Proc. Natl. Acad. Sci. USA Vol. 93, pp. 3732-3739, April 1996 Colloquium Paper

This paper was presented at a colloquium entitled "Earthquake Prediction: The Scientific Challenge," organized by Leon Knopoff (Chair), Keiiti Aki, Clarence R. Alien, James R. Rice, and Lynn R. Sykes, held February 10 and 11, 1995, at the National Academy of Sciences in Irvine, CA. Intermediate- and long-term earthquake prediction (earthquake precursors/California tectonics/earthquake statistics/seismology) LYNN R. SYKES Lamont-Doherty Earth Observatory and Department of Geological Sciences, Columbia University, Palisades, NY 10964 ABSTRACT Progress in long- and intermediate-term on those large shocks that break the entire downdip width (W) earthquake prediction is reviewed emphasizing results from ofthe seismogenic zone, i.e., the shallow part ofthe lithosphere California. Earthquake prediction as a scientific discipline is that undergoes brittle deformation (Fig. 1). Large earthquakes still in its infancy. Probabilistic estimates that segments of are sometimes called delocalize, bounded, characteristic, or several faults in California will be the sites of large shocks in plate-rupturing events. Small (i.e., unbounded or localized the next 30 years are now generally accepted and widely used. shocks) rupture only a portion of W. Several examples are presented of changes in rates of mod- Large California earthquakes include the 1906 San Fran- erate-size earthquakes and seismic moment release on time cisco, 1989 Loma Prieta, 1992 Landers, and 1966 Parkfield scales of a few to 30 years that occurred prior to large shocks. shocks. The latter is among the smallest earthquakes that A distinction is made between large earthquakes that rupture rupture the entire width W and, hence, is regarded as large in the entire downdip width ofthe outer brittle part ofthe earth's my terminology. The recent Kobe earthquake in Japan also crust and small shocks that do not. Large events occur ruptured the entire width of a major strike-slip fault (2). The quasi-periodically in time along a fault segment and happen terms large and small are not synonymous with damaging or much more often than predicted from the rates ofsmall shocks lack of damage. A number of small earthquakes have resulted along that segment. I am moderately optimistic about improv- in considerable damage and loss of life when they are located ing predictions of large events for time scales of a few to 30 close to population centers, occur at shallow depth, and shake years although little work of that type is currently underway structures with little or no earthquake resistance. Large earth- in the United States. Precursory effects, like the changes in quakes in remote regions often result in little damage. stress they reflect, should be examined from a tensorial rather The frequency-size relationship differs for small and large than a scalar perspective. A broad pattern of increased earthquakes (1). The transition from small to large events numbers ofmoderate-size shocks in southern California since occurs at about moment magnitude (Mw) 7.5 for earthquakes 1986 resembles the pattern in the 25 years before the great along plate boundaries of the subduction type but at only Mw 1906 earthquake. Since it may be a long-term precursor to a 5.9 for transform faults like the San Andreas (1). This differ- great event on the southern San Andreas fault, that area ence is mainly accounted for by the shallow dip of the plate deserves detailed intensified study. interface at subduction zones, the very steep dip of transform (strike-slip) faults, and the cooling effect of the downgoing In the mid 1960s, earthquake prediction emerged as a respect- plate at subduction zones. W typically extends from at or near able scientific problem in the United States. Although a major the surface to depths of only 10-20 km for strike-slip faults in effort to monitor the San Andreas fault in California and the California and from depths of 10-50 km for interplate thrust Alaska-Aleutian seismic zone was recommended after the events at subduction zones. great Alaskan earthquake of 1964, the war in Vietnam diverted In terms of phenomena that change prior to large earth- funds that might have been used for prediction. While the quakes, I emphasize seismic precursors. In California, seismic U.S.S.R., Japan, and China had started major programs in monitoring is more extensive than other types of geophysical prediction by 1966, very little work on the subject commenced or geochemical measurements, and the record of instrumen- in the United States until the mid to late 1970s. I have been tally recorded shocks extends back nearly 100 years. Higher involved in work on earthquake prediction and its plate stresses and larger changes in stress probably occur along fault tectonic basis and on studies of the space-time properties of zones at depths greater than several kilometers where in situ large earthquakes for about 25 years. From 1984 to 1988, Iwas monitoring is either impossible or prohibitively expensive. Chairman of the U.S. National Earthquake Prediction Eval- Earthquakes of a variety of sizes at depths where premonitory uation Council (NEPEC). This paper draws upon those expe- changes are most likely to occur, however, can be studied by riences and tries to summarize progress made in earthquake using data from local seismic networks. prediction on an intermediate term (months to 10 years) and It is my view that many large earthquakes will turn out to be long term (10-30 years). I assess what appear to be fruitful more predictable on intermediate and long time scales than lines of research and monitoring in the United States during small events. If so, this is fortunate since many very damaging the next 20 years. shocks are large by my terminology. I devote considerable Rather than discussing earthquakes on a global basis, I attention to the quasi-periodic nature of large events that emphasize mainly the plate boundary in California where rerupture specific fault segments since that property bears study and monitoring have been underway for many decades whether of some kind is to be and accurate locations of seismic events are available. I focus strongly upon prediction likely Abbreviations: M, earthquake magnitude; Mo, seismic moment; Mw, The publication costs of this article were defrayed in part by page charge moment magnitude; CFF, Coulomb failure function; NEPEC, Na- payment. This article must therefore be hereby marked "advertisement" in tional Earthquake Prediction Evaluation Council; W, downdip width; accordance with 18 U.S.C. §1734 solely to indicate this fact. L, rupture length; N, cumulative number of events. 