Earthquakes by Kaye M. Shedlock and Louis C. Pakiser

Many buildings in Charleston, South Carolina, were damaged or destroyed by the large earth­ quake that occurred August 31,1886.

For sale by the Superintendent of Documents, U.S. Government Printing Office, Wachinntnn D C. OCiAriO ne of the most frightening and Earthquakes in History destructive phenomena of nature O is a severe earthquake and its ter­ he scientific study of earthquakes rible aftereffects. An earthquake is a sud­ is comparatively new. Until the den movement of the Earth, caused by T 18th century, few factual descrip­ the abrupt release of strain that has accu­ tions of earthquakes were recorded, and mulated over a long time. For hundreds the natural cause of earthquakes was lit­ of millions of years, the forces of plate tle understood. Those who did look for tectonics have shaped the Earth as the natural causes often reached conclusions huge plates that form the Earth's surface that seem fanciful today; one popular slowly move over, under, and past each theory was that earthquakes were caused other. Sometimes the movement is grad­ by air rushing out of caverns deep in the ual. At other times, the plates are locked Earth's interior. together, unable to release the accumu­ The earliest earthquake for which lating energy. When the accumulated we have descriptive information energy grows strong enough, the plates occurred in China in 1177 B.C. The break free. If the earthquake occurs in Chinese earthquake catalog describes a populated area, it may cause many several dozen large earthquakes in China deaths and injuries and extensive proper­ during the next few thousand years. ty damage. Earthquakes in Europe are mentioned as Today we are challenging the early as 580 B.C., but the earliest for assumption that earthquakes must pre­ which we have some descriptive infor­ sent an uncontrollable and unpredictable mation occurred in the mid-16th century. hazard to life and property. Scientists The earliest known earthquakes in the have begun to estimate the locations and Americas were in Mexico in the late likelihoods of future damaging earth­ 14th century and in Peru in 1471, but quakes. Sites of greatest hazard are being descriptions of the effects were not well identified, and definite progress is being documented. By the 17th century, des­ made in designing structures that will criptions of the effects of earthquakes withstand the effects of earthquakes. were being published around the world although these accounts were often exaggerated or distorted. The most widely felt earthquakes in the recorded history of North America were a series that occurred in 1811-12 near New Madrid, Mo. A great earth­ quake, whose magnitude is estimated to be about 8, occurred on the morning of December 16,1811. Another great earth­ quake occurred on January 23,1812, and a third, the strongest yet, on February 7, 1812. Aftershocks were nearly continu­ ous between these great earthquakes and USGS scientist uses portable seismic recording continued for months afterwards. These equipment near Mount St. Helens, Washington. earthquakes were felt by people as far away as Boston and Denver. Because the The great 1906 San Francisco earthquake and fire destroyed most of the city and left 250,000 people homeless. most intense effects were in a sparsely March 27, 1964, was of greater magni­ populated region, the destruction of tude than the San Francisco earthquake; human life and property was slight. If it released perhaps twice as much energy just one of these enormous earthquakes and was felt over an area of almost occurred in the same area today, millions 500,000 square miles. The ground of people and buildings and other struc­ motion near the epicenter was so violent tures worth billions of dollars would be that the tops of some trees were snapped affected. off. One hundred and fourteen people The San Francisco earthquake of (some as far away as California) died as 1906 was one of the most destructive in a result of this earthquake, but loss of the recorded history of North America life and property would have been far the earthquake and the fire that followed greater had Alaska been more densely killed nearly 700 people and left the city populated. in ruins. The Alaska earthquake of Where Earthquakes Occur

he Earth is formed of several lay­ ers that have very different physi­ Tcal and chemical properties. The outer layer, which averages about 70 kilometers in thickness, consists of about a dozen large, irregularly shaped plates that slide over, under, and past each other on top of the partly molten inner layer. Most earthquakes occur at the boundaries where the plates meet In fact, the locations of earthquakes and the kinds of ruptures they produce help sci­ entists define the plate boundaries. There are three types of plate boundaries: spreading zones, transform faults, and subduction zones. At spread­ ing zones, molten rock rises, pushing two plates apart and adding new material at their edges. Most spreading zones are found in oceans; for example, the North American and Eurasian plates are spreading apart along the mid-Atlantic ridge. Spreading zones usually have earthquakes at shallow depths (within 30 kilometers of the surface). Transform faults are found where plates slide past one another. An example

