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

This paper was presented at a colloquium entitled "Earthquake Prediction: The Scientific Challenge," organized by LeonKnopoff (Chair), Keiiti Aki, Clarence R. Alien, James R. Rice, and Lynn R. Sykes, held February 10 and 11, 1995, atthe 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-termearthquake prediction is reviewed emphasizing results fromCalifornia. Earthquake prediction as a scientific discipline isstill in its infancy. Probabilistic estimates that segments ofseveral faults in California will be the sites of large shocks inthe next 30 years are now generally accepted and widely used.Several examples are presented of changes in rates of mod-erate-size earthquakes and seismic moment release on timescales of a few to 30 years that occurred prior to large shocks.A distinction is made between large earthquakes that rupturethe entire downdip width of the outer brittle part ofthe earth'scrust and small shocks that do not. Large events occurquasi-periodically in time along a fault segment and happenmuch more often than predicted from the rates ofsmall shocksalong that segment. I am moderately optimistic about improv-ing predictions of large events for time scales of a few to 30years although little work of that type is currently underwayin the United States. Precursory effects, like the changes instress they reflect, should be examined from a tensorial ratherthan a scalar perspective. A broad pattern of increasednumbers of moderate-size shocks in southern California since1986 resembles the pattern in the 25 years before the great1906 earthquake. Since it may be a long-term precursor to agreat event on the southern San Andreas fault, that areadeserves detailed intensified study.

In the mid 1960s, earthquake prediction emerged as a respect-able scientific problem in the United States. Although a majoreffort to monitor the San Andreas fault in California and theAlaska-Aleutian seismic zone was recommended after thegreat Alaskan earthquake of 1964, the war in Vietnam divertedfunds that might have been used for prediction. While theU.S.S.R., Japan, and China had started major programs inprediction by 1966, very little work on the subject commencedin the United States until the mid to late 1970s. I have beeninvolved in work on earthquake prediction and its platetectonic basis and on studies of the space-time properties oflarge earthquakes for about 25 years. From 1984 to 1988, IwasChairman of the U.S. National Earthquake Prediction Eval-uation Council (NEPEC). This paper draws upon those expe-riences and tries to summarize progress made in earthquakeprediction on an intermediate term (months to 10 years) andlong term (10-30 years). I assess what appear to be fruitfullines of research and monitoring in the United States duringthe next 20 years.Rather than discussing earthquakes on a global basis, I

emphasize mainly the plate boundary in California wherestudy and monitoring have been underway for many decadesand accurate locations of seismic events are available. I focus

on those large shocks that break the entire downdip width (W)of the seismogenic zone, i.e., the shallow part of the lithospherethat undergoes brittle deformation (Fig. 1). Large earthquakesare sometimes called delocalize, bounded, characteristic, orplate-rupturing events. Small (i.e., unbounded or localizedshocks) rupture only a portion of W.

Large California earthquakes include the 1906 San Fran-cisco, 1989 Loma Prieta, 1992 Landers, and 1966 Parkfieldshocks. The latter is among the smallest earthquakes thatrupture the entire width W and, hence, is regarded as large inmy terminology. The recent Kobe earthquake in Japan alsoruptured the entire width of a major strike-slip fault (2). Theterms large and small are not synonymous with damaging orlack of damage. A number of small earthquakes have resultedin considerable damage and loss of life when they are locatedclose to population centers, occur at shallow depth, and shakestructures with little or no earthquake resistance. Large earth-quakes in remote regions often result in little damage.The frequency-size relationship differs for small and large

earthquakes (1). The transition from small to large eventsoccurs at about moment magnitude (Mw) 7.5 for earthquakesalong plate boundaries of the subduction type but at only Mw5.9 for transform faults like the San Andreas (1). This differ-ence is mainly accounted for by the shallow dip of the plateinterface at subduction zones, the very steep dip of transform(strike-slip) faults, and the cooling effect of the downgoingplate at subduction zones. W typically extends from at or nearthe surface to depths of only 10-20 km for strike-slip faults inCalifornia and from depths of 10-50 km for interplate thrustevents at subduction zones.

