opportunities for decoupling amplitude is measured over several cycles, the could be reduced. The final section assesses M s:mb discrimin ant will indicate an explosion. the extent to which the most threatening eva- Furthermore, the sequence of waves simulated sion scenarios limit our capability to monitor by the explosion will only appear as an earth- seismically underground nuclear explosions. quake over a particular distance range. Con- sequently, a well-distributed network that Testing Behind the Sun records over a variety of ranges would not be fooled. In addition, such an evasion attempt or in Deep Space would create large seismic signals that other It has been suggested that the Soviet Union discriminants might recognize as being created could cheat on all test limitation treaties sim- by an explosion. ply by testing in deep space or behind the sun. The idea is that one or two space vehicles would Testing During an Earthquake go behind the sun or into deep space. A nuclear device would be detonated and an instru- The hide-in-earthquake scenario posits that ment package would record the testing infor- a small explosive test can be conducted with- mation and at a later time transmit the data out detection by detonating it shortly after a back to Earth. Some feel that such a testing nearby naturally occurring earthquake. If the scenario is both technically and economically earthquake is sufficiently large and the explo- feasible. Others feel that the technical sophis- sion is properly timed, the seismic signal of tication, risk, and uncertainty of such a test the explosion will be partially or completely exceeds the utility of any information that hidden by the larger seismic signal of the earth- could be obtained in such a manner. Such a quake. This evasion method was at one time test would bean unambiguous violation of sev- considered a challenge to seismic monitoring eral treaties, and hence, discovery would be even though the technical difficulties associ- costly to the tester. In any case, if clandestine ated with the execution have long been known testing behind the sun or in deep space is dem- to be great. For example, seismologists cur- onstrated on technical grounds to be a concern, rently have no reliable techniques for the short- the risk could addressed, albeit at considerable term prediction of the time, location, and size expense, by deploying satellites to orbit around of earthquakes, and this limitation is unlikely the sun. Such satellites equipped with detec- to be overcome in the near future. tors for thermal photons could monitor explo- ‘A discussion of M,:m~ and other discriminants is presented sions in deep space. Alternatively, the risk in chapter 5, “Identifying Seismic Events. ” 97

Recent developments in seismic instrumen- two reasons. First, a nonspherical cavity would tation and data handling have further reduced have no better and perhaps worse decoupling the feasibility of this evasion scenario. New than a spherical cavity of the same volume.2 seismic instrumentation is now capable of fil- Second, a monitoring network would have seis- tering so as to pass only high-frequency seismic stations in many directions, not just one. mic waves. Because nuclear explosions produce The presence of such stations would increase higher frequency seismic waves than earth- the risk of detection by at least one station, quakes, it is often possible to remove the ef- possibly at an enhanced level. fects of distant earthquakes and see the waves created by the explosion. For this reason and Testing in Low-Coupling Materials the difficulty of detonating an explosion at the right location and time, the hide-in-earthquake As discussed in the previous chapters, the scenario is no longer considered a credible eva- proportion of explosive energy converted into sion threat. However, because high-frequency seismic waves depends on the type of rock in seismic waves may not always be detectable which the explosion occurs. Low-coupling ma- at great distances, it maybe necessary to have terials such as dry porous alluvium have air- seismic stations within the Soviet Union to ob- filled pore spaces that absorb much of the ex- viate the hide-in-earthquake scenario at yield plosive energy. This has led to the concern that limits as small as a few kilotons. a monitoring network could be evaded by detonating an explosion in low-coupling material. Testing in a Large Underground The opportunities for such evasion are thought Cavity—Decoupling to be limited in the Soviet Union because no great thicknesses of dry alluvium are known If a nuclear explosion is set off in a suffi- to exist there. In fact, large areas of the So- ciently large underground cavity, it will emit viet Union are covered with permafrost that seismic waves that are much smaller than would produce well-coupled seismic signals. those from the same size explosion detonated Estimates of the maximum thickness of allu- in a conventional underground test. This scheme, vium in the Soviet Union indicate that it would called cavity decoupling, has been experimen- only be sufficient to muffle explosions up to tally verified at small yields. It is the consensus 1 or 2 kt. Even if such an opportunity does of geologists that significant opportunities ex- exist, alluvium is a risky medium for testing ist within the Soviet Union to construct un- because it is easily disturbed. An explosion in derground cavities suitable for decoupling low alluvium could create a subsidence crater or yield explosions. Furthermore, it is the con- other surface expression. Consequently, clan- sensus of seismologists that seismic waves can destine testing in low-coupling material is con- be muffled by this technique. Consequently, sidered feasible only for explosions below 1 or the technical capability to conduct clandestine 2 kt. decoupled nuclear tests determines the yield threshold below which treaty verification by Masking a Test With a Large seismic means alone is no longer possible with Chemical Explosion high confidence. The later sections of this chapter discuss cavity decoupling scenarios in detail. As discussed in chapter 5, chemical explosions are used routinely in the mining and con- Testing in a Nonspherical Cavity struction industries. In monitoring a low-yield or comprehensive test ban treaty, there would This evasion scenario suggests that the det- be concern that large chemical explosions could onation of an explosion in an nonspherical cavity could be used to focus the resulting seis- 2L.A. Glenn and J.A. Rial, “Blast-Wave Effects on Decoupling mic waves away from monitoring stations. With Axis-Symmetric Cavities, ” Geophysical Journal of the This evasion scenario has been dismissed for Royal Astronomical Society, October 1987, pp. 229-239. 98 be used to mask the signals from a nuclear test. decoupled in a large underground cavity and Unlike an earthquake, such explosions could the reduced seismic signal either masked with be timed to coincide with a clandestine nuclear the simultaneous detonation of a very large test. If done in combination with cavity chemical explosion or attributed to a chemi- decoupling, this evasion scenario would be a cal explosion. The combination of decoupling challenge to a monitoring network. For exam- and masking is discussed further in the de- ple, a nuclear explosion of a few kt could be tailed sections on decoupling scenarios.

PHYSICS OF CAVITY DECOUPLING An underground nuclear explosion creates If, on the other hand, the explosion occurs seismic waves with a broad range of frequen- in a hole of much greater initial volume, the cies. For purposes of seismic detection and explosive stresses at the cavity wall will be identification of small events, frequencies from smaller. This results in less flow of the rock, roughly 1 Hz to perhaps as high as 30 or 50 hence less cavity expansion and reduced coup- Hz may be important. ling to seismic waves. If the initial hole is suffi- For the lower end of this frequency range, ciently large that the stresses in the surrounding rock never exceed the elastic limit, the the amplitude or size of the seismic waves cre- seismic couplings minimized. Further increase ated by an explosion is approximately propor- of the emplacement hole size will not further tional to the total amount of new cavity vol- reduce coupling at low frequencies, and the ex- ume created by the explosion. A conventional, plosion is said to be fullly decoupled. or tamped, test is detonated in a hole whose initial volume is negligible compared to its post-test volume. Because the initial hole is Cavity construction on a scale required for small, the rock surrounding the explosion is explosion decoupling is possible in either salt driven beyond its elastic limit by the explo- of sufficient thickness or in hard rock such as sion and flows plastically. This flow results in granite. In either case, the cavity volume re- large displacements of the surrounding rock quired for full decoupling increases in propor- mass, and therefore leads to a large cavity- tion to the explosion yield and decreases as the volume increase around the explosion, and ef- strength of the rock increases. ficient generation of seismic waves.

EFFICIENCY OF CAVITY DECOUPLING Limits on Cavity Construction was sufficiently large to decouple the subse- in Salt Deposits quent 0.38 kt “Sterling” nuclear test, which was detonated in the Salmon explosion cavity.4 Large cavities suitable for decoupled nuclear Nuclear explosions create cavity volume ap- testing above 1 kt can be constructed in salt proximately in proportion to their yield. Thus, deposits either by detonating a nuclear explo- applying the yield ratio given in the Salmon/ sion of several tens of kilotons, or by solution Sterling experiment, an explosion greater than mining. For example, a stable, free-standing 14 kt would be required to create a cavity suffi- cavity was created by the U.S. “Salmon” test, cient to fully decouple a 1 kt test; similarly, a 5.3 kt explosion in a salt dome.3 This cavity an explosion greater than 140 kt would be required to create a cavity sufficiently large to ‘U.S. Department of Energy, Office of Public Affairs, Nevada Operations Office, Anncmnced United States Nuclear Tests, July 1945 Through December 1983, NVO-209 (rev.4), January 1984. 41bid. 99 fully decouple a 10 kt test. Explosive construction of the overburden (i.e. the weight of the tion of cavities adequate to decouple shots overlying rocks). If the cavity was empty, the above 1 kt would obviously be impossible to overburden pressure would not be supported accomplish clandestinely. Past nuclear tests and the stability of the cavity would be un- in the Soviet Union have produced many cavi- certain. ties suitable for decoupling, but the location Both salt domes and bedded salt regions and approximate size of most of these is known; have to be considered as candidate locations and thus evasion opportunities at these sites could be limited if activity at them is mon- for construction of decoupling cavities in salt, itored. although the mining procedure would be more complex in bedded salt deposits. To create a Solution mining on the required scale would cavity in bedded salt, solution mining of solu- also be difficult to conceal. For example, with ble layers would have to be combined with ex- present techniques it would take many months plosive mining of insoluble interbeds. or perhaps even a year of continuous opera- The creep strength of natural rock salt con- tion at high circulation rates to solution mine trols the maximum depth at which a stable a cavity adequate to fully decouple a 5 kt ex- cavity can be maintained. Cavity collapse or plosion. The technology also requires enormous major changes in cavity shape occur over time amounts of water and the disposal of enormous scales of a few months when the overburden amounts of brine, further hindering conceal- pressure at cavity depth exceeds the internal ment. However, given that such an operation pressure in the cavity by more than about 20 would be detected and monitored, it might be difficult to distinguish legitimate and evasion- MPa (200 times atmospheric pressure). This corresponds to a maximum depth of about 1 related activity. Concealment of the site would km for a stable, empty cavity. A brine-filled not be necessary if appropriate activity (min- or gas-pressurized cavity might be stable to ing of salt) exists. Consequently, areas of salt about 2 km depth. If a cavity is made by an deposits might require treaty provisions deal- explosion or by solution mining, the salt will ing with chemical explosions with magnitudes be weakened. This will be a consideration be- comparable to decoupled nuclear explosions. cause for weak salt a larger cavity is needed, Apart from the significant problems of con- than predicted for strong salt, to fully decou- cealment and resource application, there do not ple a given explosion. appear to be constraints preventing the construction, by solution mining, of cavities large enough to fully decouple explosions up to 10 Limits on Cavity Construction kt. In fact, the Soviet literature reports solu- in Hard Rock tion-mined cavities with volumes up to one million cubic meters. If such a cavity could be con- No cavities have been constructed in hard structed with a spherical shape, it would be rock on the scale of those known in salt. There 60 meters in radius. A spherical cavity with is agreement among verification experts that a 60 meter radius would have sufficient vol- decoupling cavities with radii up to about 25 ume to decouple an explosion up to 14 kt, based meters, suitable for repeated testing up to on cube root scaling of the U.S. Salmon/Ster- about 1 or 2 kt, can probably be constructed ling salt dome decoupling experiment. However, with existing technology. Repeated testing these existing large, solution-mined cavities could likely be detected well enough to get good are not spherical. They are highly elongated, locations; and the detection of repeated events irregular in shape, and filled with brine. These at the same location would be suspicious. There features reduce the size of the explosion that appear to be no known technological limita- could be decoupled in the cavity. Furthermore, tions preventing construction of cavities up the brine in the cavity supports through its to perhaps 45 meters radius, suitable for de- own hydrostatic pressure a considerable por- coupling explosions up to about 10 kt. How- ever, the long-term stability under repeated ex- As noted above, however, there is a limit to plosive loading is questionable. how deep a cavity can be maintained in salt, due to salt’s low creep strength. Thus, there In constructing a cavity of radius larger than about 25 meters, a very extensive network of are two separate issues regarding the depth long cables would be needed to strengthen and of cavities: 1) the deeper the cavity, the smaller pre-stress a large region of the rock surround- the size required to decouple an explosion of ing the cavity. Such construction would require a given yield, and 2) the deeper the cavity, the an elaborate network of additional tunnels and lower the strength of the salt and the more difficult it is to maintain an open cavity. The low shafts in the surrounding rock. The technology is untested on this scale and construction strength of salt eventually limits the depth at may be severely complicated in many areas by which a cavity can be created. Even the smallest hole below 1-2 km will squeeze shut. the presence of high compressive stresses in the rock and by joints, fractures, and other rock As discussed earlier, the limiting depth for inhomogeneities that are present in even the stability of a large, empty cavity in salt is most uniform granites. about 1 km. This depth implies that the Salmon/ Concealment of such a massive excavation Sterling cavity (at 0.82 km depth), was very operation from satellite reconnaissance or near the maximum depth for stability of an other National Technical Means would be ex- empty cavity. Consequently, the Salmon cavity tremely difficult, and thus some plausible cover size approximately sets the lower bound on operation would probably be necessary. A po- Figure 6-1 .—Minimum Cavity Size Required To tential evader would also have to consider the Decouple a 5 kt Nuclear Explosion possible leakage along joints of radioactive products such as bomb-produced noble gases. Finally, the evader would also have to be concerned that explosions might result in the unexpected collapse of the cavity and the formation of a crater on the surface. Such a possibility is not without precedent: in the 1984 “Midas Myth” test in Nevada, 14 people were hurt and one man killed during the unexpected formation of a crater above a tiny collapsed cavern at a depth of 1,400 feet.