TRANSFORM SUBDUCTION FAULT ZONE of a transform-fault plate boundary is the Earthquakes can also occur within San Andreas fault, along the coast of plates, although plate-boundary earth­ California and northwestern Mexico. quakes are much more common. Less Earthquakes at transform faults tend to than 10 percent of all earthquakes occur occur at shallow depths and form fairly within plate interiors. As plates continue straight linear patterns. to move and plate boundaries change Subduction zones are found where over geologic time, weakened boundary one plate overrides, or subducts, another, regions become part of the interiors of pushing it downward into the mantle the plates. These zones of weakness where it melts. An example of a subduc- within the continents can cause earth­ tion-zone plate boundary is found along quakes in response to stresses that origi­ the northwest coast of the United States, nate at the edges of the plate or in the western Canada, and southern Alaska deeper crust. The New Madrid earth­ and the Aleutian Islands. Subduction quakes of 1811-12 and the 1886 zones are characterized by deep-ocean Charleston earthquake occurred within trenches, shallow to deep earthquakes, the North American plate. and mountain ranges containing active volcanoes. "M How Earthquakes Happen

n earthquake is the vibration, sometimes violent, of the Earth's Asurface that follows a release of energy in the Earth's crust. This energy can be generated by a sudden dislocation of segments of the crust, by a volcanic eruption, or even by manmade explo­ sions. Most destructive quakes, however, Normal Fault. Blocks are pulled apart are caused by dislocations of the crust. The crust may first bend and then, when the stress exceeds the strength of the rocks, break and "snap" to a new posi­ tion. In the process of breaking, vibra­ tions called "seismic waves" are generat­ ed. These waves travel outward from the source of the earthquake along the sur­ face and through the Earth at varying speeds depending on the material Thrust Fault. Blocks are pushed together through which they move. Some of the vibrations are of high enough frequency to be audible, while others are of very low frequency. These vibrations cause the entire planet to quiver or ring like a bell or a tuning fork. A fault is a fracture in the Earth's Strike-Slip Fault. Blocks slide past each crust along which two blocks of the crust other have slipped with respect to each other. Faults are divided into three main groups, depending on how they move. Geologists have found that earth­ Normal faults occur in response to quakes tend to reoccur along faults, pulling or tension; the overlying block which reflect zones of weakness in the moves down the dip of the fault plane. Earth's crust. Even if a fault zone has Thrust (reverse) faults occur in response recently experienced an earthquake, to squeezing or compression; the overly­ however, there is no guarantee that all ing block moves up the dip of the fault the stress has been relieved. Another plane. Strike-slip (lateral) faults occur in earthquake could still occur. In New response to either type of stress; the Madrid, a great earthquake was followed blocks move horizontally past one anoth­ by a large aftershock within 6 hours on er. Most faulting along spreading zones December 16,1811. Furthermore, reliev­ is normal, along subduction zones is ing stress along one part of the fault may thrust, and along transform faults is increase stress in another part; the New strike-slip. Madrid earthquakes in January and February 1812 may have resulted from this phenomenon. Crust

Distance from Earth's surface to center is approximately 6,370 Inner \ kilometers (3,960 miles)

X / Thickness of mantle is approximate!) 2.380 kilometers (1,760 miles)

Thickness of Earth's crust is 0 to 70 kilometers (0 to 44 miles)

Focus of most earthquakes is in crust or upper mantle

Seismic waves

Mantle The/oca/ depth of an earthquake is earthquake is commonly described by the depth from the Earth's surface to the the geographic position of its epicenter region where an earthquake's energy and by its focal depth. originates (the focus). Earthquakes with Earthquakes beneath the ocean floor focal depths from the surface to about 70 sometimes generate immense sea waves kilometers (43.5 miles) are classified as or tsunamis (Japan's dread "huge shallow. Earthquakes with focal depths wave"). These waves travel across the from 70 to 300 kilometers (43.5 to 186 ocean at speeds as great as 960 kilome­ miles) are classified as intermediate. The ters per hour (597 miles per hour) and focus of deep earthquakes may reach may be 15 meters (49 feet) high or high­ depths of more than 700 kilometers (435 er by the time they reach the shore. miles). The focuses of most earthquakes During the 1964 Alaska earthquake, are concentrated in the crust and upper tsunamis engulfing coastal areas caused mantle. The depth to the center of the most of the destruction at Kodiak, Earth's core is about 6,370 kilometers Cordova, and Seward and caused severe (3,960 miles), so even the deepest earth­ damage along the west coast of North quakes originate in relatively shallow America, particularly at Crescent City, parts of the Earth's interior. Calif. Some waves raced across the The epicenter of an earthquake is ocean to the coasts of Japan. the point on the Earth's surface directly above the focus. The location of an