3732 Downloaded by guest on October 6, 2021 Colloquium Paper: Sykes Proc. Natl. Acad. Sci. USA 93 (1996) 3733 on the fault of Mw 6.8. Not as much is known Surface 2D) Hayward I :.: ...... about the shock of Mw.= -7.2 of 1838 that ruptured the San Andreas fault from just south of San Francisco to opposite San Jose but also is inferred to have ruptured the adjacent Loma Prieta segment to the southeast based on a comparison of shaking at Monterey in 1838 and 1906 (7, 8). Intensity reports (i.e., qualitative descriptions of seismic shaking) become more reliable after 1850. Changes in Rates of Moderate-Size Earthquakes. The fre- of moderate-size herein taken to be events of FIG. Two of and L is quency shocks, 1. types earthquakes-small large. rupture - strike of fault; W is its width 5 M < 7, where M is earthquake magnitude, has varied by length along downdip (1). as much as a factor of 20 in the Bay area during the past 150 feasible. I criticize the view (3-5) that large shocks, like small, years (6, 12, 13). From 1882 until the great 1906 shock, activity are strongly clustered, not quasi-periodic. Clearly, large shocks was very high along faults in the area out to about 75 km from are not strictly periodic. I think the important questions are those segments of the San Andreas fault that ruptured sub- how predictable and how chaotic are large shocks and on what sequently in 1906 (Fig. 2A). Those moderate-size events are time-space scales? In this review I exclude short-term predic- well enough located based on intensity reports that most, and tion (time scales of hours to months) since very little progress perhaps all, occurred on faults other than the San Andreas. has been made in that area. For lack of space I also exclude the The northernmost event in Fig. 2A, however, is not well Parkfield prediction experiment and failure of predictions enough located to ascertain on which fault it occurred. Mod- made for that area. erate activity dropped off dramatically after 1906 and re- mained low until about 1955 (Fig. 2B). Earthquakes in the San Francisco Bay Area Sykes and Nishenko (8) remarked in 1984 that moderate activity increased to the southeast of San Francisco from 1955 Several large earthquakes according to the terminology used to 1982 but in a smaller region than in the 25 years preceding herein have occurred in the San Francisco Bay area (Fig. 2) the 1906 earthquake. They concluded that that pattern might since 1836. Of those events, the greatest amount of informa- represent a long-term precursor to a future event of M = 7.0 tion is available (6-10) for the great (Mw 7.7) 1906 earthquake along the southern 75 km of the San Andreas fault of Fig. 3. that ruptured a 430-km portion of the San Andreas fault (Fig. That pattern became better developed from 1982 to 1989 (Fig. 2A), the 1989 Loma Prieta shock ofMw 6.9 that broke a 40-km 2C). The 1989 earthquake, the first large event to occur on the segment of that fault (Figs. 2C and 3), and the 1868 event (Fig. San Andreas fault in the San Francisco Bay area since 1906, was centered along that fault segment (Figs. 2C and 3). A 124°W 121° 124°W similar of moderate occurred from 1855 to 39.5° pattern activity N 1868 in the area surrounding the coming 1868 shock on the Hayward fault (Fig. 2D). Moderate-sized events shut off in the region after 1868 and did not resume for 13 years. The patterns of activity that stand out strongly in Fig. 2 are 38.5° increased rates of moderate-size shocks in the 20-30 years preceding the three large events. The size of the region of increased activity appears to scale with the length of the rupture zone of the coming large event (Fig. 2), being much 37.5° longer for the 1906 earthquake. Moderate activity decreased greatly after the 1868 and 1906 shocks. It is reasonable to ask if these changes are an artifact of either differing methods of determining M or the completeness of catalogs. The record is 36.5° complete for M - 5 since 1910 and, except in the far northern part of Fig. 2, for M > 5.5 since 1850 (6, 14, 15). The values of M prior to 1906, which are based mainly on the sizes of the felt areas of shocks, are probably underestimated with respect to more recent instrumental values (13). Thus, the large number of events in Fig. 2A prior to the 1906 shock is not an artifact of overestimating M. Most of the changes in frequency of occurrence of earthquakes in the Bay area are confined to moderate-size events. The rate of smaller earthquakes in the entire area has remained nearly constant (13). Changes in Rate of Release of Seismic Moment. Looking for changes in the cumulative number (N) of events -M, as in the previous section, suffers from the fact that small changes in the 361955-1989 9 determination of M near the lower cutoff used can affect N at 31245124°w 121o about a factor of 1.5. Since the number of small earthquakes in a follows the FIG. 2. Distribution of earthquakes of magnitude (M) 5 or larger large region relationship in San Francisco Bay area for four time intervals (6). Major active N = A - bM [1] faults are shown. Solid circles, epicenters of 1906, 1989, and 1868 log earthquakes; heavy solid lines, rupture zones of those three large and b is close to about half of the cumulative numbers of shocks; dashed lines enclose events taken to be within their precursory 1.0, areas. Arrows in A denote sense of strike-slip motion along San events are found between M and M + 0.3. Most of the seismic Andreas fault. Note very low activity near most of 1906 rupture zone moment (Mo) released in a region, however, is contained in the from 1920 to 1954 and higher activity in periods before large three few largest earthquakes. The cumulative moment release as a events. function of time, ;Mo, is not very sensitive to the lower cutoff Downloaded by guest on October 6, 2021 3734 Colloquium Paper: Sykes Proc. Natl. Acad. Sci. USA 93 (1996) A. 01/01/69 to 10/17/89 Crystal Springs San Juan San Francisco Reservoir Portola Valley Loma Prieta Bautista A A tA' _Q t t * v 0 . a 114t; 0 I K ol' o.. a I -10 P''- . .." 0a .. °.' g ' 0* - _.. *o*.0 0 a ° - o < 0 a LU 0O , . c C)Q -20

I · r r-- -

S.F. PENINSULA SEGMENT, WORKING GROUP (1988) I SSCM SEGMENT, WORKING GROUP (1988) j SYKES AND NISHENKO (1984) LINDH (1983) J B. Loma Prieta mainshock and aftershocks

A A' 0

Ir -10

LU Q -20