Cavity Size Requirement for Decoupling The minimum cavity radius required for full decoupling is proportional to the cube root of the explosion yield and inversely proportional to the cube root of the maximum pressure which the overlying rock can sustain without blowing out or collapsing. In salt, the maximum sustainable cavity pressure increases ap- To fully decouple a 5 kt explosion in salt, a spherical cavity proximately in proportion to depth. Therefore, with a radius of at least 43 meters would be required. The the minimum cavity size for decoupling is in- height of the Statue of Liberty with pedestal (240 ft) is 85°/0 versely proportional to the cube root of depth of the required diameter (282 ft). (i.e. smaller cavities will work at deeper depths). SOURCE: Office of Technology Assessment, 1988 101 cavity size for full decoupling (at the yield of meter cavity would be needed to decouple 5 the Sterling test). This implies a minimum kt. This number is a rough estimate because cavity radius of at least 25 meters for full it does not take into account the joints and decoupling of a 1 kt test in salt. Cavity require- fractures that would be present in even the ments are expected to scale as the cube root most uniform granites. The difference between of the explosion yield. For example, to fully the salt and granite estimates is due to the decouple a 5 kt explosion in salt, a spherical greater strength of granite, which might there- cavity with a radius of at least 43 meters (25 fore sustain a somewhat higher pressure. Such times the cube root of 5) would be required (fig- estimates, however, remain uncertain and ure 6-l). some experts doubt that the effective strength of granites would be greater than salt and be- In granite, a smaller cavity, perhaps around lieve that a radius comparable to that of salt 20 meters in radius, might be expected to suc- would be needed to decouple the same size ex- cessfully decouple a 1 kt explosion, while a 34 plosion in granite.

CONSTRAINTS ON DECOUPLING Decoupling Factors a larger explosion. Analysis of the seismic waves from these events led to the conclusion The reduction of seismic wave amplitudes that the low-frequency decoupling factor is ap- achievable by full decoupling is called the proximately 70. That is, a fully decoupled ex- decoupling factor. On theoretical grounds, the plosion in salt has its low-frequency seismic decoupling factor is expected to be smaller at 5 amplitude reduced by a factor of 70 compared high frequencies than at low frequencies. This to a “tamped,” or “well-coupled,” explosion expectation has been confirmed experimentally. of the same yield in salt. The 1985 Diamond The transition from low-frequency decoupling Beech/Mill Yard experiment compared decou- to high-frequency decoupling occurs over a pled and tamped nuclear explosions in tuff. In range of frequencies rather than abruptly, and this case, the observed decoupling factor was the transition frequency range depends on again 70. The 1959 Cowboy series of tests in yield. For a 1 kt explosion, seismic waves of dome salt used conventional explosives instead about 6 Hz and below can be assumed to be of nuclear explosives. While initial estimates controlled by the full low-frequency decoupling of the decoupling factor from Cowboy ranged factor, whereas seismic waves above 6 Hz will from 100 to 150, it was subsequently deter- exhibit much less decoupling. mined that conventional explosives are significantly less efficient when detonated in a large Low Frequencies cavity than when detonated under tamped conditions. When a correction was made for this Several decoupling experiments have been effect, the Cowboy data yielded an estimate carried out by the United States. Taken to- of the full low-frequency decoupling factor of gether, these experiments permit us to esti- approximately 70, in close agreement with the mate the low-frequency decoupling factor with results obtained in the nuclear experiments. considerable confidence. In the 1966 Salmon/ Sterling experiment, a smaller nuclear explo- Earlier theoretical estimates that the low- sion was detonated in the cavity created by frequency decoupling factor could be as high as 200 or greater were based on several simplifying assumptions. Seismologists are now in ‘Donald B. Larson (cd.), Lawrence Livermore National Lab- agreement that the experimentally determined oratory, Proc&”ngs of the Department of Energy Sponsored Cavity Decoupling Workshop, Pajaro Dunes, CaZiforn;a, July decoupling factor of 70 is appropriate at low fre- 29-31, 1985. quencies. .

102

High Frequencies tances, and the events were not co-located. Data from a better experiment could reduce Roughly speaking, if two explosions excite the uncertainty in the high-frequency decoupling low-frequency seismic waves whose amplitudes factor. differ by a factor F, their high-frequency seismic waves are expected to have amplitudes The evidence for reduced decoupling at high whose ratio is approximately the cube root of frequencies comes from experiments with spher- F. Thus, seismic theory predicts that the de- ical (or hemispherical) cavities. However, theo- coupling factor will be much reduced at high retical calculations show that this conclusion frequencies. The Salmon/Sterling experimental is not altered even when highly elongated data corroborate this prediction. Figure 6-2 shows the decoupling factor as a function of frequency inferred from the Salmon/Sterling experiment, with both explosions scaled to 1 kt. As already discussed, the low-frequency Figure 6-2.—Decoupling Factor as a Function decoupling factor averages about 70. However, of Frequency the experimentally observed decoupling factor begins to drop at about 6 to 8 Hz, and the 103 drop is quite sharp above about 10 Hz. At 20 Hz, the decoupling factor is down to approximately 7. This result can reasonably be extrapolated to estimate decoupling for other yields by scaling the frequency axis in figure 6-2 by the inverse of the cube root of yield. For example, a 5 kt explosion would be expected to 102 be decoupled by a factor of approximately 7 at a frequency of about 12 Hz (20 divided by the cube root of 5). These scaling considerations provide an additional argument in favor of using high-frequency recordings to extend monitoring capabilities down to lower yield levels. 10’ At this time, the exact value of the high- frequency decoupling factor is considered less certain than the low-frequency factor because the instruments recording the Salmon explosion lacked sufficient dynamic range to provide reliable data above 20 Hz. High-frequency data from the Diamond Beech/Mill Yard ex- 10° periment in tuff also show high-frequency 10° 10’ 102

decoupling factors less than 10, consistent with Frequency, Hz the Salmon/Sterling experience in salt. How- ever, interpretation of the Diamond Beech/Mill --- Theoretical _ Average Salmon/Sterling spectral Yard data in terms of decoupling is compli- ratio scaled to 1 kt ● . ● ● 95% Confidence interval cated by the facts that the decoupled event The experimentally observed decoupling factor decreases at was a factor of 100 smaller than the tamped higher frequencies.

event, the decoupling cavity was hemispheri- SOURCE: Modified from J.R. Murphy, Summary of presentation at the ARPA cal, the measurements were made at short dis- Decoupling Conference, Feb. 7, 1979. 103 cavity geometry is considered, that is, it does ated, elongated decoupling cavity may enhance not appear to be possible to increase the high- high-frequency decoupling in certain preferred frequency decoupling factor by constructing directions, but will decrease high-frequency specially shaped, air-filled cavities.6 An evacu- decoupling in other directions.

6Glenn and Rial, op. cit., footnote 2.

DECOUPLING OPPORTUNITIES IN THE SOVIET UNION Cavity construction for low-yield decoupling largest known Soviet salt dome explosions. is possible in salt domes, bedded salt, and dry The yields in table 6-1 were estimated by divid- hard rock. These geologic categories exclude ing the seismically estimated yields of the few areas of the Soviet Union. However, it is largest Soviet salt dome explosions by the generally agreed that salt domes provide the Salmon/Sterling yield ratio (5.3/0.38 = 14). The most suitable host rock for large, stable cavi- use of this ratio is justifiable because the cavity ties. Salt domes are the most suitable because volume created by a tamped explosion (in this of the homogeneity of rock salt in domes, the case 5.3 kt, Salmon) is proportional to yield relative simplicity compared to hard rock of and the largest fully decoupled explosion (in constructing stable cavities explosively or by this case 0.38 kt, Sterling) is proportional to solution mining, and the fact that numerous cavity volume. large cavities already exist in salt domes in the Table 6-1 indicates that Soviet decoupling Soviet Union. Cavities confined to a single, homogeneous salt layer in bedded salt, on the opportunities at 1 kt and above, using existing explosion-generated cavities, are limited other hand, are limited in size by the layer thickness. Assuming that the radius of a cavity to three regions: the North Caspian region, the East Siberian Basin, and a single site in Cen- in bedded salt should not exceed one-half the tral Asia. On the basis of table 6-1, there are layer thickness, decoupling opportunities are probably limited to 1 or 2 kt in bedded salt.

Vast regions of the Soviet Union are under- Table 6-1.— Numbers of Decoupled Explosions of Yield lain by salt deposits. The general distribution Greater Than 1 kt That May Be Possible in Cavities Created by Contained U.S.S.R. Underground of these deposits is indicated by the map in a figure 6-3. However, we probably do not know Explosions, 1961-86 the full extent of Soviet salt deposits. Further- Yield (kt) more, although figure 6-3 indicates those areas Areas of known salt deposits 1234 5 b where salt domes are prevalent, without ac- North Caspian region: cess to detailed subsurface geologic data it is Azgir ...... 2 4 2 0 not possible to rule out the presence of domes Astrakhan c ...... 1 — — — O Orenburg...... 2 — — — O in any area of bedded salt in the Soviet Union. Karachaganak...... — 5 — — O Lake Aralsor...... — 1 — — O Because the construction of large salt dome Ishimbay ...... — — — 1 0 cavities may be difficult to conceal, it is use- East Siberian Basin: ful to estimate the decoupling opportunity pro- NW of Lake Baikal ...... 3 2 — — O Within a few x 100 km of basin . . . 1 — — 1 0 vided by already existing cavities created by Bukhara, Central Asia (explosion used Soviet underground explosions (presumed to extinguish fire in oil well) ...... — — — 1 0

l 3 tamped). Table 6-1 summarizes this informa- Full decoupling in salt: minimum radius (meters) = 25 ● (explosive yield (kt))/ Full decoupling in hard rock” minimum radius (meters) = 20. (explosive yield tion. At each yield level, the table shows the (kt))1/3 %btained from yield of known explosion at site divided by yield ratio for number of existing holes large enough for full Salmon/Sterling = 5.3/0.38 = 14. b(or greater) decoupling. These numbers refer to cavities cMany cavities capable of being used for full decoupling at yields Of about O 5 presumed to have remained open following the kt; some could be connected. 104

...... ,”. ,.

., 105 no opportunities in existing explosion-gen- mation procedure used in constructing table erated cavities above 4 kt. It should be noted 6-1 is one that most seismologists would sup- that the potential decoupled yields estimated port, within an uncertainty of about 50 per- in table 6-1 depend critically on seismic yield cent. This uncertainty translates into 50 per- estimates made for the corresponding cavity- cent uncertainty in the decoupled yields in generating explosions. The seismic yield esti- table 6-1.

PARTIAL DECOUPLING

The size of an explosion that could be decou- Conducting a partially decoupled explosion pled is limited by the maximum size of an air- also has many risks that the potential evader filled cavity that could reasonably be created would have to consider, including the following: and remain stable. Concern has been expressed, however, that even if a nuclear device is too A. Partially decoupling a nuclear explosion large to be fully decoupled, it could perhaps is uncertain. As seen in figure 6-4, the re- be partially decoupled, thus reducing its seis- duction in seismic signal occurs along a steep curve. It would be difficult to pre- mic signal to some extent. It has been further suggested that by partially decoupling large dict from such a steep curve how much explosions, a country might be able to clan- actual decoupling there will be for a given destinely test above the 150 kt threshold level explosion. If partial decoupling is not of the Threshold Test Ban Treaty. As figure achieved, an explosion inside a cavity 6-4 illustrates, however, partial decoupling is not straightforward. Figure 6-4.— Partial Decoupling Effect Figure 6-4 is scaled for the case of a 1 kt ex- — plosion and shows how the size of the seismic signal is affected by partial decoupling. As the cavity size first increases, the seismic signal actually gets larger, reaching a maximum for a cavity about 2 meters in radius. The large seismic signal is produced at first in a small I cavity because less energy goes into melting rock and more energy is transmitted into seis- . mic waves. For the case of a 1 kt explosion, the radius has to exceed about 4 meters before any reduction of the seismic signal occurs, and must exceed about 6 meters to obtain reduction by more than a factor of 2. After that, further reduction occurs rapidly. If the relationship for a 1 kt explosion is extrapolated to larger explosions, the radius (in meters) of I I I I I I I I I ● 1 I I the cavity to begin partial decoupling = 6 * 0.1 1.0 10 100 [size of explosion (kt)]1/3. For a 10 kt explosion, Initial cavity radius (M) for a 1 kt explosion the size of the cavity required to begin partial The effect of partial decoupling scaled for the case of a decoupling would be 6 * 101/3 = 13 meter ra- 1 kt explosion, dius; for 150 kt, the radius would have to ex- SOURCE: Modified from R.W. Terhune, C.M. Snell, and H.C. Rodean, ‘r Enhanced Coupling and Decoupling of Underground Nuclear Explosions, ” LLNL ceed 32 meters. Report UCRL-52806, Sept. 4, 1979, 106

might actually produce seismic signals plosion would have to be detonated in a larger than a well-coupled explosion. cavity near the maximum possible depth B Partial decoupling creates greater pres- to minimize the pressure on the cavity sure on the wall of a cavity thin- full wall. Risks of deformation or collapse in- decoupling. For example, a 20 kt explo- crease with both the yield and the depth sion set off in a cavity suitable for full of the cavity. A risk trade off would be decoupling of 10 kt will result in a dou- involved: the desire to minimize the es- bling of the cavity pressure compared to cape of bomb-produced gases leads the that for the 10 kt shot. Partial decoupling evader to construct a cavity as deep as damages the cavity wall and this makes possible, whereas construction difficul- it more difficult to be confident that no- ties, the time needed for construction, and ble gases and other bomb-produced iso- the risk of cavity deformation or collapse topes will not leak out of the cavity, reach all become increasing problems at greater the surface, and be detected. A 20 kt ex- depths.