Tsunami destruction on Kamehameha Avenue on Hilo's waterfront, 1946. (Photograph provided by the U.S. Army Corps of Engineers.) Liquefaction, which happens when caused major subsidence, fracturing, and loosely packed, water-logged sediments horizontal sliding of the ground surface lose their strength in response to strong in the Marina district in San Francisco. shaking, causes major damage during earthquakes. During the 1989 Loma Liquefaction of sands and debris caused major Prieta earthquake, liquefaction of the damage throughout the Marina district in San soils and debris used to fill in a lagoon Francisco during the Loma Prieta earthquake.

10 Landslides triggered by earthquakes saw that the bluff had broken back often cause more destruction than the approximately 300 feet southward from earthquakes themselves. During the 1964 its original edge. Additional slumping of Alaska quake, shock-induced landslides the bluff caused me to return to my car devastated the Turnagain Heights resi­ and back southward approximately 180 dential development and many down­ feet to the corner ofMcCollie and town areas in Anchorage. An observer Turnagain Parkway. The bluff slowly gave a vivid report of the breakup of the broke until the corner of Turnagain unstable earth materials in the Turnagain Parkway and McCollie had slumped Heights region: I got out of my car, ran northward. northward toward my driveway, and then

Many homes were damaged by landslides trig­ gered by the 1964 Alaska earthquake (above) and the 1989 Loma Prieta shock (left).

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Measuring Earthquakes Body waves are of two types, com- pressional and shear. Both types pass through the Earth's interior from the he vibrations produced by earth­ quakes are detected, recorded, and focus of an earthquake to distant points measured by instruments called on the surface, but only compressional T waves travel through the Earth's molten seismographs. The zig-zag line made by a seismograph, called a "seismogram," core. Because compressional waves trav­ el at great speeds and ordinarily reach reflects the changing intensity of the the surface first, they are often called vibrations by responding to the motion of the ground surface beneath the instru­ "primary waves" or simply "P" waves. P ment From the data expressed in seis- waves push tiny particles of Earth mater­ mograms, scientists can determine the ial directly ahead of them or displace the time, the epicenter, the focal depth, and particles directly behind their line of the type of faulting of an earthquake and travel. can estimate how much energy was Shear waves do not travel as rapidly released. through the Earth's crust and mantle as The two general types of vibrations do compressional waves, and because they ordinarily reach the surface later produced by earthquakes are surface waves, which travel along the Earth's they are called "secondary" or "S" waves. Instead of affecting material surface, and body waves, which travel directly behind or ahead of their line of through the Earth. Surface waves usually travel, shear waves displace material at have the strongest vibrations and proba­ right angles to their path and are there­ bly cause most of the damage done by fore sometimes called "transverse" earthquakes. waves. Focus

13 The first indication of an earthquake particular location. is often a sharp thud, signaling the arrival The Richter Scale, named after Dr. of compressional waves. This is followed Charles F. Richter of the California by the shear waves and then the "ground Institute of Technology, is the best known roll" caused by the surface waves. A scale for measuring the magnitude of geologist who was at Valdez, Alaska, earthquakes. The scale is logarithmic so during the 1964 earthquake described that a recording of 7, for example, indi­ this sequence: The first tremors were cates a disturbance with ground motion hard enough to stop a moving person, 10 times as large as a recording of 6. A and shock waves were immediately quake of magnitude 2 is the smallest noticeable on the surface of the ground. quake normally felt by people. Earth­ These shock waves continued with a quakes with a Richter value of 6 or more rather long frequency, which gave the are commonly considered major; great observer an impression of a rolling feel­ earthquakes have magnitudes of 8 or ing rather than abrupt hard jolts. After more on the Richter scale. about 1 minute the amplitude or strength The Modified Mercalli Scale ex­ of the shock waves increased in intensity presses the intensity of an earthquake's and failures in buildings as well as the effects in a given locality in values rang­ frozen ground surface began to occur ... ing from I to XII. The most commonly After about 31/2 minutes the severe used adaptation covers the range of in­ shock waves ended and people began to tensity from the condition of "I Not felt react as could be expected. except by a very few under especially The severity of an earthquake can be favorable conditions," to "XII Damage expressed in several ways. The magni­ total. Lines of sight and level are distort­ tude of an earthquake, usually expressed ed. Objects thrown upward into the air." by the Richter Scale, is a measure of the Evaluation of earthquake intensity can be amplitude of the seismic waves. The made only after eyewitness reports and moment magnitude of an earthquake is a results of field investigations are studied measure of the amount of energy and interpreted. The maximum inteasity released an amount that can be esti­ mated from seismograph recordings. The intensity, as expressed by the Modi­ fied Mercalli Scale, is a subjective measure that describes how Strong a ^arSe earthquakes cause more damage east of the Rocky Mountains; this map shows areas that suffered major architectural damage (striped areas) and minor damage (dotted shock was areas) during the magnitude-8 earthquakes in New Madrid and San Francisco and the felt at a smaller but still damaging quakes in Charleston and San Fernando.