160 140 120 100 80 60 40 20 0 DISTANCE (KM) FIG. 3. Seismicity along the San Andreas fault, 1969-1989, from north of San Francisco at left to San Juan Bautista at right (10). The size of symbols increases with magnitude. (A) Brackets indicate fault segments forecast by various authors as discussed in text. SSCM is Southern Santa Cruz Mountains segment of fault. (B) Rectangles give location of 1989 rupture zone as inferred from geodetic data (11). Distance is measured to northwest along fault from San Juan Bautista. in M but is to the values of Mo for the largest few events to a fault segment being a so-called asperity (i.e., a difficult sampled. place to rupture) and hence to its being a segment with a long -Mo was computed for shocks of M - 5 prior to the three repeat time and particularly large M. Nevertheless, a quanti- large events in Fig. 2 and before the Mw 6.0 earthquake of 1948 tative understanding of why a segment ruptures with a certain in southern California (6). YMo was calculated only for shocks Mw is lacking. This is an area in which considerable progress within the precursory areas outlined in Fig. 2. Those areas could be made in understanding fault mechanics during the were chosen qualitatively to include most of the region in next 20 years. which major changes in activity occur with time. They extend The term "characteristic earthquake" has been used in out to about the same distance in Fig. 2C where the rate of various ways in the literature to describe the slip behavior of small shocks was found to differ significantly before and after large shocks. One view based on detection of prehistoric the 1989 earthquake (16, 17). .Mo increases nearly exponen- earthquakes in trenches was that large events are nearly exact tially with time prior to each of those four large earthquakes duplicates of previous prehistoric events in terms of displace- with a time constant T of 4-11 years. ment as a function of distance (L) along a fault (19). Such Thus, the release ofMo, like the frequency of moderate-size models suffer, however, from cumulative displacement over events, is concentrated in the latter part of the time interval many cycles of large shocks being nonuniform along a major between large shocks along a given fault segment. Several fault, a violation of the idea that long-term plate motion is other examples of high rates of moderate activity preceding nearly uniform along strike. One of the common features of large earthquakes are given in ref. 6. Thus, changes in N and large earthquakes along plate boundaries is that a fault seg- Mo qualify as intermediate- to long-term seismic precursors. ment may break by itself one time but in conjunction with one Probabilities of Large Shocks Along Segments of San or more adjacent segments another time. The Loma Prieta Andreas Fault. During the last 15 years, a consensus has segment of the San Andreas fault ruptured by itself in 1989, developed among workers studying the San Andreas fault that with the adjacent Peninsular segment in 1838, and with yet stresses are built up as a result of the relative motion of the several additional segments to the north in 1906 (7, 8). This Pacific and North American plates and that fault segments that type of behavior is also common for large thrust earthquakes ruptured a relatively small amount in their last large earth- at subduction zones. This undoubtedly contributes to varia- quake are more likely to rerupture sooner than segments that tions in individual repeat times of large events. Thus, the idea experienced relatively large displacement (8-10, 15, 18). For that large shocks are like preceding ones in rupturing the same example, the Parkfield segment of the San Andreas fault has fault segment and in having the same displacement as a ruptured historically about every 22 years in shocks of about function of L is clearly not correct. Mw 6 with an average displacement of 0.5-1.0 m. Some other Another hypothesis is that a given fault segment ruptures in segments of the San Andreas rupture in shocks of M,w 7.5 a large event with a certain "characteristic displacement" that with displacements of several meters and repeat times of differs from one fault segment to another but remains the same 100-400 years. Changes in fault strike, presence of a major for that segment regardless of whether it breaks alone or in compressive (transpressive) fault step, relatively low fluid conjunction with another segment (20). This model permits pressures at depth, and unusually large W probably contribute repeat times to differ among segments but for the cumulative Downloaded by guest on October 6, 2021 Colloquium Paper: Sykes Proc. Natl. Acad. Sci. USA 93 (1996) 3735

displacement along a fault to be the same when averaged over made by others to the director of the U.S. Geological Survey. many cycles of large earthquakes. A variation of this hypothesis He appointed a "working group" of scientists to assess the is the time-predictable model, wherein the displacement in probabilities of large and damaging earthquakes in California successive events varies by as much as 1.5-2 and the time and asked NEPEC to review its report prior to publication in interval between large shocks is proportional to the slip in the 1988 (9). That study was updated for the Bay area in 1990 (10) event that precedes it. Laboratory studies of frictional sliding and for southern California in 1995 (17). Each study assigned on precut rock surfaces lend support to this model in that the 30-year probabilities to each fault segment considered. stress level just before large slip events is a constant and the For the southernmost 90 km of the 1906 fault break, the 1988 time interval to the next shock is proportional to the stress drop Working Group adopted a compromise position between the (or displacement) in the preceding event. results obtained from surface displacements and geodetic Lindh (18) and Sykes and Nishenko (8) performed the first data. Relatively little attention was paid to the question of time-varying probabilistic estimates that segments of four whether either the 1838 or 1865 shocks broke the southern part active faults in California would be sites of large earthquakes of that zone. They assigned a probability of 0.2 for the entire during 30- and 20-year periods, respectively. For large earth- 90-km segmentbreaking in an M 7 event. They made a separate quakes along each segment, their methodology involved iden- calculation, however, for the southernmost 35-km segment of tifying the date of the last event, the average and SD of that zone (denoted SSCM in Fig. 3) for which they assigned a recurrence times, and an estimate of M for each segment. In 0.3 probability of its being the site of an M 6.5 earthquake. both papers, wide use was made of the time-predictable model Evaluation ofpredictions. Of the various long-term predic- and of time intervals between large historic and prehistoric tions, the prediction of Lindh (18) comes closest to forecasting earthquakes. the length of rupture L for the 1989 earthquake and its location Long-Term Forecasts of 1989 Earthquake. In the few years (Fig. 3) and in assigning a relative high 30-year probability. His preceding the 1989 shock, a consensus had developed that the predicted magnitude of 6.5, however, was significantly smaller southern 75-90 km of the 1906 rupture zone, which had than the Mw 6.9 of the event itself. The predicted Mw of 7.0 of experienced smaller slip than that to the north of San Fran- Sykes and Nishenko (8) was more accurate; their average cisco in 1906, was more likely to rupture sooner than other 20-year probability was relatively large but their predicted L, segments and in an earthquake of smaller M than that of 1906. 