MONITORING CAPABILITIES CONSIDERING EVASION The previous two chapters discussed the help. Countering the various evasion scenarios various thresholds for the detection and iden- needs to be approached through a combination tification of seismic events within the Soviet of seismic methods, treaty constraints, and Union. These thresholds, combined with the other monitoring methods that reduce the feasibility of successfully conducting clandes- difficulties and uncertainties of applying seis- tine decoupled nuclear explosions, effectively mic monitoring methods to every conceivable determine what levels of nuclear test restric- test situation. Specifically, the structure of any tions can be monitored with high confidence. treaty or agreement should create a testing environment such that a combination of pro- As discussed in chapters 4 and 5, well-run hibitions, inspections, and seismic methods seismic monitoring networks can detect and will provide the desired high levels of verifica- identify underground nuclear explosions with tion capability. Examples of the type of treaty yields well-below 1 kt if no attempt is made constraints that have been proposed to im- to evade the monitoring network. However, a prove the capability of various monitoring net- country wanting to test clandestinely above works include the following: the allowed threshold would presumably attempt to reduce the size of the seismic signal Limitations on Salvo-Fired Chemical Explo- created by the explosion. For example, a coun- sions: All large salvo-fired chemical explo- try might attempt an evasion scenario where sions above a certain size (depending on the explosion is secretly decoupled in a large the threshold being monitored and the underground cavity and the muffled signals area) would be limited and announced well are then masked by or attributed to a large in advance with inspections/monitoring to chemical explosion that is simultaneously det- be conducted on-site by the monitoring onated under the guise of legitimate industrial side at their discretion. activity. The problem for the monitoring net- Limitations on Ripple-Fired Chemical Ex- work is to demonstrate a capability to distin- plosions: All ripple-fired chemical explo- guish such an evasion attempt from the back- sions above a certain size (depending on ground of frequent earthquakes and legitimate the threshold being monitored and the industrial explosions that occur at low yields. area) would be announced in advance with The monitoring burden placed on the seis- a quota of on-site inspections available to mic network by various evasion scenarios can the monitoring party to be used at their be greatly lessened if seismology gets some discretion. 107

. Limitations to One Inspected and Cali- coupled nuclear explosion with a yield of about brated Test Site: All tests would be con- 1 kt. Consequently, clandestine nuclear explo- ducted within the boundaries of one de- sions above 1 kt would need to be decoupled fined test area in hard rock or below the to evade the monitoring network. Several con- water table. Further, several calibration siderations limit the threshold at which such tests recorded by in-country monitoring clandestine nuclear tests might be attempted. stations would be allowed with yields spanning the threshold yield. Inspections Holes large enough to decouple explosions of the test site would be allowed before above a few kilotons would have to be made enactment of the treaty to ensure that no in salt domes. Almost all of the known salt large cavities suitable for decoupling were dome regions of the U.S.S.R. and regions that present. have any known types and thicknesses of salt deposits are situated in areas of low natural ● On-going Test Site Inspections: A yearly quota of on-site inspections by the moni- seismicity and good seismic transmission. The toring party would be allowed at the des- detection of seismic events from such areas ignated test site. would probably be better than average. ● Joint On-site Inspections of Sites of Possi- Even if an explosion were successfully decou- ble Violations: A yearly quota of on-site pled and the seismic signal muffled down be- inspections would be allowed at sites des- low the 90 percent identification threshold, it ignated by the monitoring party with might still be identified. Decoupled explosions prompt access by a U.S.-Soviet technical produce seismic signals that are very team. explosion-like. Because there is no breaking of ● Country-wide Network Calibration Tests: rock or tectonic release, the signals from decou- Agreement to conduct a number of large pled explosions do not look like earthquakes. chemical explosions, of both salvo and This makes the identification of a detected ripple-fired types, would be allowed to event as a decoupled explosion likely. Even evaluate signal propagation characteris- though the magnitude of the clandestine test tics and detection-identification capabil- is below the identification threshold, the test ities in particular critical areas. would in many cases still be well-detected and If these types of testing constraints can be located. Also, note that the identification negotiated within a treaty, reduced thresholds threshold is for 90 percent identification, that could effectively be monitored. Keeping in is, 90 percent of all events above this magni- mind the various types of networks and nego- tude will be positively identified. There is no tiated treaty constraints that are possible, the sharp boundary between identification and following sections give a sense of the treaty non-identification. Even if the seismic magni- thresholds that could be monitored. tude from a specific event fell somewhat below the identification threshold, there is a good chance that it would be identified. As discussed Monitoring Capability Within The earlier, the identification threshold is largely U.S.S.R. Using No Internal Stations set by the problem of distinguishing large chemical explosions from small decoupled nu- The threshold for detecting and locating seis- clear explosions, so that treaty constraints for mic events within the Soviet Union (90 per- handling large chemical explosions would be cent probability of detection at four or more very helpful. stations), using a seismic network with no internal stations, is at least as low as mb 3.5 (ch. The largest air-filled cavity that could rea- 4). The associated threshold for the identifica- sonably be created and remain stable would tion of 90 percent of all seismic events within fully decouple a nuclear explosion of no more the Soviet Union is at least as low as mb 4.0 than about 10 kt. While large explosions of up

(ch. 5). An mb of 4.0 corresponds to a well- to 20 or more kt could theoretically be partially 108 decoupled to produce a seismic signal below Almost all of the known salt dome regions the cautious identification threshold, such eva- of the U.S.S.R. and regions that have any sion scenarios are considered implausible be- known types and thicknesses of salt deposits cause of the practical considerations of con- are situated in areas of low natural seismicity tainment and predicting the decoupling. In and good seismic transmission. The exceptions fact, evasion scenarios for explosions above 10 include salt deposits in the Caucasus, Tad- kt are not considered credible by most experts. jikistan, and near the Chinese border. An in- This means that the monitoring of the Soviet ternal monitoring network should involve the Union with only an external network can be placement of more seismic stations at closer accomplished down to a threshold of about 10 spacing in those few areas. In addition, those kt. However, for accurate monitoring of a 10 areas are all near the southern border of the kt treaty, all experts agree that it would be U.S.S.R. where the detection and identifica- desirable to have stations within the Soviet tion thresholds either are currently better or Union for accurate yield estimation, plus treaty can be made better than the average identifi- restrictions for handling the identification of cation threshold by monitoring from nearby large chemical explosions in areas where de- countries (i.e., Turkey for Caucasus) and sta- coupling could take place. tions inside the U.S.S.R. Many seismologists feel that the discrimi-

Monitoring Capabilities Within The nation threshold of mb 3.5 is too cautious a prediction for the capability of an internal seis- U.S.S.R. With Internal Stations mic network. This identification threshold is The detection threshold (90 percent probabil- mostly set by the large numbers of chemical ity of detecting a seismic event at four or more explosions that occur below this level. The limitations imposed by identifying chemical explo- stations) is mb 2.() - 2.5 using a seismic network with internal stations (ch. 4). The associ- sions can be approached in two ways: first, ated threshold for the identification of 90 per- limiting them by treaty (limiting their size and cent of all seismic events is at least as low as requiring on-site observers and monitoring) and second, by further developing techniques mb 3.5 (ch. 5) and could be reduced depending on what provisions are negotiated to handle to make use of the expected differences be- chemical explosions. This identification thresh- tween the signals created by distributed ripple- old corresponds to a well-coupled nuclear ex- fired chemical explosions and the concentrated plosion with a yield below 1 kt. point explosions characterizing decoupled nuclear tests. While chemical explosions in the

Seismic stations within the Soviet Union U.S.S.R. of mb 3.0 are likely to be more com- would permit lower thresholds to be monitored mon than those of mb 3.5, the monitoring need by reducing the opportunities for evasion. only be concerned with those chemical explo-

Decoupling is possible for explosions with sions of mb 3.0 and larger that are located in yields below 10 kt. Consequently, the network areas of known or possible salt domes. This of internal stations would be designed primar- excludes very large areas of the Soviet Union. ily to reduce the opportunities for decoupling. A monitoring network with stations internal The most challenging evasion scenario for such to the Soviet Union should concentrate on a network would be the situation where a small areas of poor transmission and areas where (1-5 kt) nuclear explosion is decoupled and the decoupling opportunities would be possible. reduced seismic signal masked by or attributed Through such a strategy and with constraints to a simultaneous detonation of a legitimate on chemical explosions, many predict that the industrial explosion. As noted above, several identification threshold will be closer to mb considerations limit the threshold at which 3.0. This would significantly reduce the size such clandestine nuclear tests might be at- of decoupled explosions that could be clandes- tempted. tinely attempted. 109

All of the considerations so far have not Small differences of opinion concerning mon- made allowances for increased verification ca- itoring capability will always remain because pability afforded by high-frequency recording. parts of the debate are comparable to discus- The recent N.R.D.C. recordings to very high sions of “half-full” versus “half-empty’ glasses frequencies at distances of 200-650 km from of water. Some will review the complex opera- three chemical explosions with yields of 0.01 tion of seismic monitoring and will conclude -0.02 kt are very impressive in this regard. that a country could cheat if any step in the From these data, it appears that explosions process is uncertain. The chain is only as good with yields comparable to a fully decoupled 2.6 as its weakest link. Others will review the com- -3.8 kt explosion (corresponding to magnitude plicated evasion scenarios that have been mb 3.0) in areas of good transmission will pro- postulated and conclude that evasion is too dif- duce large signals with frequencies of 10-20 ficult and uncertain to be credible. Cautious Hz. This is also a frequency band in which the assumptions about seismic monitoring capa- decoupling factor will be small. bility and generous assumptions about the likelihood of successfully conducting clandes- The decoupling reduction that is assumed tine decoupled nuclear explosions can be com- for these evasion scenarios is a factor of 70. bined to produce the conclusion that even with If the monitoring system has even a modest an internal network an explosion of up to 10 capability to record frequencies as high as 10 or 20 kt could be partially decoupled in the -15 Hz, the effectiveness of the decoupling largest hole (capable of fully decoupling a 10 would be greatly reduced. Figure 6-2 indicates kt explosion) to create a seismic signal below the m 3.5 identification threshold. On the a decoupling factor of 30 for frequencies of 1 b -2 Hz and 50 as averaged from 1 to 5 Hz. At other hand, generous assumptions about mon- high frequencies, the decoupling factor will itoring capabilities and favorable assumptions probably be reduced from 70 to below 10. about uncertainty and the role of other intelli- Smaller decoupling factors will result in a lower gence gathering systems can be combined to (better) threshold for the detection and iden- produce the conclusion that even explosions tification of decoupled explosions of a given of a fraction of a kiloton fully decoupled can yield. be effectively monitored with high confidence. Considering all of the arguments, however, a few general statements can still be made con- Decoupling combined with masking remains cerning monitoring capability with an inter- a challenging evasion scenario even with a nal seismic network. high-quality internal network. Opportunities for such evasion, however, would be limited Most experts agree that a high-quality net- by the many practical considerations described work of internal stations combined with strin- above and throughout this report. Attempt- gent treaty constraints, could monitor a thresh- ing evasion by this complicated scenario would old of around 5 kt. Differences of opinion range entail further risk when viewed in conjunction from 1 to 10 kt and are due to judgments about with all types of intelligence gathering, rather the level of constraints that can be negotiated than purely as a problem for seismic discrimi- into the treaty and what levels of motivation nation. Detected seismic events in areas of pos- and risk the Soviet Union would be willing to sible decoupling would be suspicious and pre- take to test clandestinely slightly above the sumably focus attention. On-site inspections threshold. Experts further agree that below could play an important role as opportunities 1-2 kt, monitoring would become much more for cheating could be still further reduced by difficult because additional methods of evasion negotiated agreements requiring prior announce- are possible. Explosions of 1 or 2 kt could be ment and possible on-site inspections of large decoupled not only in salt but also in other me- chemical explosions in areas of potential de- dia such as granite and alluvium. At present, coupling. there is not a consensus that an internal net- 110 work would be capable of positively identify- perience of low-yield monitoring within the So- ing with high confidence all such evasion at- viet Union together with a high level of nego- tempts. If such a capability is possible, it will tiated supplementary measures to limit certain require demonstration through practical ex- evasion opportunities. Chapter 7 Estimating the Yields of Nuclear Explosions CONTENTS

Page Introduction...... 113 Measuring the Size of Seismic Signals...... 113 Determining Explosive Yield From Seismic Magnitude ...... 115 Sources of Uncertainty...... ,.116 Bias Correction for Soviet Nuclear Explosions...... 117 Reducing Uncertainties ...... 120 Random Uncertainty...... 120 Systematic Uncertainty ...... ,122 Yield Estimation Capabilities ...... 122 Soviet Test Practices and Test Ban Compliance ...... 124

Box Box Page 7-A. Calculations of the Six Largest East Kazakhstan Explosions. By Sykes et al., Based on Unclassified Data ...... 124

Figures Figure No. Page 7-l. Computation of P-Wave Magnitude ...... 114 7-2. Explosions in Granite ...... 115 7-3. Yield Data From the Nevada Test Site ...... 116 7-4. Yield Estimate Distribution ...... 116 7-5. Schematic Illustration of Attenuation-Related Magnitude Bias ...... 118 7-6. Comparisons of Explosions in Eurasia With Explosions in North America...... 119 7-7. Uncertainty in Yield Estimates ...... 121

7-8. mb Versus Time for Large Soviet Explosions ...... 125 7-9. U.S.S.R. Nuclear Tests 1966-86 According to Sykes and Davis ...... 126 Chapter 7 Estimating the Yields of Nuclear Explosions

For treaties that limit the testing of nuclear weapons below a specific threshold, the yield of the explosion must also be measured.

INTRODUCTION Once a seismic event has been detected and the United States and the Soviet Union calcu- identified as a nuclear explosion, the next step late explosive yields in the same way. Only that is to estimate the yield of the explosion. This part of the total energy released in a nuclear is particularly important for monitoring trea- explosion that is immediately available (the so- ties that limit the testing of nuclear weapons called prompt energy release) is counted in the below a certain threshold. The process of esti- yield, and there does not appear to be any gen- mating the yields of Soviet explosions involves erally accepted precise definition of what energy three steps: 1) calculate the magnitude of the release time scale is considered “prompt.” Also seismic signal; then, 2) make corrections to ad- a complication arises in determining the yield just for the different geology at each test site; of underground nuclear explosions due to and finally, 3) convert the magnitude into a energy released by interaction of the neutrons yield estimate. from the explosive device with the surrounding ground. Consequently, there might be The final yield measure describes the explo- slight differences in how the United States and sive energy of a nuclear explosion in terms of the Soviet Union measure yields. kilotons, where 1 kiloton (kt) was originally defined as the explosive power equivalent to Yields can be estimated not only through 1,000 tons of TNT. This definition was found seismic means, but also by other methods. The to be imprecise,’ however, and so it was agreed other methods are based on analysis of the nu- in the United States during the Manhattan clear byproducts of the explosion (radiochem- project that the term “kiloton” would refer to ical methods) or measurements of the speed the release of 1012 calories of explosive energy. of the shockwave generated by the explosion 12 in the surrounding rock (hydrodynamic meth- 1 kiloton = 1,000,000,000,000 = 10 calories ods). Neither radiochemical nor hydrodynamic While this convention is also followed in the methods are currently used by the United Soviet Union, it does not necessarily mean that States to measure routinely the yields of So- viet explosions because they require access to *The definition is not precise for two reasons. First, there is some variation in the experimental and theoretical values of the test site during the test, and in the case the explosive energy released by TNT (although the majority of radiochemical methods, may require infor- of values lie in the range from 900 to 1,100 calories per gram). mation about the weapon design that could re- Second, the term “kiloton” could refer to a short kiloton (2x106 pounds), a metric kiloton (2.205x106 pounds), or a long kiloton veal sensitive information concerning the char- (2.24x10 6 pounds). acteristics of the weapon.