In terms of phenomena that change prior to large earth-quakes, I emphasize seismic precursors. In California, seismicmonitoring is more extensive than other types of geophysicalor geochemical measurements, and the record of instrumen-tally recorded shocks extends back nearly 100 years. Higherstresses and larger changes in stress probably occur along faultzones at depths greater than several kilometers where in situmonitoring is either impossible or prohibitively expensive.Earthquakes of a variety of sizes at depths where premonitorychanges are most likely to occur, however, can be studied byusing data from local seismic networks.

It is my view that many large earthquakes will turn out to bemore predictable on intermediate and long time scales thansmall events. If so, this is fortunate since many very damagingshocks are large by my terminology. I devote considerableattention to the quasi-periodic nature of large events thatrerupture specific fault segments since that property bearsstrongly upon whether prediction of some kind is likely to be

Abbreviations: M, earthquake magnitude; Mo, seismic moment; Mw,moment magnitude; CFF, Coulomb failure function; NEPEC, Na-tional Earthquake Prediction Evaluation Council; W, downdip width;L, rupture length; N, cumulative number of events.

The publication costs of this article were defrayed in part by page chargepayment. This article must therefore be hereby marked "advertisement" inaccordance with 18 U.S.C. §1734 solely to indicate this fact.

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SurfaceI :.: ... ..........................................................=

FIG. 1. Two types of earthquakes-small and large. L is rupturelength along strike of fault; W is its downdip width (1).feasible. I criticize the view (3-5) that large shocks, like small,are strongly clustered, not quasi-periodic. Clearly, large shocksare not strictly periodic. I think the important questions arehow predictable and how chaotic are large shocks and on whattime-space scales? In this review I exclude short-term predic-tion (time scales of hours to months) since very little progresshas been made in that area. For lack of space I also exclude theParkfield prediction experiment and failure of predictionsmade for that area.

Earthquakes in the San Francisco Bay Area

Several large earthquakes according to the terminology usedherein have occurred in the San Francisco Bay area (Fig. 2)since 1836. Of those events, the greatest amount of informa-tion is available (6-10) for the great (Mw 7.7) 1906 earthquakethat ruptured a 430-km portion of the San Andreas fault (Fig.2A), the 1989 Loma Prieta shock ofMw 6.9 that broke a 40-kmsegment of that fault (Figs. 2C and 3), and the 1868 event (Fig.

39.5°N

38.5°

37.5°

36.5°

124°W 121° 124°W

361955-1989 9

31245 121124°w 121o

FIG. 2. Distribution of earthquakes of magnitude (M) 5 or largerin San Francisco Bay area for four time intervals (6). Major activefaults are shown. Solid circles, epicenters of 1906, 1989, and 1868earthquakes; heavy solid lines, rupture zones of those three largeshocks; dashed lines enclose events taken to be within their precursoryareas. Arrows in A denote sense of strike-slip motion along SanAndreas fault. Note very low activity near most of 1906 rupture zonefrom 1920 to 1954 and higher activity in periods before large threeevents.

2D) on the Hayward fault of Mw 6.8. Not as much is knownabout the shock of Mw -7.2 of 1838 that ruptured the SanAndreas fault from just south of San Francisco to opposite SanJose but also is inferred to have ruptured the adjacent LomaPrieta segment to the southeast based on a comparison ofshaking at Monterey in 1838 and 1906 (7, 8). Intensity reports(i.e., qualitative descriptions of seismic shaking) become morereliable after 1850.Changes in Rates of Moderate-Size Earthquakes. The fre-

quency of moderate-size shocks, herein taken to be events of5 - M < 7, where M is earthquake magnitude, has varied byas much as a factor of 20 in the Bay area during the past 150years (6, 12, 13). From 1882 until the great 1906 shock, activitywas very high along faults in the area out to about 75 km fromthose segments of the San Andreas fault that ruptured sub-sequently in 1906 (Fig. 2A). Those moderate-size events arewell enough located based on intensity reports that most, andperhaps all, occurred on faults other than the San Andreas.The northernmost event in Fig. 2A, however, is not wellenough located to ascertain on which fault it occurred. Mod-erate activity dropped off dramatically after 1906 and re-mained low until about 1955 (Fig. 2B).