75 km, was too large. The latter discrepancy is reduced While most workers focused upon approximately the region somewhat if the 12-km rupture zone of the 1990 Chittenden that ruptured in 1989, estimates of its size and probability of earthquake (Fig. 3), which extended the 1989 rupture to the rupture varied substantially. southeast, is added to that of 40 km for the 1989 shock, as Lindh (18) estimated a 30-year probability of 47-83% for a determined from geodetic data and the distribution of early 45-km segment of Fig. 3 rupturing in an event of M 6.5. In aftershocks. talking to him and other U.S. Geological Survey scientists While refs. 9 and 22 forecast an event of M 7, their 30-year prior to the 1989 earthquake, it was clear to me that they probabilities were low and their forecast of 90 km for L was too believed that segment had ruptured in an event ofM 6.5 in 1865 large. I take those forecasts to be incorrect. Likewise, the and the 1838 shock had not broken the southernmost 50 km or rupture zone predicted by the 1988 Working Group (9) for the so of the 1906 rupture zone. Sykes and Nishenko (8) argued, SSCM segment only overlaps half of that of the 1989 shock; its however, that the 1838 shock ruptured that segment and used predicted Mwas too small, and the 30-year probability was only both the time interval 1906-1838 and estimates of slip along 0.3. I agree with Savage (24) that the latter prediction is of that segment in 1906 to calculate a high probability of rupture doubtful validity in terms of forecasting the 1989 event. He is in an event ofMw 7.0 for the period 1983-2003. They indicated incorrect, however, in calling his own paper "Criticisms of a large uncertainty, however, in their probability estimates. A Some Forecasts of the National Earthquake Prediction Coun- subsequent comparison of felt areas for the 1865 and 1989 cil" (24). NEPEC did review the report of the working group shocks indicates that the former did not occur along the San (9) in terms of its general scientific validity but NEPEC itself Andreas fault (7). does not make predictions. Considerable debate ensued from 1984 until the Loma While the title of the summary article in Science on the 1989 Prieta earthquake in October 1989 about the amount of earthquake by staff of the U.S. Geological Survey (25) refers displacement in 1906 along the southernmost 75 km of the to it as "an anticipated event," the predictions of the two 1906 rupture zone. The likelihood of a large event was debated earliest papers (8, 18) were more accurate than the consensus at meetings of NEPEC and in the literature (21, 22). Scholz estimates of the 1988 Working Group. Prior to the event (21) argued that the segment had a more east-west trend (i.e., responsible agencies of the federal and state governments was a transpressional feature) and slipped only 1-1.4 m in 1906 installed little additional monitoring equipment and took few compared to the 2.5-4 m typical of rupture on the Peninsular measures to mitigate the effects of a large earthquake. segment to the northwest. By using geodetic data from before Improvements in understanding in hindsight. All of the long- and after the 1906 shock, Thatcher and Lisowski (22) argued term predictions made prior to the 1989 event assumed the that slip in 1906 along the entire southernmost 90-km segment same value of W (Fig. 1) for various parts of the San Andreas of Fig. 3 was 2.6 ± 0.3 m and its 30-year probability of rupture, fault in the Bay area. It is clear that the 1989 shock ruptured while high compared to fault segments north of San Francisco, to a greater depth and hence a greater W than was assumed in was low for the remainder of the 20th century. those calculations. That could, in fact, have been anticipated During my chairmanship, NEPEC reviewed the long-term from the greater depths of small earthquakes close to the potential of major faults in California. In 1987 I asked mem- Loma Prieta rupture from 1969 to 1989 (Fig. 3). Likewise, it bers of NEPEC to rate fault segments and areas considered to should be expected that large future earthquakes along sec- have a relatively high potential of being sites of large earth- tions of the San Andreas fault with deeper than normal quakes in terms of priority for further instrumentation and activity, such as near San Francisco (Fig. 3) and in southern study (23). I and other members of NEPEC were concerned California near San Gorgonio Pass, will release greater than that a dense monitoring network consisting of a variety of normal Mo per unit length along strike. instruments was deployed in the United States only at Park- Likewise, the 1990 report (10) increased the slip rate field and that such monitoring need to be carried out in several assigned to the Peninsular and Loma Prieta sections ofthe San areas to have a reasonable chance of observing precursors to Andreas fault, leading to smaller calculated repeat times for a large earthquake within a few decades. NEPEC reports its large shocks by using the time-predictable model. The poten- findings about the scientific validity of earthquake predictions tial slip accumulated as strain between 1906 and 1989 [i.e., 83.5 Downloaded by guest on October 6, 2021 3736 Colloquium Paper: Sykes Proc. Natl. Acad. Sci. USA 93 (1996) years x (19 ± 4 mm/year) = 1.6 + 0.3 m]. A reexamination (transpressional) part of the Loma Prieta zone is encountered. of the amount of displacement in 1906 across the fault in They suggest that the orientation of the slip vector in the 1989 Wright tunnel (km 51 in Fig. 3), the only place where slip was event, parallel to the line of intersection of the two fault measured at depth along the southern 75 km of the 1906 segments, was not fortuitous and that it permits slip to occur rupture zone, gives 1.7-1.8 m (26). Assuming the Loma Prieta on the two faults without opening subsurface voids. When segment ruptured in large earthquakes in 