MEASURING THE SIZE OF SEISMIC SIGNALS

2 At present, U.S. estimates of Soviet yields waves, Rayleigh waves, and Lg waves. The are generally made using seismic waves re- various magnitudes are averages based on corded at teleseismic distances (distances greater than 2,000 km). Seismic magnitudes 2A description of the various types of seismic waves is pre- can be determined from the amplitudes of P sented in Chapter 3, The Role of Seismology. 113 114

recordings at several stations. The magnitudes Ms = log (A/T) + D, are then converted to explosive yields using where A and T are the amplitude and the formulas developed through past experience. period measured off long-period vertical com- The formulas used are based on testing experi- ponent seismic recordings, again in nanometers ence at the Nevada test site and at the test and seconds, and D is a distance-dependent cor- site operated in the Sahara by France in the rection term for Rayleigh waves. 1960s. The three magnitude measures most often used in yield estimation are: the P-wave The magnitude measure derived from meas- magnitude mb the surface wave magnitude urements of Lg waves is computed from the

Ms, and the Lg-wave magnitude mb(Lg). formula

The mb is computed from measurements of mb (Lg) = 5.0 + log [A(10km)/110], P-wave recordings by the use of the formula where A( 10 km) is the maximum sustained am- m plitude of L on short-period vertical records b = log (A/T) + B. g in nanometers extrapolated backwards to a dis- As illustrated in figure 7-1, A is the largest tance 10 km from the source by dividing by P-wave amplitude in nanometers (0.000000001 the geometrical spreading factor of d-5/6, where meters) measured peak-to-peak from a seismic d is the source-to-receiver distance, and by the short-period recording during the first few se- estimated attenuation along the path. The em- conds of the P wave and correcting it for the pirical mb(Lg) v. log Yield (Y) relationship also instrument magnification, T is the duration of includes a small second-order term, giving one cycle of the wave in seconds near the point 2 on the record where the amplitude was meas- mb(Lg) = 3.943 + 1.124 log Y -0.0829 (log Y) ured (for P waves the period is typically 0.5 for explosions in water saturated rocks such to 1.5 seconds), and B is a distance-dependent as those at the Nevada Test Site. correction term that compensates for the change of P-wave amplitudes with distance. The mb magnitude is routinely used at teleseismic distances for yield estimation because The surface wave magnitude is determined P waves are detectable at large distances, even by measuring the Rayleigh-wave amplitude for small seismic events. This measure can near the point where the dominant period of almost always be obtained for any seismic the wave is nearest to 20 seconds on long- event that is detected. The measurement of period vertical component records. The for- Ms requires a larger event, because Rayleigh mula used is waves are small for nuclear explosions. For ex-

plosions below 50 kt, Ms may be missed al-

together at teleseismic distances. The Lg am- Figure 7-1.–Computation of P-Wave Magnitude plitude is similarly weak for small explosions. Consequently, it maybe important for seismic stations to be close to the explosion if surface

waves and Lg waves are to be used for yield esti- A mation of explosions less than 50 kt. This is one of the reasons why seismic stations within the territories of the treaty participants are desirable. The distance correction factors can be quite variable regionally, and hence, some of the seconds magnitude-yield relationships will need to be o 1 2 3 4 5 adjusted for different regions. Measurement made on P waves to obtain the magnitude of In addition to the conventional surface wave a seismic event. The peak-to-peak amplitude (A) in the first few seconds of the P wave is corrected for the instrument magnitude Ms, a new measure of source magnification at the dominant period T. strength for surface waves is coming into wide 715

use. Called seismic moment (MO), it is an esti- explosion by geological processes. The release mate of the strength of a compressional (ex- of built-up stress by the explosion creates a plosion-like) force at the explosion site. Seis- surface wave pattern similar to that observed mic moment gives a direct description of the for earthquakes, which is seen superimposed force system, acting in the Earth, that would on the signals of the explosion. Characteris- make seismic waves of the size and shape ac- tics of an earthquake, such as Love waves and tually recorded. The advantage of using seis- reversed polarities in the Rayleigh waves, are mic moment is that the computation can cor- often observed from a nuclear explosion, in- rect for the estimated effects of contamination dicating release of pm-existing stress. If not of the seismic signals due to earthquake-like removed, this release of natural stress by the motion triggered by the explosion. This is use- explosion, called tectonic release, can distort ful because nuclear explosions often release yield estimates obtained from conventional

stress that has been built up in the area of the Ms.

DETERMINING EXPLOSIVE YIELD FROM SEISMIC MAGNITUDE

Once the seismic magnitude measurements ured seismic magnitude can be described using have been made, the next step is to relate the an equation of the general form magnitude measurements to the yield of the explosion. Because we know the actual yields M = A + B log Y + Bias Correction only of U.S. tests and some French nuclear explosions (the Soviets have announced yields where M is a magnitude measure (or moment) from surface waves, body waves, or L waves, only for a few of their tests), our knowledge g is based on data other than Soviet data. The A and B are constants that depend on which actual data used to derive this relationship are magnitude measure is used, and Y is the yield shown in figure 7-2. The relationship between in kilotons. The specific constants used by the the yield of a nuclear explosion and the meas- United States for these calculations are classified. The “Bias Correction” term is an ad- Figure 7-2.—Explosions in Granite justment made to correct for the differences in how efficiently seismic waves travel from the various test sites. This correction is par- I ticularly important for mb, because short- I period body waves are strongly affected by the physical state (especially temperature) of the medium through which they travel. The empirical magnitude-yield relationships

for mb that are used to estimate yields at in- accessible test sites in the U.S.S.R. and elsewhere have been revised several times during the last two decades. These revisions were improvements in yield formulas and computa- tional procedures to correct for such problems 4.0 as difficulties in merging magnitude sets from

Yield (kt) different station configurations and instruments, clipping (limiting the maximum record- Data for explosions in granite from which the magnitude v. yield equation is derived. able amplitudes) of large signals by the record- SOURCE: Modified from Air Force Technical Application Center. ing systems, and not correctly accounting for 116

differences in the geology at different test sites. linear relationship between mb and log(yield). The early magnitude-yield formulas were based After the factors listed above were properly on the simplifying assumption that all nuclear considered, however, it became obvious that explosions in granite at any site follow a simple bias corrections for each test site were needed.

SOURCES OF UNCERTAINTY Under the ideal conditions of a perfectly uni- One reason for this variation is the small- form and symmetrical Earth, it would be pos- scale geologic contrasts in the Earth that cause sible to estimate yields of nuclear explosions focusing and scattering of seismic waves. Fo- at any site from measurements at a single cusing effects near the recording seismometers seismic station. In practice, however, seismic can create differences in estimated magnitudes magnitudes and magnitude-yield plots show even when the stations are closely spaced. scatter. Using data from the International Focusing effects near the explosion can cause Seismological Centre3 as an example, individ- broad regional variations of seismic amplitudes ual mb measurements typically have a stand- so that seismic observatories over whole con- ard deviation of 0.3 to 0.4 magnitude units be- tinents may observe higher or lower average fore station corrections are applied. When amplitudes than the global average. Fortu- station corrections are applied, the standard nately, the uncertainty introduced by focusing deviation is reduced to 0.1 to 0.15 units. Fig- effects can be reduced by averaging measure- ure 7-3 illustrates typical scatter in a magni- ments from numerous stations if the stations tude-yield plot. are well distributed around the test sites in both distance and direction. ‘The International Seismological Centre is an organization In addition to the scatter due to focusing, based in England that gathers data for the research commu- geological structures under individual stations nity from thousands of seismic stations operated all over the globe. may amplify seismic waves. Such effects may

Figure 7-3.— Yield Data From the Nevada Test Site Figure 7-4. —Yield Estimate Distribution 7.0 95% of total area —

6.0 o 75 150 225 300 375

Yield estimate. in kilotons

5.0 50 % of

4.0

Yield (kt)

M b versus yield for explosions at the Nevada Test Site. The Yield estimate, in kilotons scatter is characteristic of yield measurements when only P-wave magnitudes are used. Probability of yield estimates for a 150 kt explosion meas- SOURCE: Modified from Air Force Technical Applications Center. ured with a factor-of-2 uncertainty. 117 be corrected for by applying statistically de- is a typical empirical magnitude-yield curve rived station corrections that compensate for obtained from actual data at the Nevada test any such local effects. site that shows the measurements do not follow a single line but scatter around it because After averaging many measurements and applying appropriate corrections, estimates of of measurement errors and variations in rocks surrounding the explosions. the yield of a nuclear explosion are expected to be distributed about the “true” value in the manner indicated in figure 7-4. The horizontal Some of the uncertainty described above is axis in this figure is the yield estimate while due to variations in how well explosions are the vertical axis is the probability that the esti- coupled to the surrounding rock. Also, explo- mate is correct. The area under the curve be- sion depth can influence the amplitudes of the tween 2 yield values represents the probabil- seismic waves emitted, as can variations in the ity that the actual yield is in this interval (the physical properties of the Earth. For inacces- percentage chance is 100 times the probabil- sible test sites, these effects result in increased ity and the total area under the curve is 1, giv- uncertainty in estimating yields. However, if ing a 100 percent chance that some value of data were exchanged and calibration shots per- magnitude will be measured). This figure shows formed, corrections could be made that would that it is most likely that the central yield value greatly reduce the uncertainty. Nevertheless, (150 kt in this case) will be close to the actual there will always be some uncertainty in esti- value and that outcomes become increasingly mates of the yields of Soviet explosions, as in less likely the larger the difference between the estimates of any physical quantity. This is not estimated value and the central value (see chap- unique to seismology. Some uncertainty will ter 2 for a more detailed discussion of uncer- exist no matter what type of measurement sys- tainty and what it represents). The yield dis- tem is used. Such uncertainty should not nec- tribution is asymmetric due to the normal essarily be considered to represent opportuni- distribution of mb and the logarithmic rela- ties for cheating. Chapter 2 discusses the tionship between the yield of the explosion and meaning of the various uncertainties and their the measured seismic magnitude. Figure 7-3 implications for cheating.

BIAS CORRECTION FOR SOVIET NUCLEAR EXPLOSIONS In estimating the yields of Soviet explosions, der Nevada and many other tectonically ac- a major concern is how well the magnitude- tive regions. Regions of high temperatures yield formula for U.S. tests can be applied to change the elastic and absorptive properties Soviet test sites. Geophysical research has of the rocks, causing a large loss in the ampli- shown that seismic P waves traveling through tudes of seismic waves traveling through them. the Earth’s mantle under the main U.S. test Similar phenomena are thought to occur un- site in Nevada (and many other areas of the der the French test sites in Algeria and the world as well) are severely attenuated when Pacific, though not under either the Soviet test compared to most other continental areas, sites in Kazakh and Novaya Zemlya or the U.S. especially those with no history of recent plate test sites in Mississippi and Amchitka. If the tectonic movements. If not corrected for, this P-wave magnitudes observed from U.S. tests attenuation will cause a sizable systematic er- in Nevada are used as a basis for estimating ror in estimates of the yield of Soviet ex- yields, most Soviet explosions which have been plosions. exploded in areas where the upper mantle is cool and there is little attenuation of P waves The apparent reason for this attenuation is will appear considerably larger than they ac- the high temperature in the upper mantle un- tually are. ———

118

The evidence for such attenuation effects In addition, there is a large amount of inde- comes from many studies, including: pendent geophysical evidence supporting the notion of anomalously high temperatures un- ● comparisons of P-wave amplitudes ob- der most of the western United States. This served at the Nevada test site with obser- evidence includes: vations made at other sites in areas un- ● measurements of anomalously high heat derlain by colder mantle; flow, ● studies of short period S-wave amplitudes, ● measurements of electrical conductivity, which are very sensitive measures of man- and tle temperature variations; ● ● the low velocity P waves (Pn) and the ab- studies of the frequency content of both sence of S waves (S ) that propagate just P and S waves, i.e., the relative loss of high n under the Earth’s crust. frequency energy in waves traveling through the upper mantle in both direc- These “symptoms” of high attenuation have tions under Nevada; and been observed in many other areas of the world ● studies of P- and S-wave velocities, which and are recognized as such by most geophysi- are also influenced by temperature. cists. The sketch in figure 7-5 illustrates how

Figure 7-5.—Schematic Illustration of Attenuation. Related Magnitude Bias

Signal from Signal from Soviet test U.S. test

Soviet Union’s Us. test site test site

Seismic body waves crossing parts of the upper mantle with high temperatures become anonymously reduced in amplitude. Seismic signals from the Soviet Union’s test site appear much larger than signals from an identical explosion conducted at the U.S. test site. SOURCE: Modified from Defense Advanced Research Projects Agency. 119 this attenuation is created in the Earth and Figure 7-6.—Comparisons of Explosions in Eurasia affects estimates of the size of the wave source. With Explosions in North America Seismic body waves crossing the hatched high 7.0 attenuation zones in the upper mantle are reduced in amplitude and high frequency components of wave motion relative to waves that bypass such zones. 6.0 Magnitudes derived from Rayleigh waves and Lg waves are less influenced by temperature variations in the mantle because they travel along the surface and largely bypass the 5.0 high attenuation zones in the upper mantle. A plot of P-wave magnitudes (rob) against surface wave magnitudes (Ms) should, therefore, show the attenuation of P waves relative to I I 1 I 3.0 4.0 5.0 6.0 surface waves. By plotting the Ms - mb ratio M of explosions for different test areas, the at- s tenuation indifferent regions can be compared. Figure 7-6 shows the results of an early study Explosions ● Eurasia o North America that compared the Ms - mb for explosions in Explosions with the same M value have lower m values in Eurasia with the M - m for explosions in S b s b North American than in Eurasia. This led to early specula- North America. It can be seen that the two tion about the existence of high attenuation zones in the groups are offset, with explosions of the same upper mantle.