Sykes and Nishenko (8) remarked in 1984 that moderateactivity increased to the southeast of San Francisco from 1955to 1982 but in a smaller region than in the 25 years precedingthe 1906 earthquake. They concluded that that pattern mightrepresent a long-term precursor to a future event ofM = 7.0along the southern 75 km of the San Andreas fault of Fig. 3.That pattern became better developed from 1982 to 1989 (Fig.2C). The 1989 earthquake, the first large event to occur on theSan Andreas fault in the San Francisco Bay area since 1906,was centered along that fault segment (Figs. 2C and 3). Asimilar pattern of moderate activity occurred from 1855 to1868 in the area surrounding the coming 1868 shock on theHayward fault (Fig. 2D). Moderate-sized events shut off in theregion after 1868 and did not resume for 13 years.The patterns of activity that stand out strongly in Fig. 2 are

increased rates of moderate-size shocks in the 20-30 yearspreceding the three large events. The size of the region ofincreased activity appears to scale with the length of therupture zone of the coming large event (Fig. 2), being muchlonger for the 1906 earthquake. Moderate activity decreasedgreatly after the 1868 and 1906 shocks. It is reasonable to askif these changes are an artifact of either differing methods ofdeterminingM or the completeness of catalogs. The record iscomplete forM - 5 since 1910 and, except in the far northernpart of Fig. 2, forM > 5.5 since 1850 (6, 14, 15). The values ofM prior to 1906, which are based mainly on the sizes of the feltareas of shocks, are probably underestimated with respect tomore recent instrumental values (13). Thus, the large numberof events in Fig. 2A prior to the 1906 shock is not an artifactof overestimating M. Most of the changes in frequency ofoccurrence of earthquakes in the Bay area are confined tomoderate-size events. The rate of smaller earthquakes in theentire 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 theprevious section, suffers from the fact that small changes in thedetermination ofM near the lower cutoff used can affectN atabout a factor of 1.5. Since the number of small earthquakesin a large region follows the relationship

log N = A - bM [1]

and b is close to 1.0, about half of the cumulative numbers ofevents are found betweenM andM + 0.3. Most of the seismicmoment (Mo) released in a region, however, is contained in thefew largest earthquakes. The cumulative moment release as afunction of time, ;Mo, is not very sensitive to the lower cutoff

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A. 01/01/69 to 10/17/89

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B. Loma Prieta mainshock and aftershocks

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SYKES AND NISHENKO (1984)

LINDH (1983) J

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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 SantaCruz Mountains segment of fault. (B) Rectangles give location of 1989 rupture zone as inferred from geodetic data (11). Distance is measuredto northwest along fault from San Juan Bautista.

in M but is to the values of Mo for the largest few eventssampled.-Mo was computed for shocks ofM - 5 prior to the threelarge events in Fig. 2 and before theMw 6.0 earthquake of 1948in southern California (6). YMo was calculated only for shockswithin the precursory areas outlined in Fig. 2. Those areaswere chosen qualitatively to include most of the region inwhich major changes in activity occur with time. They extendout to about the same distance in Fig. 2C where the rate ofsmall shocks was found to differ significantly before and afterthe 1989 earthquake (16, 17). .Mo increases nearly exponen-tially with time prior to each of those four large earthquakeswith a time constant T of 4-11 years.

Thus, the release of Mo, like the frequency of moderate-sizeevents, is concentrated in the latter part of the time intervalbetween large shocks along a given fault segment. Severalother examples of high rates of moderate activity precedinglarge earthquakes are given in ref. 6. Thus, changes in N andMo qualify as intermediate- to long-term seismic precursors.