1838, 1906, and 1989 isostacy is taken into account, they conclude that observed gives a mean repeat time of 76 ± 11 years. uplift rates are consistent with long-term slip on this section of Inferences from geodetic data. The Loma Prieta benchmark the San Andreas fault occurring in 1989-type events. Thus, the is the only one that was remeasured in the 1880s after the 1865 displacement field of the 1989 earthquake does not appear to and 1868 earthquakes and again soon after the 1906 shock that be anomalous for the geometry of the restraining bend. was close to the fault segment that broke in 1989 (27). While its average displacement in 1906 was 1.2 m, the 95% confi- Large Events as a Quasi-Periodic Process dence limits are 0.35 and 2.0 m (27). Simple dislocation models assuming slip on a vertical San Andreas fault about 3 km from Deficit of Small Shocks Along San Andreas Fault. It has that benchmark give about 2.5 m of slip on that fault segment become increasingly clear in the last decade that an extrapo- when rupture is assumed to extend from 0 to 10 km (27) and lation of Eq. 1 as determined from small earthquakes along a about 2.3 m when it extends to 18 km, the maximum depth of given fault segment seriously underestimates the rate of oc- rupture in 1989. Slip deduced in 1906 depends critically upon currence of large events along the same feature (19, 33, 34). what fault(s) is (are) assumed to have ruptured, uncertainties Thus, large events account for nearly all of the strain energy in the sparse geodetic data and W. Dislocation models using and seismic moment release along that fault segment. This is data from the much denser horizontal geodetic network that demonstrated very clearly for earthquakes along most seg- existed in 1989 (28) yield a displacement for the Loma Prieta ments of the San Andreas fault. For the several decades for benchmark that differs from the observed (7) by a factor of 1.4. which a complete record is available, the rate ofseismic activity Thus, it is clear in retrospect that not as much weight should at the M - 3 level has been at a very low level for those have been given to geodetic data in estimating long-term segments that ruptured in the great historic earthquakes of probabilities. 1812 and 1857 in southern California and for its southernmost Was the 1989 earthquake the event predicted? Many geosci- segment (Fig. 4), which last broke in a great event about 1690 entists were surprised that the 1989 shock did not produce a clear primary break at the earth's surface. That expectation arose from widely published photographs of fences and roads 34K ^\S^~A that were offset in 1906 along those portions of the fault that traverse more level ground farther north and evidence of offset 36' at the surface in several other large California earthquakes. The southern portion of the 1906 rupture zone, however, traverses mountainous terrain and is the site of a major sinistral (i.e., transpressional) fault offset. Since surface area is not conserved as fault displacement accumulates in many large events, it is the site of considerably tectonic complexity and vertical deformation. Primary faulting at the surface along that 34' segment appears to have been as rare in 1906 as in 1989 (29). Inversion of various seismological data sets for the 1989 earthquake led to models that differ in the amount and sense of slip as a function ofL and W(Fig. 1). Most authors assumed, however, the same best-fitting planar rupture surface that was deduced from geodetic data soon after the earthquake (11) 32° and varied only the rake and slip as a function of L and W, not the strike and dip. All four probablyvary in the transpressional offset, and slip probably occurred on more than one fault as judged from aftershocks and vertical displacements in 1989 (14, 30, 31). While one of the inversions of seismic data indicates slip was negligible at depths shallower than 8 km, others do not. I put greater reliance on the loci of aftershocks and the inversion of geodetic data (14, 28, 30-32), which indicate that significant slip in 1989 extended to a shallow depth of 2-5 km and was spatially complex. One extreme model is that the 1906 and 1989 shocks ruptured different faults-the former, a vertical fault from 0 to 10 km, and the latter, a nearby steeply dipping fault from 10 to 18 km (14, 27). Evidence that rupture in 1989 was as shallow as 2-5 km indicates that a small to negligible Wis still available for generating a sizable event at a shallower depth on a steeply dipping fault. An event of Mw 6.5 still could take place in the upper 5 km along a shallow-dipping thrust fault to the north- east of the San Andreas fault (7, 31). Shaw et al. (32) used the distribution of aftershocks of the 1989 focal evidence of defor- event, mechanisms, geological FIG. 4. Earthquakes of M > 5 in southern California from Cali- mation in the last few million years, and balanced cross sections fornia Institute of Technology catalog. (A) 1977 through 1985. (B) to derive models of fault and fold structure at depth in the 1985 through 1994. Thin solid lines, active faults; heavier line (dashed Loma Prieta zone. They conclude that fault strike and dip where multibranched and poorly delineated in San Gorgonio Pass), change from southeast to northwest as the restraining San Andreas fault. Downloaded by guest on October 6, 2021 Colloquium Paper: Sykes Proc. Natl. Acad. Sci. USA 93 (1996) 3737 (9, 20). Similarly, such activity has remained at a very low level Large Earthquakes Are Not a Clustered Process. Davison for that portion of the 1906 rupture zone that did not break in and Scholz (34) examined the frequency of moderate-size 1989 (Fig. 2) and for many decades before 1989 for the Loma earthquakes for segments of the Alaska-Aleutian plate bound- Prieta segment (10, 13, 14). ary and found that the Mo of large segment-rupturing events From 1907 to 1995, no earthquakes of M > 6 have occurred was much higher than predicted from an extrapolation of along the San Andreas fault itself for the entire 430-km length smaller events using Eq. 1. Kagan (5) states that their result was of the rupture zone of the 1906 shock with but one exception, biased by uncertainties in b value, saturation of the magnitude the 1989 earthquake of Mw 6.9. If Eq. 1 were correct, about 8 used, and poor knowledge of repeat times of large events. The events ofM > 6 would be expected to have occurred during the absence of events of M > 6 and the very small number of interval from 1906 to 1989 for the Loma Prieta segment for shocks of M > 5 along the San Andreas from 1906 to 1989, reasonable values of the slope, b (i.e., those close to 1.0), and however, cannot