Ms value having lower mb values in North SOURCE: Modified from P.D. Marshall and P.W. Basham, Geophysical Journal America than in Eurasia. Results like this led of the Royal Astronomical Society, 1972, vol. 28, pp. 431-458. to early speculation about the existence of high attenuation zones in the upper mantle. reduced by using seismic moment instead of

In general, if the P-wave magnitudes are surface wave magnitude (Ms) for yield esti- plotted against the Rayleigh or Lg magni- mation. tudes for the Nevada and Soviet test sites, the 2 sets of data are offset by about 0.3 to 0.4 Various government-supported scientific magnitude units (or an amplitude factor of panels of seismologists, after considering the about 2 for P waves).4 The most likely expla- totality of the geophysical evidence, have nation for this offset is the reduction of P-wave repeatedly recommended during the last dec- magnitudes due to attenuation at the Nevada ade that U.S. yield estimates of Soviet explosions be reduced by subtracting a larger “Bias test site. Such data constitute additional sup- Correction” term from the magnitudes to ac- port for the idea of an attenuation bias for P count for the attenuation effect on mb. As a waves. Offsets can also be brought about by result, the bias correction has been increased other factors such as contamination of the M S on several occasions over the last decade as measurement by tectonic release. However, new scientific evidence indicated that such such contamination can be detected by the strong Love waves the release generates, and changes are appropriate. The size of the bias correction was determined simply by averaging the correction in- “See for example P. Il. Marshall and P. W. Basham, “Dis- crimination Between Earthquakes and Underground Explosions ferred from a number of independent or semi- Employing an Improved M. Scale, ” GeophysicaZJoumal of the independent estimates of the attenuation ef- libyzdAstronomicaJ Soa”ety, vol. 28, pp. 431-458,1972, and Otto fect made by different researchers. Most evi- W. Nuttli, “L~ Magnitudes of Selected East Kazakhstan Un- derground Explosions, ” BulZetin of the Seismological Society dence for an attenuation “bias” has been in- of Ame~”ca, vol. 76, No, 5, pp. 1241-1252, October 1986. direct thus far, although the evidence from

76-584 0 - 88 : QL 3 - 5 120 global seismic studies and seismological experi- cil. Data from these stations will help improve ence gives strong support to the idea. More estimates of the bias correction and assess the direct measurements of this bias may soon be- efficiency of seismic wave propagation at high come available. The United States and Soviet frequencies to regional distances. Union have recently agreed on experiments to calibrate seismic yield estimation methods The bias correction is currently used as a sim- through measurements at each others test ple, yield-independent adjustment to the in- sites. Explosions will be measured with seis- tercept, A, in the rob-log Y curve. The value mic methods and the yields confirmed inde- currently used by the U.S. Government is in- pendently by hydrodynamic methods. In addi- tended to be the most appropriate value for tion, several seismic stations have been set up yields near the 150 kt threshold of the 1974 recently in the Soviet Union near the Kazakh Threshold Test Ban Treaty. A different bias test site by a group of U.S. scientists supported may be appropriate for yields that are either through the Natural Resources Defense Coun- much larger or much smaller.

REDUCING UNCERTAINTIES The estimated yield of an underground of nu- Random Uncertainty clear explosion, like any quantity derived from measurements, has some error associated with Different methods of yield estimation have it. The error comes from a variety of sources. different accuracies and uncertainties. At the Some of the error is considered to be random Nevada test site, the most precise method uses in that it varies unpredictably from one meas- P-wave magnitudes (rob). Less precise meth- urement to another. Other errors are not ran- ods use Lg waves (mb(Lg)), surface waves (MS), dom but are systematic. Systematic errors are and seismic moment ( M s) At the Nevada test those that always act in the same way, for ex- site, yields estimated from mb alone have a ample, the bias between test sites. If system- random uncertainty factor of 1.45 at the 95 atic errors are understood, corrections can be percent confidence level, whereas those from made to reduce or eliminate the error. mb(Lg) have an uncertainty factor of 1.74, and those from Mo have an uncertainty factor of The distinction between random and system- 2.13. atic errors, however, has no clear boundary. If everything about the Earth and seismic Recently, the Defense Advanced Research waves were known, almost all errors in seis- Projects Agency (DARPA) has been able to mology would be systematic. In general, ran- reduce the random uncertainty in seismic yield dom errors usually turn out to be systematic estimates at the Nevada test site by combin- errors once the reason for the error is under- ing measurements made by the three differ- stood. However, if the systematic errors are ent methods. Scientists have shown, using not understood, or if there are lots of system- data from U.S. explosions at the Nevada test atic errors all operating indifferent ways, then site, that the random errors of the three types the systematic errors are often approximated of magnitude measures for a given event can as random error. In such cases, random uncer- be considered statistically independent. Con- tainties are inflated to encompass the unex- sequently, an improvement in the accuracy of plained systematic uncertainties.5 yield estimates can be achieved by combining several methods to produce a “unified” mag- ‘As discussed in Chapter 2, random errors do not provide opportunities for cheating. However, if systematic errors are found nitude measure. By forming a weighted aver- to be 1) of sufficient size, 2) usable for an advantage by one side, age of the three magnitudes, a “unified seis- and 3) unrecognized as being systematic by the other side, then mic magnitude” with an uncertainty factor of such errors can be exploited under some situations. A treaty 1.33 (figure 7-7) has been derived. Most seis- should, therefore, contain provisions to reduce the uncertainty of yield estimates and counter evasion opportunities. mologists believe that if this method were now 121

Figure 7-7.—Uncertainty in Yield Estimates

6.2 95% 1.74 Factor 1.33

5.4

100 200 300400 / / Yield (kt) 2.13

1 I 100 200300400

Yield (kt) Uncertainty in yield estimates can be greatly reduced through the use of a unified seismic yield estimate. On the left are three plots of mb, mb(Lg), and MO versus yield at the Nevada Test Site. On the right is a similar plot of the unified seismic yield estimate versus the actual yields. The 95 percent uncertainty factors are shown to the right of each plot. SOURCE: Modified from Defense Advanced Research Projects Agency.

applied to estimating yields at the Soviet test Nevada test site (a factor of 1.3) is the system- site in Eastern Kazakh, the uncertainty would atic uncertainty or bias correction. As we will be reduced to a factor of 1.6- 1.5 for explosions see later, however, this systematic uncertainty around 150 kilotons.6 What limits the uncer- can be reduced through calibration shots. tainty from being reduced to the level of the The expected precision given above are only for explosions where all waves are used for the estimation. For smaller explosions, the re- ‘Consider, as an example, the situation where there are 3 sta- gional Rayleigh and L waves are not always tistically independent methods of calculating the yield of an explosion, all of which (for the sake of this example) have a fac- strong enough to travel the long distances re- tor of 2 uncertainty in a log normal distribution: quired to reach seismic stations outside the So- # of Methods Resulting Uncertainty viet Union. Consequently, for monitoring low- 1 2.0 yield explosions, stations within the Soviet 2 1.6 3 1.5 Union may be necessary to obtain the improved By combining methods, the resulting uncertainty can be re- accuracy of the “unified seismic yield” esti- duced. This methodology, however, can only reduce the random mation method. The relationship between mag- uncertainty. Systematic uncertainty such as differences between the test sites will remain and limit the extent to which the un- nitude and yield for the stations within the So- certainty can be reduced unless calibration is performed. viet Union will also have to be established. —

122

As noted above, the formulas derived from Systematic Uncertainty the Nevada test site data that describe the relationships between yield and m (L ) and mo- The yield estimation precision described b g above for teleseismic data are limited because ment MO are not directly applicable to the So- viet test site in Kazakh without some as yet of systematic uncertainties. As discussed unknown adjustments. The values of these ad- above, the systematic uncertainty can be re- justments can be determined if stations are duced by calibrating the test site. Calibrating placed within the Soviet Union and the Soviet a test site involves exploding devices whose test sites are calibrated. Test site calibrations yields are either known or accurately deter- suitable for lower yields could only take place mined by independent means, and then meas- after an internal network is installed. If the uring the magnitudes at a large number of mb(Lg) and MO v. yield curves are suitably monitoring stations. By doing so, the yields calibrated, the absolute yields of explosions of other events can be determined by compar- at Kazakh should be measurable with the “uni- ing the amplitudes of the seismic waves at com- fied seismic method” just as accurately as at mon seismic recording stations with those the Nevada Test Site. originating from the events with known yields. The above analysis applies only to explosions This approach reduces the systematic uncer- at known test sites observed by a large set of tainties caused by having to estimate the vary- well-distributed seismic stations for which the ing properties of the rocks surrounding the appropriate station corrections and bias cor- explosion and any focusing effects near the ex- rection have been determined. The accuracy plosion sites. As long as these factors remain with which the yields of “off-site” nuclear tests approximately unchanged within a geologi- could be estimated would be less than that cally uniform area, the calibration improves stated above. Therefore, to maintain high ac- the estimation of yields. curacy in yield estimation, nuclear testing should be prohibited outside specified, cali- The sizes and numbers of geophysically dis- brated test sites. tinct subdivisions in any test site depend on the geological structures of the area. A spe- Most yield estimation research has concen- cific calibration maybe valid only for a limited trated on yields around 150 kt, so the accuracy area around the shot if, at larger distances, the that could be achieved by seismic methods at rock properties and focusing effects change. lower yields is not yet well known. In any fu- The distances over which the relevant condi- ture low threshold test ban treaty, it might be tions change vary, depending on the local ge- expected that the initial uncertainties in yield ology. Testing areas that are large or contain estimation for explosions below 10 kt would varying geology would obviously need more be large. These uncertainties would then be re- calibration shots than areas that are geologi- duced as more data were gathered, as our cally uniform. If calibration were performed knowledge of wave propagation properties for at the Soviet test site, the expected seismic various paths in the monitored regions was re- yield estimation capability would be compara- fined, and as calibration information was ob- ble to the existing seismic capability at the Ne- tained. vada Test Site.

YIELD ESTIMATION CAPABILITIES

In considering the capability of all methods is radiochemical methods. Radiochemical meth- of yield estimation, it must be kept in mind ods of yield estimation have an uncertainty of that it is never possible to determine a yield about 10 percent (a factor of 1.1). Also, experi- without some uncertainty. The standard against mental devices often detonate with yields that which yield estimation methods are measured are slightly different from what was predicted. 123

This uncertainty in predicted yield was recog- Test Site. In fact, Soviet seismologists nized during the negotiations of the Thresh- have told U.S. seismologists that they are old Test Ban Treaty and provisions were estab- able to estimate yields seismically at their lished for unintended breaches (see ch. 2). own test site with only a factor of 1.2 un- The yields of Soviet underground explosions certainty. ● Small Explosions: For small explosions can be seismically estimated with a much better capability than the factor-of-2 uncertainty (below 50-kt), the regional seismic waves 7 may not always be strong enough to travel that is commonly reported. New seismic methods have greatly improved yield estimation ca- long distances to seismic stations outside pabilities. Further improvements would occur the Soviet Union. Consequently, seismic if the test sites were calibrated and, for small stations within the Soviet Union may be necessary (in addition to calibration) to ob- tests, if stations were present within the So- tain the 1.3 factor of uncertainty from viet Union during the calibration. The capa- combined seismic methods for explosion bilities depending on these variables can be summarized as follows: with yields below 50 kt. At yields below 10 kt small variations of the physical envi- ● Without Calibration: For large explosions ronment may produce greater uncertainty. (above 50 kilotons) seismic yield estima- Therefore, at yields below 10 kt, the un- tion could be improved with the additional certainty may be inherently greater. use of the other methods including: sur- A 1.3 factor of uncertainty (for yields above face waves, Lg waves, and seismic moment. Through such a combined method, 50 kt) is the claimed capability of the hydrodynamic yield estimation method using CORR- it is estimated that without calibration So- 8 TEX data that has been proposed as an alter- viet yields can be seismically measured native means for improving yield estimation. with present resources to a factor of 1.6 Consequently, hydrodynamic yield estimation to 1.5 uncertainty. will not provide a significantly superior yield ● With Calibration: Further reductions in the uncertainty of yield estimates can be estimation capability over what could be ob- accomplished if the Soviet test site were tained through well-calibrated seismic means (also a 1.3 factor of uncertainty). Hydrody- calibrated. At a defined, well-calibrated namic yield estimation is, however, one of the Soviet test site, it is estimated that yields methods that could be used to provide inde- could be seismically measured with the pendent estimates of the yields of calibration same factor of 1.3 uncertainty that is found for seismic estimates at the Nevada shots to improve seismic methods. Once a test site was calibrated using hydrodynamic methods, there would be no need to continue the ‘See, for example, VerifyingIVuclear TestingLim.itations: Pos- use of those intrusive methods. sible U.S.-Soviet Cooperation (Washington, DC: U.S. Depart- ment of State, Bureau of Public Affairs, Special Report No. 152, Aug. 14, 1986) ‘See appendix, Hydrodynamic Methods of Yield Estimation. 124

SOVIET TEST PRACTICES AND TEST BAN COMPLIANCE

Specific concern over compliance with test 1. The mb of several Soviet tests at their ban treaties has been heightened with findings Shagan River (E. Kazakhstan) test site are

by the Reagan Administration that: significantly l arger than the mb for U.S. tests with yields of 150 kt. Soviet nuclear testing activities for a number of tests constitute a likely violation of the 2. The pattern of Soviet testing indicates legal obligations under the Threshold Test that the yields of Soviet tests increased Ban Treaty.9 after the first 2 years of the treaty. Such findings are presumably based on net The validity of the first argument is depen- assessments of all sources of data. In measur- dent on how the Soviet yields are calculated. ing yields near the 150 kt limit of the Thresh- Because of the uncertainty in measuring the old Test Ban Treaty, however, seismic evidence yields of Soviet tests using only mb and be- is considered the most reliable basis for esti- cause of differences in opinion as to what the mating the yields of Soviet underground nu- correct bias value for Soviet tests should be, 10 clear explosions. It is, therefore, the seismic there is disagreement as to whether the mb evidence that has received particular attention. values of the largest Soviet tests do, in fact, represent violations of the 150 kt limit of the Concern about whether the Soviet Union is Threshold Test Ban Treaty. For example, when actually restricting its testing to a maximum calculations such as those in table 7-1 are made yield of 150 kt is motivated by two arguments: using both mb and MS measurements and a bias correction of 0.35, they indicate that the few remaining yields estimated as above 150 “’The President’s Unclassified Report on Soviet Noncompli- kt are well within the expected random scat- ance with Arms Control Agreements, ” transmitted to the Con- ter, and do not support claims of a violation. gress Mar. 10, 1987. Conclusion of the Defense Intelligence Agency Review Psnel The second argument is dependent on as- as stated in a letter from Roger E. Batzel, Director of Lawrence Livermore National Laboratory to Senator CMbome Pen on sumptions about probable Soviet behavior. Feb. 13, 1987. Two years after the signing of the Threshold