14 The January 17,1994, earthquake at Northridge, California, caused this collapse of a major highway inter­ change. (Photograph by James W. Dewey, USGS.) experienced in the Alaska earthquake of construction in the area shaken by the 1964 was X; damage from the San quake. Francisco and New Madrid earthquakes The Loma Prieta earthquake of 1989 reached a maximum intensity of XI. demonstrated a wide range of effects. Earthquakes of large magnitude do The Santa Cruz mountains suffered little not necessarily cause the most intense damage from the seismic waves, even surface effects. The effect in a given though they were close to the epicenter. region depends to a large degree on local The central core of the city of Santa surface and subsurface geologic condi­ Cruz, about 24 kilometers (15 miles) tions. An area underlain by unstable away from the epicenter, was almost ground (sand, clay, or other unconsoli- completely destroyed. More than 80 dated materials), for example, is likely to kilometers (50 miles) away, the cities of experience much more noticeable effects San Francisco and Oakland suffered than an area equally distant from an selective but severe damage, including earthquake's epicenter but underlain by the loss of more than 40 lives. The great­ firm ground such as granite. In general, est destruction occurred in areas where earthquakes east of the Rocky Mountains roads and elevated structures were built affect a much larger area than earth­ on unstable ground underlain by loose, quakes west of the Rockies. unconsolidated soils. An earthquake's destructiveness The Northridge, California, earth­ depends on many factors. In addition to quake of 1994 also produced a wide magnitude and the local geologic condi­ variety of effects, even over distances of tions, these factors include the focal just a few hundred meters. Some build­ depth, the distance from the epicenter, ings collapsed, while adjacent buildings and the design of buildings and other of similar age and construction remained structures. The extent of damage also standing. Similarly, some highway spans depends on the density of population and collapsed, while others nearby did not

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Volcanoes and Predicting Earthquakes Earthquakes he goal of earthquake prediction is to give warning of potentially arthquakes are associated with volcanic eruptions. Abrupt Tdamaging earthquakes early increases in earthquake activity enough to allow appropriate response to E the disaster, enabling people to minimize heralded eruptions at Mount St. Helens, Washington; Mount Spurr and Redoubt loss of life and property. The U.S. Volcano, Alaska; and Kilauea and Mauna Geological Survey conducts and sup­ Loa, Hawaii. The location and move­ ports research on the likelihood of future ment of swarms of tremors indicate the earthquakes. This research includes field, movement of magma through the vol­ laboratory, and theoretical investigations cano. Continuous records of seismic and of earthquake mechanisms and fault tiltmeter (a device that measures ground zones. A primary goal of earthquake tilting) data are maintained at U.S. research is to increase the reliability of Geological Survey volcano observatories earthquake probability estimates. in Hawaii, Alaska, California, and the Ultimately, scientists would like to be able to specify a high probability for a Cascades, where study of these records enables specialists to make short-range specific earthquake on a particular fault predictions of volcanic eruptions. These within a particular year. Scientists esti­ warnings have been especially effective mate earthquake probabilities in two in Alaska, where the imminent eruption ways: by studying the history of large of a volcano requires the rerouting of earthquakes in a specific area and the international air traffic to enable air­ rate at which strain accumulates in the planes to avoid volcanic clouds. Since rock. 1982, at least seven jumbo jets, carrying Scientists study the past frequency more than 1,500 passengers, have lost of large earthquakes in order to deter­ power in the air after flying into clouds mine the future likelihood of similar of volcanic ash. Though ah1 flights were large shocks. For example, if a region able to restart their engines eventually has experienced four magnitude 7 or and no lives were lost, the aircraft suf­ larger earthquakes during 200 years of fered damages of tens of millions of dol­ recorded history, and if these shocks lars. As a result of these close calls, an occurred randomly in time, then scien­ international team of volcanologists, tists would assign a 50 percent probabili­ meteorologists, dispatchers, pilots, and ty (that is, just as likely to happen as not controllers have begun to work together to happen) to the occurrence of another to alert each other to imminent volcanic magnitude 7 or larger quake in the eruptions and to detect and track vol­ region during the next 50 years. canic ash clouds. But in many places, the assumption of random occurrence with time may not be true, because when the strain is released along one part of the fault sys­ tem, it may actually increase on another part. Four magnitude 6.8 or larger earth­ A sudden increase in earthquake tremors signaled the beginning of a series of eruptions at Redoubt quakes and many magnitude 6-6.5 Volcano in 1989-90. shocks occurred in the San Francisco