be attributed to those uncertainties. about 80 events ofM > 5. During that period the Loma Prieta A possibility is the 1906 and 1989 events broke either segment experienced one complete cycle of large shocks. different faults separated by a few kilometers or different Low activity along 1989 rupture zone. From 1910, when the depth ranges for the same fault. I argued earlier that both are catalog ofM > 5 becomes complete, until the 1989 shock, only unlikely. Even if each event occurred on a different nearby 10 events of5 M< < 6 occurred along or near the southeastern fault, both involved substantial strike-slip motion and released 75 km (0-75 km in Fig. 3) of the 1906 rupture zone (14, 15). shear strain energy, not from a fault surface, but from a volume Epicentral locations more precise than a few kilometers only of rock that extends outward about 75 km from each rupture become available starting in 1969 (14). Of the 3 events from zone. Hence, the drop in strain energy associated with strike- 1969 until the 1989 mainshock, the 2 Lake Elsman earthquakes slip motion on northwest-trending faults in the Loma Prieta of 1988 and 1989 are well enough located that they clearly region is similar for most of that volume of rock. occurred on a nearby fault, one of steep but opposite dip The hypothesis that large and small events differ in many (northeast) to the one that ruptured in the Loma Prieta shock. of their properties is supported by simple dynamical models of The other, in 1974, occurred well to the east of the San faults (37, 38) that can be run on computers for thousands of Andreas on the Busch fault. Prior to 1960 epicentral locations cycles of large events and by observations of the frequency for that area are more uncertain than 10 km (14). Four of the of occurrence of avalanches of various sizes on a large sandpile remaining 7 events occurred during that period; the other (39). In both cases small events follow a distribution like Eq. three occurred in 1963, 1964, and 1967. A special study of the 1 but large events occur much more often than an extrapola- 1963 shock (35) indicates that it occurred close to the San tion from small events predicts. Andreas fault but southeast of Parajo gap (15 km in Fig. 3) A catalog of global shallow earthquakes ofMw > 7 from 1900 beyond the 1989 rupture zone and along that part of the San to 1990 indicates a change in b value in Eq. 1 from 1.5 for events Andreas where fault creep takes place at the surface and small of Mw 2 7.5 to 1.0 for smaller shocks (1). In that study N was to moderate shocks have been more numerous historically a cumulative count-i.e., the number of events greater than or (14). The 1964 event was well enough located (36) to ascertain equal to Mw. While such a cumulative number, of course, that it occurred east of the 1989 rupture zone. The 1967 shock cannot decrease as Mw is reduced, the number per magnitude ofM 5.6 occurred close to the 1989 rupture zone but no special (or moment) interval does decrease as b changes from 1.5 to study of its aftershocks or mechanism was published. 1.0. The interval distribution for the global catalog exhibits a Thus, of the 10 events of M - 5 from 1910 until the 1989 maximum at Mw 7.5 followed by a minimum at a somewhat mainshock, none occurred on the coming rupture zone itself smaller Mw. Since interplate thrust events dominate the global during the 20 years for which precise locations are available, catalog for Mw > 7, this behavior is appropriate to those types the 1967 shock may have been on it, and large uncertainties of events. The fact that b is about 1.5 for shocks of Mw> 7.5 exist in the locations of four events between 1910 and 1959. does not mean that large thrust events are rare but merely that Hence, 0-5 shocks of M > 5 occurred along the Loma Prieta the distribution of large earthquakes, when summed (1) over rupture zone itself during almost a complete earthquake cycle many different segments ofplate boundaries, fits Eq. 1 but with as opposed to 80 predicted from Eq. 1. Also, the rupture zone a different slope than small events. (It does mean that shocks of the 1989 shock appears to have been very quiet even at the of M 9.5 are rare.) The maximum and minimum in the global level of the smallest earthquakes detected from 1969 until the distribution are not as extreme as for a single fault segment 1989 mainshock (14, 31). since W Mw, and Mo usually differ among segments, resulting Peninsular segment. Likewise, for the Peninsular segment of in the two extrema being smeared out when summed over the San Andreas fault (60-120 km in Fig. 3), an extrapolation many fault segments. of Eq. 1 predicts about 30 events of M > 5.5 and 10 of M > Kagan and Jackson (3) concluded that shocks remaining in 6 between 1838 and 1906, the dates that segment ruptured in several earthquake catalogs (after removal of events involved large events (Mw > 7). The historic record probably is com- in short-term clustering like aftershocks) are characterized by plete for that region for M > 5.5 since about 1850 but not for clustering, not quasi-periodic behavior. They examined the smaller shocks (13, 15). Only 3 events of 5.5 - M < 6 and none Harvard catalog for events of Mw > 6.5, claiming Mw 6.5 is ofM > 6 occurred along that segment from 1850 until the 1906 large enough to be a plate rupturing (i.e., a large) shock. Since earthquake (6, 13, 15). Only a single event ofM > 5 occurred that catalog is dominated by thrust events at convergent plate near that segment since 1910 (Fig. 2). Its mechanism, involving boundaries, however, Mw 7.5 is an appropriate lower bound for mainly dip-slip motion (15), suggests that it was not located on large shocks (1). That and the other catalogs of shallow events the San Andreas fault. Thus, the record of M > 6 for the they examined are dominated by small, not large, events. Thus, 145-year period since 1850 and that of M > 5 since 1910 are the clustering properties that they find are pertinent to the at least a factor of 10 lower than rates predicted from shocks former, but not the latter. of Mw > 7 by using Eq. 1. Some examples of the clustering of large events do exist. Southern California. Fig. 4 shows events ofM> 5 in southern Adjacent segments of a major fault often rupture in large California for a recent 18-year period. Activity was very low events separated by days to years. The 1984 earthquake on the (i.e., a single event) for the San Andreas fault itself. The Calaveras fault and the 1989 Loma Prieta event 30 km from it historic record of the last 100 years indicates similar low levels may be considered clustered events but not on the same fault. of activity for