Box 7-A.—Calculations of the Six Largest East Kazakhstan Explosions. By Sykes et al., Based on Unclassified Data

6 Largest East Kazakhstan Explosions 1976-1986 Yield from Yield averaged from

Date mb only mb & MS 23 June 79...... 152 149 14 Sept 80 ...... 150 150* 27 Dec 81 ...... 176 161 4 July 82 ...... , ...... 158 158* 14 July 84 ...... 140 140* 27 Oct 84...... 140 140* Average: 152.7 (± 13.4 kt) Average: 149.7 (± 8.8 kt)

(*based on mb only; no Ms determined) All estimated yields are well within the uncertainty expected for observance of the150 kt threshold limit. SOURCE: Calculations of the six largest East Kazakhstan explosions made by Sykes et al., based on unclassified data. Body wave measurements from International Seismological Centre Bulletins and United States Geological Survey Reports. Station corrections determined to be 0.02 to 0.04 from mean. Surface wave calculations made by Sykes et al. Calibration corrected for bias using a value of 0.35 to make body wave data consistent with surface wave data. 125

Test Ban Treaty, the size of the largest Soviet anon-technical consideration, it can be argued explosions at their eastern Kazakh test site that if the Soviets had tested above the limit increased markedly (see figure 7-8).11 This in- of the Threshold Test Ban Treaty at the Kazakh crease has been interpreted by some to infer test site, they would never have offered to al- that the Soviets have been violating the 150 low the United States to calibrate their test kt threshold limit in the later tests. The argu- site using CORRTEX and Soviet test explo- ment assumes that the Soviets were testing sions. The calibration will reduce the uncer- up to the limit for the first 2 years and, there- tainty of yield estimates, a reduction that ap- fore, by inference, have been testing above the plies to past as well as future explosions and limit in violation of the treaty ever since 1978. hence can provide more accurate evidence con- Alternate interpretations for this apparent cerning past compliance. yield increase have been offered. It has been Because of the statistical nature of all yield pointed out that a similar pattern of testing occurred at Kazakh for the 2 years prior to the estimates, the question of compliance can be addressed best not by looking at individual treaty. It has also been speculated that this increase in yields may reflect a Soviet decision tests but rather by examining the entire pat- to move their high yield testing from the Novaya tern of Soviet testing. It is particularly useful Zemlya test site to the Kazah test site.12 As to compare the testing programs of the United States and the Soviet Union. It can be seen from figure 7-9 that if a bias value lower than ~There w= &IO speculation that this increase was coincident with a change in the U.S. official method of yield estimation. 0.35 is used, there appears to have been about For example, Jack Anderson, “Can’t Tell If Russia Cheats On 10 (out of over 200) Soviet tests since the sign- Test Ban,” The Waslu”ngton Post, Aug. 10, 1982, p. C15. ing of the Threshold Test Ban Treaty in 1974 ‘*See for example, “Nuclear Test Yields” (Letter to the Edi- tor), J. F. Evemden and L. R. Sykes, Science, vol. 223, Feb. 17, with yield central values above the 150 kt 1984, p. 642. threshold limit. When the same method of yield

Figure 7-8.— mb Versus Time for Large Soviet Explosions

*o ● . E

5.0 ‘

4.4 “ ‘

I I I I 1 I 1965 1966 1967 1968 1969 1970 1971 1972 1973 1974 1975 1976 1977 1978 1979 1980 198’1 1982 1983

. Kazakhstan O Novaya Zemlya

The m b versus time for all large Novaya Zemlya and Kazakhstan explosions. It can be seen that a large increase of the maximum yield for explosions at the Eastern Kzazkh test site occurred about 2 years after the Threshold Test Ban Treaty was signed.

SOURCE: T.C. Bache, S.R. Bratt, and L.B. Bache, “P Wave Attenuation mb Bias, and The Threshold Test Ban Treaty,” SAIC-86-1647, submitted to AFGL, March 1966, p. 5. 126

Figure 7-9.—U.S.S.R. Nuclear Tests 1966-86 According to Sykes and Davis 5,000

2,000

1,000

500

100

20 1966 1967 1968 1969 1970 1971 1972 1973 1974 1975 1976 1977 1978 1979 1980 1981 1982 1983 1984 1985

● Kazakhstan O Novaya Zemlya These yields were calculated by combining P-wave and surface-wave magnitudes and using a bias correction of 0,35. Bars denote the estimated standard deviations of the estimates. The few tests that do appear to have exceeded 150 kt are well within the expected scatter. If a lower bias correction is applied and only P-wave determinations are used, then slightly higher yield estimates will result and additional central values will be greater than 150 kt.

SOURCE: Modified from L.R. Sykes and D.M. Davis, Scientific America, vol. 258, No. 1, January 1987, pp. 29-37.

estimation is applied to U.S. tests, approxi- All of the estimates of Soviet tests are within mately the same number of U.S. tests also ap- the 90 percent confidence level that one would pear to be above the 150 kt threshold limit. expect if the yields were 150 kt or less. Exten- This, however, does not mean that one or the sive statistical studies have examined the dis- other or both countries have cheated; nor does tribution of estimated yields of explosions at it defacto mean that seismology is an inade- Soviet test sites. These studies have concluded quate method of yield estimation. It is inher- that the Soviets are observing a yield limit. ent in any method of measurement that if both The best estimate of that yield limit is that countries are testing up to the yield limit, the it is consistent with compliance with the 150 estimated yields of some tests will have cen- kt limit of the Threshold Test Ban Treaty .13 tral values above the yield limit. Because of the uncertainty of measurements using any method, it is expected that about half the So- I$such gtatistic~ studies have been carried out extensively viet tests at 150 kt would be measured as by Lawrence Livermore National Laboratory, The conclusion of these studies was reported in open testimony before the Sen- slightly above 150 kt and the other half would ate Armed Services Committee on Feb. 26, 1987 by Dr. Milo be measured as slightly below 150 kt. Nordyke, Leader of the Treaty Verification Program. Appendix Appendix Hydrodynamic Methods of Yield Estimation

Hydrodynamic methods could be used to complement seismic methods of yield estimation

Introduction Table A-1.—Characteristic Times in the Evolution of a Shock Wave Caused by an Underground The yield of an underground nuclear explosion Nuclear Explosiona may be estimated using so-called hydrodynamic Event in the evolution Time (µs) for a Time (µs) for a methods. These methods make use of the fact that of the shock wave 1 kt explosion 150 kt explosion Beginning of the Self-Similar larger explosions create shock waves that expand b Strong Shock Interval – -10 faster than the shock waves created by smaller ex- Beginning of the Transition plosions. Three steps are involved in making a yield Interval c...... -20 -80 estimate. First, the properties of the geologic me- Beginning of the Plastic dia at the test site that may affect the expansion Wave Intervald...... -2,000 -10,000 aFOr an idealized Spherically-syrnrnelrlc exploslon In granite of the shock wave are determined. Second, the ex- bf+ere fhe shock wave is assumed 10 become self-similar when If reaches a radws of 1 m (see pansion of the shockwave caused by the explosion text) No time is gwen for a 1 kt explosion because the shock wave caused by such an exploslon fyplcally weakens before it has time to become self -s!mllar of interest is measured during the hydrodynamic coeflned as the time when the denslfy just behind the shock front falls to 80 percent Of Its maXU’rWM phase, when the ambient medium behaves like a limiting value d~fined as the time when the speed of the shock wave falls to 120 perCent Of the plaStlC Wave fluid. Finally, the yield of the explosion is estimated speed In reality, granite undergoes a phase transition sllghlly before the shock speed reaches by fitting a model of the motion of the shock front this value, a complication that has been neglected In this Illustration to measurements of the motion. SOURCE’F K, Lamb, “Hydrodynamic Methodsof Yield Estlmatlon ‘‘ Report Iorthe U S Congress Although the algorithms used by different indi- office of Technology Assessment, Feb 15, 1988 viduals or groups can (and usually do) differ in detail, most of the algorithms currently in use are of four basic types: insensitive interval scaling, a shock wave forms and begins to move outward. similar explosion scaling, semi-analytical modeling, At this time the speed of the shock wave is much and numerical modeling. Before considering these greater than the speed of sound in the undisturbed algorithms and their application to test ban veri- ambient medium, the pressure behind the shock fication, it will be helpful to have in mind how the wave is predominantly thermal pressure, and the shock wave produced by an underground nuclear ratio of the density behind the shock wave to the explosion evolves during the hydrodynamic phase density in front is close to its limiting value. This and how this evolution is affected by the proper- is the strong shock interval.1 ties of the ambient medium. If the shock wave envelops a mass of material much greater than the mass of the nuclear charge Shock Wave Evolution and casing while it is still strong, and if energy transport by radiation can be neglected, the shock The hydrodynamic evolution of the shock wave wave will become self-similar, expanding in a par- produced by a large, spherically symmetric explo- ticularly simple way that depends only weakly on sion underground may be usefully divided into the properties of the medium.’ The time at which three different intervals. These are listed in table A-1, along with the times after detonation at which ‘See Ya. B. Zel’dovich and Yu. P. Riazer, Physics of Shock Waves they begin for 1 and 150 kiloton (kt) explosions in and High-Temperature Phenomena (New York, NY: Academic Press, granite. The characteristics of these intervals 1967 ~nglish Translation]), ch. XI. As the strength of a shock wave follow. is increased, the ratio of the material density immediately behind it to the material density immediately in front of it generally increases, until a value of the ratio is reached beyond which an increase in the strength Self-Similar Strong-Shock Interval of the shock wave produces little or no further increase in the density of the post-shock material. This density ratio is referred to as the limit- At the very earliest times, the energy of the ex- ing density ratio. In typical rocks, pressures behind the shock front of about 10-100 Mbar are needed to produce a density ratio close to the plosion is carried outward by the expanding limiting value. weapon debris and by radiation. Soon, however, ‘Ibid., ch. I and XII.

129 130 the motion becomes self-similar depends in part Theoretical models and experimental data show on the design of the nuclear charge and diagnostic that the evolution of the shock wave in all three equipment and on the size of the emplacement hole. intervals depends on such properties of the rock As the shockwave expands, it weakens and slows, as its chemical composition, bulk density, plastic the density behind the shock front drops, and the wave speed, and degree of liquid saturation. These wave enters a transition interval in which the mo- properties vary considerably from one rock to tion is no longer self-similar. No time is given in another. As a result, the shock wave generally de- table A-1 for the beginning of the self-similar velops differently in different rocks. For example, strong-shock interval for a 1 kt explosion because the characteristic radius at which the shock wave the shock wave produced by such an explosion produced by a 150 kt explosion changes from a typically weakens before it has time to become self- strong, self-similar wave to a plastic wave5 varies similar. from about 30 meters in wet tuff to over 60 meters in dry alluvium. Transition Interval As the shock wave weakens and slows, it enters Measuring the Position of the a broad transition interval in which the thermal Shock Front pressure is not much greater than the cold pressure of the medium. The motion of the shockwave Several techniques have been used to measure changes only gradually and so the time at which the position of the shock front as a function of time. the transition interval is said to begin is purely con- During the 1960s and early 1970s, extensive meas- ventional. In this report the shockwave is consid- urements were made using the so-called SLIFER ered to have entered the transition interval when technique. 6 In the mid-1970s an improved tech- the density just behind the shock front has fallen nique, called CORRTEX, was developed.7 This is to 80 percent of its maximum limiting value. The the technique the Reagan administration has pro- speed of the shock front is only a few times greater posed as a new technique to monitor the Thresh- than the relevant sound speed-the speed of the old Test Ban Treaty (TTBT).8 so-called plastic wave—in the medium over much In the CORRTEX technique, an electrical sens- of the transition interval and hence the motion of ing cable is lowered into a vertical hole to a depth the shock wave in this interval is more sensitive greater than the depth at which the nuclear explo- to the properties of the medium than it is in the sion will take place, typically hundreds of meters strong shock interval.3 for explosives with yields near 150 kt. The hole may be the one in which the nuclear explosive is placed Plastic-Wave Interval (the emplacement hole) or one or more other holes (so-called satellite holes) that have been drilled spe- In the absence of phase transitions and other cifically for this purpose. The latter geometry is complications,4 the shock wave weakens and slows shown in figure A-1. If satellite holes are used, they still further, entering an interval in which the pres- must be drilled at the proper distance(s) from the sure behind the shock front is predominantly the emplacement hole, typically about ten meters for cold pressure of the compressed ambient medium yields near 150 kt. Then, if the sensing cable is and the shock speed is close to the plastic wave strong enough that it is not crushed by other dis- speed in the medium. Again, the motion of the shock wave changes only gradually and so the time at which the plastic-wave interval is said to begin Y3ee F. K, Lamb, ACDIS WP-2-87-2 (University of Illinois Program is purely conventional. In this report the shock in Arms Control, Disarmament, and International Security, Urbana, wave is considered to have entered the plastic-wave IL, 1987). ‘M. Heusinkveld and F. Holzer, Review of Scientific Instruments, 35, interval when its speed is less than 120 percent of 1105 (1964). SLIFER is an acronym for “Shorted Location Indication the plastic wave speed in the medium. In practice, by Frequency of Electrical Resonance.” 7C.F. Virchow, G.E. Conrad, D.M. Holt, et.al., Review of Scientific phase transitions and other effects complicate the Instruments, 51,642 (1980) and Los Alarnos National Laboratory pub- evolution of the shock wave in this interval for lic information sheet on CORRTEX (April 1986). CORRTEX is an acro- rocks of interest. nym for “Continuous Reflectometry for Radius v. Time Experiments. ” W.S. Department of State, Bureau of Public Affairs, “U.S. Policy Re garding Limitations on Nuclear Testing, ’’Special Report No. 150, Au- gust 1986; and U.S. Department of State, Bureau of Public Affairs, ‘Ibid., pp. 741-744, “Verifying Nuclear Testing Limitations: Possible U.S.-Soviet Coopera- ‘Ibid., ch. XII. tion,” Special Report No. 152, Aug. 14, 1986. 131