17 Bay region during the 75 years between and slip to a new position. Scientists 1836 and 1911. For the next 68 years measure how much strain accumulates (until 1979), no earthquakes of magni­ along a fault segment each year, how tude 6 or larger occurred in the region. much time has passed since the last Beginning with a magnitude 6.0 shock in earthquake along the segment, and how 1979, the earthquake activity in the much strain was released in the last region increased dramatically; between earthquake. This information is then 1979 and 1989, there were four magni­ used to calculate the time required for tude 6 or greater earthquakes, including the accumulating strain to build to the the magnitude 7.1 Loma Prieta earth­ level that results in an earthquake. This quake. This clustering of earthquakes simple model is complicated by the fact leads scientists to estimate that the prob­ that such detailed information about ability of a magnitude 6.8 or larger earth­ faults is rare. In the United States, only quake occurring during the next 30 years the San Andreas fault system has ade­ in the San Francisco Bay region is about quate records for using this prediction 67 percent (twice as likely as not). method. Another way to estimate the likeli­ Both of these methods, and a wide hood of future earthquakes is to study array of monitoring techniques, are being how fast strain accumulates. When plate tested along part of the San Andreas movements build the strain in rocks to a fault. For the past 150 years, earthquakes critical level, like pulling a rubber band of about magnitude 6 have occurred an too tight, the rocks will suddenly break average of every 22 years on the San

Using a two-color laser to detect movement along a fault near Parlfied, California.

18 Andreas fault near Parkfield, California. after the expected earthquake; to issue a The last shock was in 1966. Because of short-term prediction; and to develop the consistency and similarity of these effective methods of communication earthquakes, scientists have started an between earthquake scientists and com­ experiment to "capture" the next Park- munity officials responsible for disaster field earthquake. A dense web of moni­ response and mitigation. This project has toring instruments was deployed in the already made important contributions to region during the late 1980s. The main both earth science and public policy. goals of the ongoing Parkfield Earth­ quake Prediction Experiment are to re­ cord the geophysical signals before and

San Andreas fault in the Carrizo Plain, central California.

19 Scientific understanding of earthquakes is of vital importance to the Nation. As the population increases, expanding urban development and construction works encroach upon areas susceptible to earth­ quakes. With a greater understanding of the causes and effects of earthquakes, we may be able to reduce damage and loss of life from this destructive phenomenon.

U.S. Geological Survey Information Services P.O. Box 25286 Denver, CO 80225

A statue of Christ at Cemetery Hill overlooks the ruined town ofYungay, Peru. It and a few palm trees were all that remained standing after the May 31,1970, earthquake.

20 H U.S. GOVERNMENT PRINTING OFFICE: 1998-674-728 :££9 TT^yfe.. As the Nation's principal conservation agency, the Department of the Interior JSasgm has responsibility for most of our nationally owned public lands and natural r" ' 'Y^a and cultural resources. This includes fostering sound use of our land and water 5*\rJ / resources; protecting our fish, wildlife, and biological diversity; preserving the ^~J%/ environmental and cultural values of our national parks and historical places; rt*&' and providing for the enjoyment of life through outdoor recreation. The Department assesses our energy and mineral resources and works to ensure that their development is in the best interests of all our people by encouraging stewardship and citizen participation in their care. The Department also has a major responsibility for American Indian reservation com­ munities and for people who live in island territories under U.S. administration.

(Cover photographs, clockwise from top left): Mexico City, Mexico, 1985; Coalinga, Calif., 1983;Northridge, Calif., 1994; Anchorage, Alaska, 1964; San Francisco, Calif., 1906; Loma Prieta, Calif, 1989.