the San Andreas even though the Mojave Individual segments of major faults, however, rarely, if ever, segment to the north of Los Angeles ruptures in large shocks rerupture in large events within a short time. Kagan and about every 130 years (9, 20). Jackson (4) state "earthquakes in the near future will be Downloaded by guest on October 6, 2021 3738 Colloquium Paper: Sykes Proc. Natl. Acad. Sci. USA 93 (1996) similar in location and mechanism to those of the recent past." large earthquake in the Bay area, however, occurs off its That proposition, however, is pertinent to small earthquakes. rupture zone on nearby faults. This is an important lesson for Most, and perhaps all, large events along a given fault segment prediction and a factor that needs to be incorporated in future occur quasi-periodically in time. The fault segments that computer modeling. rupture in large events that I examined are parts of very active These and three other findings make me moderately opti- faults, the main loci of plate motion. Whether large shocks in mistic for intermediate- and long-term prediction. areas of complex multibranched faulting, as in Asia, occur (i) Rates of relative plate motion are virtually constant from quasi-periodically is yet to be ascertained. a few to a few million years. Plate motion is the driving engine that leads to the buildup of elastic stresses that are released in Recent Buildup of Activity in Southern California earthquakes along plate boundaries. Accurate estimates of long-term rates of deformation have become available for Fig. 4 shows earthquakes of M - 5 in southern California for many active faults by using space geodesy. the periods 1977-1985 and 1986-1994. In the first 9-year (ii) The key nonlinearity in the earthquake process appears period, no shocks of that size occurred on or close to the San to be associated with the stick-slip frictional force at fault Andreas fault, while in the second interval, activity occurred interfaces (37). Most of that effect is concentrated during or on both sides of the San Andreas fault along a 200-km-long close to the rupture time of large shocks. Fortunately, socially zone in the transverse ranges and the northern Los Angeles useful prediction needs to be attempted only for the next large basin. That pattern of activity, especially the occurrence of event, not several such shocks into the future where nonlinear several earthquakes of M > 6, resembles that in the 25 years effects become cumulative. A better understanding of the before the 1906 earthquake (Fig. 2A). It was not centered near nature of rupture in the last large event seems crucial to the Landers earthquake. long-term prediction. Better modeling of fault interactions The possibility that the recent pattern of activity is a should permit a choice of which fault segments will rupture long-term precursor to a great earthquake along the southern either separately or together in the next large earthquake along San Andreas fault deserves serious study and debate. Segments the San Andreas fault in southern California. of the fault in that region have not ruptured in great earth- (iii) I foresee progress being made by recognizing that quakes since either 1812 or about 1690 (9, 20). Also, changes precursory processes are tensorial in character, not scalar. in stress generated by the Landers sequence of shocks resulted Stress (and its evolution with time), which is basic to an in portions of the fault in San Gorgonio Pass moving closer to understanding of the earthquake process, is a second-order failure by about 10 years (40). The San Andreas fault under- tensor. This would explain why increases as well as decreases goes a complex compressional left step in San Gorgonio Pass in seismicity have been reported as precursors. While activity that is much larger than that in the Loma Prieta region. Much in most of the San Francisco Bay area decreased greatly soon remains to be learned about the distribution of faults at depth, after the great 1906 earthquake, the moderate activity that did possible changes in seismic activity, the loci of volumes of take place in the next few decades was largely concentrated weak- and strong-rock, fluid pressures, and the state of stress. south and west of the end of the rupture zone (Fig. 2B) in areas An intensified effort is needed to understand that area in that are predicted from dislocation models to have, in fact, detail, which did not happen before the 1989 earthquake. moved closer to failure (15). Damage and loss of life may not be greatest in large events such Moderate-size shocks are modulated in their occurrence by as one on the southern San Andreas. Moderate-size shocks changes in the Coulomb failure function (ACFF) at the time of that are part of the buildup to a great earthquake but located large events (6, 15-17, 31, 40). The ratio of the numbers of closer to centers of population may cause the largest catas- small events before to that after the 1989 earthquake for trophes. The Northridge event of 1994 may turn out to be such individual nearby fault segments changed in accord with an example. predictions of ACFF by using dislocation models (16, 17, 31). These changes occurred for fault segments extending out to Discussion and Summary 75-100 km from the 1989 rupture zone. This corresponds to ACFF > 0.01 to 0.03 MPa, where 1 MPa = 10 bars. Those Time-varying probabilistic estimates of large earthquakes for changes are about a factor of 10 times larger than those segments of several active faults in California are now in their generated by earth tides. Thus, changes in rates of small second generation (10, 20) and are generally accepted and earthquakes may become more useful for prediction when widely used. Debate continues about the width of the proba- individual fault segments are analyzed separately. bility function to use either in general or for specific segments. Changes in the distribution of moderate-size events in the Since those predictions are for 30-year periods, however, the Bay area of the past 150 years can be explained in terms of a probability gain with respect to a random distribution in time drop in stress at the time of large earthquakes and the slow is only about a factor of 1.5-3. Those long-term forecasts, buildup of stress with time. The 1906 shock created a broad which have replaced the earlier seismic gap concepts of the area of reduced shear stress (or CFF) and suppressed mod- 1970s, help to focus scarce scientific resources on specific areas erate activity for many decades until stresses were gradually and conversely to indicate segments unlikely to rupture in the restored by plate motion. The southern Calaveras fault was one next few decades. While progress can be expected over the next of the first to return to the pre-1906 stress level and was the site 20 years in improving those types of forecasts, probability gains of some of the earliest moderate activity prior to 1989 (6, 15, likely will remain less than 5-10. 