Figure A-1 .—Use of the CORRTEX Technique

CORRTEX CORRTEX recorder recorder

Surface Surface

Sensing Sensing cable cable

tic nostic /

Working point point I // \ Shock front / progression \ /

Typical cable emplacement Moving shock wave from in satellite hole nuclear detonation crushes and shortens cable

SOURCE: U.S. Department of State, Bureau of Public Affairs, “U.S. Policy Regarding Limitations on Nuclear Testing,” Special Report No 150, August 1986 132

turbances but weak enough that it is crushed by Insensitive Interval Scaling the pressure peak at the shock front, it will be electrically shorted close to the point where the shock Once measurements of the radius of the shock front intersects the cable (see figure A-l). As the front as a function of time are in hand, an estimate shock front expands with time, the changing dis- of the yield of the explosion can be made by com- tance from the surface to the shallowest point at paring the measurements with a model of the mo- which the shock wave intersects the sensing cable tion of the shock front away from the center of the is measured at preset time intervals by electrical explosion. The simplest algorithm currently in use equipment attached to the cable and located above is insensitive interval scaling. This is the algorithm ground. The CORRTEX technique is much less af- that the Reagan administration has proposed to fected by disturbing early signals from the explo- use in analyzing CORRTEX data as an additional sion than were earlier techniques. new method of monitoring compliance with the 150 The time at which the explosion begins is taken kt limit of the TTBT. to be the time at which the first signal, produced Insensitive interval scaling is based on the as- by the electromagnetic pulse (EMP) from the ex- sumption that the radius of the shock wave for an plosion, arrives at the CORRTEX recorder. If the explosion of given yield is independent of the explosion is spherically symmetric, the length of medium during a certain interval in time and ra- the unshorted cable decreases rapidly and dius called here the insensitive interval. Indeed, smoothly with time as the shock front expands studies indicate that for the collection of rocks away from the center of the explosion and the ra- within U.S. test experience (mostly silicates), rock dius of the shock front at a given time can be cal- properties are correlated in such a way that there culated using simple geometrical equations. If the is a time during the transition interval when the explosion is not spherically symmetric, due to the radius of the shock front produced by an explosion of given yield varies relatively little from one shape of the canister, the design of the nuclear 9 charge, or inhomogeneities in the ambient medium, rock to another. the interpretation of CORRTEX data is more com- In using insensitive interval scaling, the shock plicated and could be ambiguous or misleading un- wave sensing cable must be placed close enough der the conditions encountered in treaty verifica- to the center of the explosion that it samples the tion. Problems of this kind can be prevented by insensitive interval. Yield estimates are then de- cooperative agreements, as discussed below. rived by fitting a simple empirical formula, called the Los Alamos Formula, to the shock radius An error of 1 meter in the measured distance of 10 the crushing point from the center of the explo- versus time data in this interval. The Los Alamos sion will cause an error of about 50 kt in the yield Formula is a power law that approximates the ac- estimate, for yields near 150 kt. Thus, an accurate tual radius versus time curve during the insensi- survey of the satellite hole is required in order to tive interval. This is illustrated by figure A-2, make an accurate yield estimate. Surveys are cur- which compares the Formula with a model of the rently made with special laser or gyroscopic equip- evolution of the shock wave produced in granite ment. In some yield estimation algorithms, the by a spherically-symmetric point explosion with lateral displacement from the center of the explo- a yield of 62 kt. sion can be treated as one of the unknowns in estimating the yield. The rocks for which the United States had good data or models are the dry alluvium, partially saturated tuff, saturated tuff, granite, ba- salt, and rhyolite at the test sites used, almost all of which are at the Yield Estimation Algorithms Nevada Test Site. At present, the reason for the correlation of rock properties that gives rise ta the insensitive interval is not well understmd Hydrodynamic methods of yield estimation are from a fundamental physical point of view. Moreover, it is known that evolving as research aimed at gaining a better un- the radius of the shock wave in this interval is very different for other very different kinds of rocks. Thus, the existence of an insensitive in- derstanding of underground explosions and im- terval must be established by test experience or modeling, and is only proving yield estimation methods continues. At assured for certain geologic media. 1’The Los Alamos Formula for the shock radius in meters is R(t) =a present, four basic types of algorithms are com- .W%(t/W1/3)6, where W is the yield of the explosion in kilotons, t is the monly in use. In order to simplify their descrip- elapsed time since the beginning of the explosion in milliseconds, and tion, the explosion will be assumed to be spheric- a and /3 are constants. Different values of a and 6 have been used by different individuals and groups and have changed with time. The values ally symmetric (the complications that can arise of a and /3 used here are 6.29 and 0.475 (see M. Heusinkveld, Journal if it is not will be addressed later). of Geophysictd Research, 87, 1891, 1982). 133

Figure A-2.—Comparison of the Los Alamos Formula Figure A-3.—Application of the Los Alamos Formula With a Semi= Analytical Model of the Shock Wave in to an Explosion in Granite Granite Caused by a Spherically asymmetric Point 200 Explosion With a Yield of 62 Kilotons 1000 — Granite Model

— Los Alamos Formula ■ ✎

100

.0001 .001 .01 .1 1 10 100 1000 Time (ins)

SOURCE: F.K. Lamb, “Hydrodynamic Methods of Yield Estimation,” Report for the US. Congress, Office of Technology Assessment, Feb. 15, 1988.

1 1 1 2 3 4 5 In practice, the Los Alamos Formula is usually first fit to a broad interval of radius versus time Time (ins) SOURCE: F.K. Lamb, “Hydrodynamic Methods of Yield Estimation,” Report for data that is thought to include the insensitive in- the U.S. Congress, Off Ice of Technology Assessment, Feb. 15, 1988. terval. The result is a sequence of yield estimates. Due to the departure of the Formula from the actual radius versus time curve at both early and late times, the sequence of yield estimates typically mic interval is indicted in figure A-3 by the two forms a U-shaped curve. This is illustrated in fig- vertical bars at the bottom of the figure. In this ure A-3, which shows the sequence of yield esti- example, the assumptions of the algorithm are mates obtained by applying the Formula to the satisfied and the average of the yield estimates relatively high-quality SLIFER data from the that lie within the algorithmic interval is very close Piledriver explosion in granite. If the assumptions on which the algorithm is based are satisfied, the yield estimates near the bottom of the curve ap- Table A-2.—Algorithmic Intervals for Various Yieldsa proximate the actual yield of the explosion, In the Time Time interval (ins) Radius interval (m) usual form of the algorithm, only the radius versus 1 0.1-0.5 2-4.5 time data that fall within a certain predetermined 10: : : : : : : : : : : : : : : : 0.2-1.1 4.5-1o interval chosen on the basis of previous experience 50...... 0.4-1.8 8-17 100...... 0.5-2.7 10-21 (the so-called algorithmic interval) are actually used 150...... 0.5-2.6 11-24 to make the final yield estimate. The length of the aThe time intewals used by various individuals and groups varies. Throughout algorithmic interval and the time at which it oc- this report the algorithmic interval Is taken to be from O.l W’~ milliseconds to 1/3 0,5W1~ milliseconds after the beginning of the explosion, where W is in kilotons, curs are both proportional to W , where W is the SOURCE: F.K. Lamb “Hydrodynamic Methods of Yield Estimation” Report for yield of the explosion (see table A-2). The algorith- the U.S. Congress, Office of Technology Assessment, Feb. 15, 1988. 134

to the announced yield of 62 kt. Studies indicate Figure A-4.—Application of the Los Alamos Formula that yield estimates made using this algorithm to a Low-Yield Explosion in Alluvium have a precision of about a factor of 1.2 at the 95 percent confidence level for spherically-symmetric explosions with yields greater than 50 kt conducted at the Nevada Test Site (NTS). Estimates of the accuracy of the algorithm made by comparing results with the usually more accurate radiochemi- . cal method are similar. According to official state- O.8 . ■ ments, the insensitive interval algorithm is . expected to be accurate to within a factor of 1.3 ● . at Soviet test sites for explosions with yields . greater than 50 kt in media within U.S. test ex- . perience.” Some scientists believe that the uncer- . . tainty would be somewhat larger. 0.7 . The insensitive interval algorithm does not work . as well if the assumptions on which it is based are not satisfied. This is illustrated in figure A-4, which shows the yield estimates obtained by fitting the . . Los Alamos Formula to good-quality SLIFER . data from atypical low-yield explosion in alluvium. 0.6 . . In this example the radius and time data have been . . scaled using the actual yield so that the derived . yield should be 1 kt. However, the yield estimates given by the Los Alamos Formula are systematically low, ranging from 30 to 82 percent of the actual yield, and do not forma U-shaped curve. The 0.5 m . . average of the yield estimates that lie within the . algorithmic interval is about 60 percent of the actual yield. The overall appearance of the yield . versus time curve shows that the assumptions of the algorithm are not satisfied. A common misconception has been that the al- 0.1 0.3 0.5 0.7 09 1.1 gorithmic interval lies within the strong shock re- Time (scaled ms) gion and that the relative insensitivity of yield esti- SOURCE: F.K. Lamb, “Hydrodynamic Methods of Yield Estimation,” Report for mates to the properties of the medium stems from the US. Congress, Office of Technology Assessment, Feb. 15, 1988. this.12 As explained earlier, radius versus time data in the algorithmic interval would indeed be rela- appropriate for a strong shock wave. The Los tively independent of the medium if this were so, Alamos Formula is, as noted earlier, an empirical and would follow a power-law curve similar to the relation, which was obtained by fitting a power- Los Alamos Formula. However, the shock wave law expression to data from a collection of explo- is not strong during the algorithmic interval, be- sions in a variety of different rocks and approxi- cause in this interval the shock speed is only a few mates actual radius versus time curves over a por- times the speed of sound and the post-shock pres- tion of the transition interval. sure is much less than the pressure required to achieve the maximum limiting density. Indeed, the Similar Explosion Scaling exponent of time usually used in the Los Alamos If a given explosion occurs in the same medium Formula is significantly greater than the value as a previous explosion at a different site, and if “U.S. Department of State, op. cit., footnote 8. radius versus time data and the yield are available “For example, “The accuracy of the method is believed to be relatively, but not wholly, independent of the geologic medium, provided for the previous explosion, then the yield of the the satellite hole measurements are made in the ‘strong shock’ region given explosion can be estimated by similar explo- . . . “ (Ibid.) This misconception may have arisen from the fact that the sion scaling. The reason is that for explosions in interval formerly used to estimate the yields of nuclear explosions in the atmosphere using hydrodynamic methods is within the strong shock the same medium, the radius versus time curve de- region. pends only on the yield of the explosion, and this 135 dependence is known and is simple.13 Hence, an radius versus time curves that extend over much estimate of the yield of the given explosion can be of the shock wave evolution, making it possible to made by comparing the two sets of radius versus base yield estimates not only on data from the tran- time data. This algorithm can make use of data sition interval but also data from later phases of outside the insensitive interval and works well if the shock wave evolution. In practice, the yield the ambient media at the two explosion sites are estimates obtained using such a procedure are sufficiently similar. However, in practice it has fairly sensitive to the equation of state of the am- sometimes proved difficult to ascertain whether bient medium, which is known with sufficient ac- the relevant properties of the media are similar curacy for only a few geologic media. If adequate enough to give the desired accuracy. Similar ex- equation of state data are lacking, numerical mod- plosion scaling has been proposed as a supplement eling may not be warranted. to insensitive interval scaling for TTBT verifi- In summary, the shockwave produced by an un- cation. derground nuclear explosion propagates differently in different media and different geological Semi-Analytical Modeling structures. As a result, knowledge of the ambient medium and local geological structures is required Semi-analytical modeling is another approach in order to make accurate yield estimates using that is useful for studying the evolution of shock hydrodynamic methods. Several different yield- waves in geologic media and for estimating yields. estimation algorithms have been developed. These In this approach both the properties of the ambient algorithms, like those based on seismic methods, medium and the motion of the shock front are involve some complexity and require sophistica- treated in a simplified way that nevertheless in- tion to understand and apply correctly. Some key cludes the most important effects. The result is a terms that have been introduced in this discussion relatively simple, semi-analytical expression for the are listed and explained in table A-3. radius of the shock front as a function of time. If the required properties of the ambient medium are known and inserted in this expression, the yield Application to Monitoring Treaties of an explosion can be estimated by fitting the expression to measurements of the shock wave mo- 14 Assuring Accuracy tion with time. Semi-analytical algorithms can in principle make use of more of the data than can Ambient Medium.-The physical properties and the insensitive interval algorithm and can also be geologic structure of the ambient medium enter used to estimate the uncertainty in the yield caused directly into yield estimates based on hydro- by uncertainties in the properties of the ambient dynamic methods. Incorrect assumptions about medium. the average properties of the ambient medium may bias the yield estimate, decreasing its accuracy, Numerical Modeling while small-scale variations will cause scatter in If a treatment that includes the details of the the radius versus time data, decreasing the preci- equation of state and other properties of the am- sion of the yield estimate. Thus, it is important to gather information about the types of rock present bient medium is required, or if the explosion is asymmetric, modeling of the motion of the shock at the test site and their properties, including their front using numerical hydrocodes may be neces- chemical composition, bulk density, and degree of 15 liquid saturation, as well as the speed of sound in sary. In principle, such simulations can Provide the ambient medium and any specific features of “See H. L. Brode, Annual Review of Nuclear Science, 18, 153-202 the local geologic structure that could affect the (1968). yield estimate. Availability of the required data “For an early semi-analytical model, see M. Heusinkveld, Report UCRL-52648 (Lawrence Livermore National Laboratory, Livermore, CA, would need to be assured by appropriate coopera- 1979). For improved semi-analytical models, see F. K. Lamb, ACDZS tive measures. WP-87-2-1 (University of Illinois Program in Arms Control, Disarma- ment, and International Security, Urbana, IL, February 1987); W. C. Some information about the geologic medium at Moss, Rep. UCRL-96430Rev. 1 (Lawrence Livermore National Labora- the test site could be obtained by examining the tory, Livermore, CA, July 1987); and R. A. Axford and D. D. Helm, contents of the hole drilled for the CORRTEX sens- Proc. NEDC (Los Alamos, October 1987).

15 For a discussion of numeric~ models currently in use ~ F. K. L~b, ing cable. Verification could be improved by coop- “Monitoring Yields of Underground Nuclear Tests, ” Threshold Test erative arrangements that would also allow obser- Ban Treaty and Peaceful Nuclear Explosions Treaty, Hearings Before vation of the construction of the emplacement hole, the Comnu”ttee on Foreign Relations, Um”ted States Senate (Washing- ton, DC: U.S. Government Printing Office, 1987), pp. 359-370. removal and examination of rock core or rock frag- 136

Table A-3.–Glossary of Hydrodynamic Yield order to cover the insensitive interval (see table Estimation Terms A-2). As a result, yield estimates can be affected by the arrangement of the nuclear charge and the Term/Explanation canister or canisters containing it and the diagnos- Strong Shock Interval: The interval in radius and time during tic equipment.17 In particular, any properties of the which the speed of the shock wave is much greater than the speed of sound in the unshocked medium experimental set-up or the surrounding geologic Transition Interval: The interval in radius and time outside media that cause the shock front to be distorted the strong shock interval in which the speed of the shock at the radii of interest could affect the accuracy wave approaches the speed of sound in the unshocked of the yield estimate. The reason is that a CORR- medium Plastic Wave Interval: The interval in radius and time outside TEX sensing cable measures only the depth of the the transition region in which the speed of the weakening shallowest point where a pressure wave first shock wave is approximately the plastic wave speed crushes it, at a single lateral displacement from SLIFER Technique: A technique for measuring the position the explosion. Thus, unambiguous interpretation of the shock wave expanding away from an underground of the data may become difficult or impossible if explosion by determining the resonant frequency of an electrical circuit that includes a sensing cable placed in the explosion is not spherically symmetric. a hole in the ground near the site of the explosion For example, explosions of nuclear charges in CORRTEX Technique: A technique for measuring the posi- tunnels may be accompanied by complicated (and tion of the shock wave expanding away from an under- unanticipated) energy flows and complex shock ground explosion by determining the round-trip travel time of electrical pulses sent down a sensing cable placed in wave patterns. If significant energy reaches the a hole in the ground near the site of the explosion sensing cable ahead of the ground shock and shorts Insensitive Interval Scaling: A yield estimation algorithm in it before the ground shock arrives, the CORRTEX which the Los Alamos Formula is fit to measurements of data will describe that flow of energy and not the the position of the expanding shock wave as a function of time during the algorithmic interval motion of the ground shock. Alternatively, the mo- Los Alamos Formula: The empirical formula used in the in- tion of the ground shock itself could be sufficiently sensitive interval scaling algorithm to make yield estimates distorted that interpretation of the shock position by fitting to shock radius versus time data data becomes ambiguous or misleading. As Algorithmic Interval: The special time interval used in insen- another example, a large canister or double explo- sitive interval scaling during which the radius of the shock wave is relatively insensitive to the ambient medium; usu- sion could short the CORRTEX cable in such away ally assumed to be 0,1-0.5 scaled milliseconds after the that only part of the total yield is sensed over most beginning of the explosion of the interval sampled by the CORRTEX cable, Similar Explosion Scaling: A yield estimation algorithm in as shown in figure A-5. The physical size of which data obtained from a previous explosion in the same medium are scaled to fit measurements of the position lrThe imwrt~ce of these disturbing effects is less for high-yield than of the expanding shock produced by the explosion under for low-yield explosions. consideration SOURCE: F.K. Lamb “Hydrodynamic Methods of Yield Estimation” Report for Figure A-5.—Effect of Nuclear Test Design on the US. Congress, Office of Technology Assessment, Feb. 15, 1988, Shock Wave Radius Measurements Using CORRTEX Equipment ments from the wall of the emplacement hole, ex- CORRTEX Recorder amination of any logs or drill core from existing exploratory holes, removal and examination of rock core or rock fragments from the walls of existing exploratory holes, and if necessary, construction of new exploratory holes. There is precedent for such cooperative arrangements in the Peaceful Nuclear Explosions Treaty (PNET), which explicitly established the hydrodynamic method as one of the monitoring methods that could be used for large salvos and specified verification measures like these.16 Test Geometry. –CORRTEX data must be taken very close to the center of the explosion in

1aArms control md Disarmament Agreements (U.S. Arms Control SOURCE: F.K. Lamb, “Hydrodynamic Methods of Yield Estimation,” Report for and Disarmament Agency, Washington, DC, 1982). the U.S. Congress, Office of Technology Assessment, Feb. 15, 1988. 137

canisters and diagnostic lines-of-sight tend to pose tion to treaty monitoring made by these methods. more of a problem for nuclear directed-energy In summary, the accuracy that could be achieved weapons than for traditional nuclear weapons.18 using hydrodynamic methods to estimate the In using hydrodynamic methods to estimate the yields of underground nuclear explosions depends yields of one’s own tests, the design and placement on the amount of information that can be gathered of the nuclear charge and related equipment are about the medium in which the explosion occurs, known and can be taken into account. This is not and the nature and extent of cooperative arrange- necessarily the case when monitoring the nuclear ments that can be negotiated to optimize the place- tests of another party. Cooperative agreements to ment of sensing cables and to limit disturbing make possible optimal placement of sensing cables effects. and to exclude nuclear test geometries that would Hydrodynamic methods for estimating the significantly disturb the yield estimate would yields have not yet been studied as thoroughly or therefore be required.19 as widely as the seismic methods currently in Such agreements could, for example, limit the use, although they have been examined more length of the canister containing the nuclear charge thoroughly than some seismic methods that have and the cross-sectional dimensions of the emplace- been proposed for the future. Tests and simulations ment hole, and mandate filling of the nuclear to identify troublesome configurations have been charge emplacement hole with certain types of ma- carried out, but only a few explosions have been terials. Such agreements could also provide for ob- monitored with the CORRTEX sensing cable in a servation of the emplacement of the nuclear charge satellite hole.21 Given the possibility that and the stemming of the emplacement hole, con- hydrodynamic yield estimation may have to be firmation of the depth of emplacement, and limi- used to monitor treaty compliance in an adversar- tations on the placement of cables or other equip- ial atmosphere, the possibility of deliberate efforts ment that might interfere with the CORRTEX to introduce error or ambiguity, and the tendency measurement. For test geometries that include for worst-case interpretations to prevail, additional ancillary shafts, drafts, or other cavities, additional research to reduce further the chances of confu- measures, such as placement of several sensing ca- sion, ambiguity, spoofing, or data denial would be bles around the weapon emplacement point, may very useful. be required to assure an accurate yield estimate. For tunnel shots, sensing cables could be placed Minimizing Intrusion in the tunnel walls or in a special hole drilled toward the tunnel from above. Again, there is precedent Hydrodynamic yield estimation methods are for such cooperative measures in the PNET.20 more intrusive than remote seismic methods for The restrictions on the size of canisters and diag- several reasons: nostic lines-of-sight that would be required even 1. Personnel from the monitoring country would with the sensing cable placed in a satellite hole be present at the test site of the testing coun- would cause some interference with the U.S. nu- try for perhaps 10 weeks or so before as well clear testing program at NTS. However, these re- as during each test, and would therefore have strictions have been examined in detail by the U.S. an opportunity to observe test preparations. The presence of these personnel would pose nuclear weapon design laboratories and the De- 22 partment of Energy, and have been found to be some operational security problems. manageable for the weapon tests that are planned 2. The exterior of the canister or canisters con- for the next several years. In assessing whether taining the nuclear charge and diagnostic hydrodynamic methods should be used beyond this equipment must be examined to verify that period, the disadvantages of the test restrictions the restrictions necessary for the yield esti- must be weighed against the potential contribu- mate to be valid are satisfied. For tests of nuclear directed energy weapons, this examination could reveal sensitive design information 23 ‘8 See Sylvester R. Foley, Jr., Assistant Secretary for Defense Pro- unless special procedures are followed. grams, Department of Energy, letter to Edward J. Markey, Congress- man from Massachusetts, Mar. 23, 1987. 1’S. S. Hecker, Threshold Test Ban Treaty and Peaceful Nuclear Ex- *’Op. cit., footnote 8. Approximately 100 tests have been carried out plosions Treaty, Hearings Before the Comnu”ttee on Foreign Relations, with the CORRTEX cable in the emplacement hole, and SLIFER data United States Senate (Washington, DC: U.S. Government Printing Of- from satellite holes are available for several tens of earlier explosions. fice, 1987), pp. 50-61 and 226-235; M. D. Nordyke, Ibid., 67-71 and 278- IZR. E, Batzel, Threshold Test Ilan . . ., op. cit., footnote 19, PP. 48+0 285; Foley, ibid and 210-225, and Foley, op. cit., footnote 18. 200p. cit., footnote 16. 21 Batzel, ibid. 138

3. Sensing cables and electrical equipment will From the point of view of the United States, pos- tend to pick up the electromagnetic pulse sible advantages of being able to use hydrodynamic (EMP) generated by the explosion. A detailed yield estimation methods at Soviet test sites in- analysis of the EMP would reveal sensitive include the additional information on yields that this formation about the design and performance would provide, establishment of the principle of of the nuclear device being tested. on-site inspection at nuclear test sites, and the pos- Intrusiveness could be minimized by careful at- sibility of collecting data on the ambient media and tention to monitoring procedures and equipment. geologic structures at Soviet test sites. Obviously, For example, the electrical equipment required can the larger the number of explosions and the greater be designed to avoid measuring sensitive informa- the number of test sites monitored, the more in- tion about the nuclear devices being tested. CORR- formation that would be obtained. Possible dis- TEX equipment has been designed in this way, and advantages for the United States include the po- the United States could insist that any Soviet tential difficulty of negotiating routine use of equipment used at NTS be similarly designed. The hydrodynamic methods at Soviet test sites, which security problems posed by opportunities to ob- could impede progress in limiting nuclear testing, serve test preparations are more severe for nuclear and the operational security problems at NTS directed energy weapon tests, since they tend to caused by the presence of Soviet monitoring per- have more and larger complex diagnostic systems sonnel there. and canister arrangements which, if fully revealed Peaceful Nuclear Explosions Treaty.–As it to the Soviets, might disclose sensitive informa- stands, the PNET does not provide for use of tion. The United States has determined that the hydrodynamic yield estimation except for salvos Soviet personnel and activities that would be re- in which the “planned aggregate yield” is greater quired at NTS to monitor U.S. tests would be than 150 kt.26 Thus, if the TTBT is modified to al- acceptable both from a security standpoint and low hydrodynamic yield estimation for all weapon from the standpoint of their effect on the U.S. test tests with planned yields above a certain value, the program. Detailed operational plans have been de- purpose of the modification could in principal be veloped to accommodate such visits without ad- circumvented by carrying out weapon tests as verse impact on operations.24 “peaceful” nuclear explosions of “planned yield” less than or equal to 150 kt, unless the PNET is Specific Applications also modified to close this loophole. Low-Threshold Test Ban Treaty. -Underground Threshold Test Ban Treaty.–As noted earlier, nuclear explosions as small as 1 kt produce shock hydrodynamic yield estimation has been proposed waves that evolve in the same way as those by the Reagan administration as a new routine produced by explosions of larger yield. However, measure for monitoring the sizes of nuclear tests, such explosions can and usually are set off at shal- in order to verify compliance with the 150 kt limit low depths and can be set off in alluvium. As a re- of the TTBT. To reduce the cost and intrusiveness sult, the motion of the ground can be markedly of such verification, it could be restricted to tests different from that on which standard hydro- with expected yields greater than some threshold dynamic yield estimation methods are based, caus- that is an appreciable fraction of 150 kt. Hydro- ing a substantial error in the yield estimate (see dynamic measurement of the yields of one or more figure A-4). There can also be significant variations nuclear explosions at each country’s test site or in the motion of the ground shock from explosion sites has also been suggested as a method of 25 to explosion under these conditions. calibrating seismic yield estimation methods. In addition, serious practical, operational, and engineering problems arise in trying to use hydrodynamic methods to estimate the yield of

24 FOley, Op. cit., footnote 18- such a small explosion. For one thing, the sensing ZfiIt has ban sugge9ted that the nuclear calibration chwges to be det- cable must be placed very close to the nuclear onated at the test sites could be provided by the monitoring country. charge (see table A-3). Drilling a satellite hole If they were, a hydrodynamic estimate of the yield might not be needed, since, as explained inch. 7, the yields of certain types of nuclear charges within 3 meters of the emplacement hole to the are accurately reproducible. Knowledge of the surrounding medium and depths of typical nuclear device emplacement, as geologic structure would still be needed to provide assurance that the would be required in order to use hydrodynamic coupling of the explosion to seismic waves was understood. However, some way would have to be found to provide assurance that no sensitive information about the design of the nuclear weapons of either country would be revealed. ‘eArms . . . , op. cit., footnote 16. 139 methods to estimate the size of an explosion with at the present time hydrodynamic yield estimation a yield near 1 kt, would be challenging, to say the methods could not be used with confidence to mon- least. The need for such close placement would ne- itor compliance with threshold test bans in which cessitate further restrictions on the maximum size the threshold is less than several tens of kilotons. and orientation of the canister used to contain the Comprehensive Test Ban.–As their name im- nuclear charge and diagnostic instrumentation. plies, hydrodynamic yield estimation methods Such restrictions might be deemed an unaccept- have been developed to measure the sizes of unable interference with test programs. However, use derground nuclear explosions. They are a poten- of small canisters with numerous diagnostic lines- tially valuable component of a cooperative program of-sight to the detonation point could disturb the to monitor limits on yields, but are neither intended CORRTEX measurements. Because the shock nor able to detect, identify, or measure the yields wave radii to be measured are much smaller at low of unannounced or clandestine nuclear tests. Thus, yields, survey errors become much more important. they are not applicable to monitoring a comprehen- Possible solutions to these problems have not sive test ban. yet been carefully and thoroughly studied. Thus,