17). Major changes in the space-time distribution of moderate- The pair of Lake Elsman earthquakes of M 5.3 and 5.4 that size earthquakes have occurred in the San Francisco Bay area occurred 16 and 2.5 months before the 1989 mainshock may be during the time intervals between large shocks. Computer interpreted as an intermediate-term precursor. Both occurred modeling of earthquakes (37, 38) and studies of avalanches on on a fault dippingsteeply to the northeast 22 km from the 1989 a large sandpile (39) show many similarities to those found for rupture zone. Like the 1989 event, they involved strike slip and shocks in the Bay area. All indicate that large events occur reverse slip (14, 31). No other moderate-size events occurred much more frequently than predicted by extrapolating rates of on or near the 1989 rupture zone since the shocks of 1964 and small events. In each case the rate of moderate-size events 1967. As shear stress was restored in the region, some of the increases before large events and the dimension of the region last places to be returned to the pre-1906 level of CFF were of increased activity increases with the size of the coming large faults very close to the San Andreas, like the Lake Elsman event. Much, if not all, of the activity that builds up prior to a fault. Those events resulted in the release of short-term (5 day) Downloaded by guest on October 6, 2021 Colloquium Paper: Sykes Proc. Natl. Acad. Sci. USA 93 (1996) 3739 warnings. While they may have had some value in terms of 7. Tuttle, M. P. & Sykes, L. R. (1992) Bull. Seismol. Soc. Am. 82, public preparedness, they were, in fact, false alarms. 1802-1820. If it had been realized that the Lake Elsman events were on 8. Sykes, L. R. & Nishenko, S. P. (1984) J. Geophys. Res. 89, a nearby but different fault, an intermediate-term warning 5905-5927. would have been more appropriate. They probably indicated 9. Working Group on California Earthquake Probabilities (1988) the return of stresses in that area to pre-1906 levels rather than U.S. Geol. Surv. Open-File Rep. 88-398, 1-62. the initiation of accelerated 10. Working Group on California Earthquake Probabilities (1990) precursory slip on the San An- U.S. Geol. Surv. Circ. 1053, 1-51. dreas fault itself. Another example of an intermediate-term 11. Lisowski, M., Prescott, W. H., Savage, J. C. & Johnson, M. J. S. seismic precursor is the northward growth in aftershock ac- (1990) Geophys. Res. Lett. 17, 1437-1440. tivity in the Joshua Tree earthquake sequence in southern 12. Tocher, D. (1959) Calif. Div. Mines Spec. Rep. 57, 39-48 and California between its mainshock on April 23 and the Landers 125-127. shock of June 28, 1992 (41). While these precursors are subtle 13. Ellsworth, W. L. (1990) U.S. Geol. Surv. Prof. Paper 1515, 153- in character, they, and other examples, indicate that precursory 187. phenomena likely exist on time scales of months to a decade. 14. Olson, J. A. & Hill, D. P. (1993) U.S. Geol. Surv. Prof Paper How earthquake prediction is and has been viewed in the 1550-C, C3-C16. United States has a number of parallels to skepticism about 15. Jaum6, S. C. & Sykes, L. R. (1996)J. Geophys. Res. 101, 765-789. continental drift and paleomagnetism prior to the late 1960s. 16. Reasenberg, P. A. & Simpson, R. W. (1992) Science 255, 1687- Like them, prediction invokes strong views about what prob- 1690. lems are "worth on." 17. Simpson, R. W. & Reasenberg, P. A. (1994) U.S. Geol. Surv. Prof. working Earthquake prediction has Paper 1550-F, F55-F89. suffered in this regard; only 10-20 scientists in the U.S. are 18. Lindh, A. G. (1983) U.S. Geol. Surv. Open-File Rep. 83-63, 1-5. currently working on intermediate-term prediction. Work in 19. Schwartz, D. P. & Coppersmith, K. J. (1984) J. Geophys. Res. 89, prediction also has suffered from a general belief that only 5681-5698. short-term predictions would have social value. While not 20. 1994 Working Group on the Probabilities of Future Large possible now, a well-founded 5-year prediction could be of Earthquakes in Southern California (1995) Bull. Seismol. Soc. greater value since serious mitigation measures could be Am. 85, 379-439. undertaken. 21. Scholz, C. H. (1985) Geophys. Res. Lett. 12, 717-719. Several observations of precursors have turned out upon 22. Thatcher, W. & Lisowski, M. (1987) J. Geophys. Res. 92, 4771- reexamination to be artifacts of either environmental changes 4784. affecting instruments or changes in earthquake catalogs that 23. Schearer, C. F. (1988) U.S. Geol. Surv. Open-File Rep. 88-37, are of human, not natural, origin. A superficial application of 296-300. the ideas of chaos has led some to conclude that 24. Savage, J. (1991) Bull. Seismol. Soc. Am. 81, 862-881. earthquakes 25. U.S. Geological Survey Staff (1990) Science 247, 286-293. are not predictable. Several workers active in studying earth- 26. Prentice, C. S. & Ponti, D. J. (1994) Eos Trans. Am. Geophys. quakes as an example of deterministic chaos, however, are Union 75, 343 (abstr.). moderately optimistic about prediction. Long- and interme- 27. Segall, P. & Lisowski, M. (1990) Science 250, 1241-1244. diate-term prediction are areas where I think progress is 28. Snay, R. A., Neugebauer, H. C. & Prescott, W. H. (1991) Bull. possible in the next 20 years. Much remains to be done in Seismol. Soc. Am. 81, 1647-1659. understanding the physics of earthquakes and the role of fluid 29. Prentice, C. S. & Schwartz, D. P. (1991) Bull. Seismol. Soc. Am. pressures at depth in fault zone and in deploying dense 81, 1424-1479. networks of a variety of observing instruments. 30. Marshall, G. A., Stein, R. S. & Thatcher, W. (1991) Bull. Seismol. Soc. Am. 81, 1660-1693. I thank J. Deng, S. Jaum6, C. Scholz, and B. Shaw for critical 31. Seeber, L. & Armbruster, J. G. (1990) Geophys. Res. Lett. 17, comments and discussions. This workwas supportedbygrants from the 1425-1428. U.S. Geological Survey, the National Science Foundation, and the 32. Shaw, J. H., Bischke, R. & Suppe, J. (1994) U.S. Geol. Surv. Prof. Southern California Earthquake Center (SCEC). This is Lamont- Paper 1550-F, F3-F21. Doherty Earth Observatory Contribution 5486 and SCEC contribu- 33. Wesnousky, S. G., Scholz, C. H., Shimazaki, K. & Matsuda, T. tion 319. (1984) Bull. Seismol. Soc. Am. 74, 687-708. 34. Davison, F. 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