Probabilities of Large Shocks Along Segments of SanAndreas Fault. During the last 15 years, a consensus hasdeveloped among workers studying the San Andreas fault thatstresses are built up as a result of the relative motion of thePacific and North American plates and that fault segments thatruptured a relatively small amount in their last large earth-quake are more likely to rerupture sooner than segments thatexperienced relatively large displacement (8-10, 15, 18). Forexample, the Parkfield segment of the San Andreas fault hasruptured historically about every 22 years in shocks of aboutMw 6 with an average displacement of 0.5-1.0 m. Some othersegments of the San Andreas rupture in shocks of M,w 7.5with displacements of several meters and repeat times of100-400 years. Changes in fault strike, presence of a majorcompressive (transpressive) fault step, relatively low fluidpressures at depth, and unusually large W probably contribute

to a fault segment being a so-called asperity (i.e., a difficultplace to rupture) and hence to its being a segment with a longrepeat time and particularly large M. Nevertheless, a quanti-tative understanding of why a segment ruptures with a certainMw is lacking. This is an area in which considerable progresscould be made in understanding fault mechanics during thenext 20 years.The term "characteristic earthquake" has been used in

various ways in the literature to describe the slip behavior oflarge shocks. One view based on detection of prehistoricearthquakes in trenches was that large events are nearly exactduplicates of previous prehistoric events in terms of displace-ment as a function of distance (L) along a fault (19). Suchmodels suffer, however, from cumulative displacement over

many cycles of large shocks being nonuniform along a majorfault, a violation of the idea that long-term plate motion isnearly uniform along strike. One of the common features oflarge earthquakes along plate boundaries is that a fault seg-ment may break by itself one time but in conjunction with oneor more adjacent segments another time. The Loma Prietasegment of the San Andreas fault ruptured by itself in 1989,with the adjacent Peninsular segment in 1838, and with yetseveral additional segments to the north in 1906 (7, 8). Thistype of behavior is also common for large thrust earthquakesat subduction zones. This undoubtedly contributes to varia-tions in individual repeat times of large events. Thus, the ideathat large shocks are like preceding ones in rupturing the samefault segment and in having the same displacement as afunction of L is clearly not correct.Another hypothesis is that a given fault segment ruptures in

a large event with a certain "characteristic displacement" thatdiffers from one fault segment to another but remains the samefor that segment regardless of whether it breaks alone or inconjunction with another segment (20). This model permitsrepeat times to differ among segments but for the cumulative

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displacement along a fault to be the same when averaged over

many cycles of large earthquakes. A variation of this hypothesisis the time-predictable model, wherein the displacement insuccessive events varies by as much as 1.5-2 and the timeinterval between large shocks is proportional to the slip in theevent that precedes it. Laboratory studies of frictional slidingon precut rock surfaces lend support to this model in that thestress level just before large slip events is a constant and thetime interval to the next shock is proportional to the stress drop(or displacement) in the preceding event.Lindh (18) and Sykes and Nishenko (8) performed the first

time-varying probabilistic estimates that segments of fouractive faults in California would be sites of large earthquakesduring 30- and 20-year periods, respectively. For large earth-quakes along each segment, their methodology involved iden-tifying the date of the last event, the average and SD ofrecurrence times, and an estimate ofM for each segment. Inboth papers, wide use was made of the time-predictable modeland of time intervals between large historic and prehistoricearthquakes.Long-Term Forecasts of 1989 Earthquake. In the few years

preceding the 1989 shock, a consensus had developed that thesouthern 75-90 km of the 1906 rupture zone, which hadexperienced smaller slip than that to the north of San Fran-cisco in 1906, was more likely to rupture sooner than othersegments and in an earthquake of smallerM than that of 1906.While most workers focused upon approximately the regionthat ruptured in 1989, estimates of its size and probability ofrupture varied substantially.Lindh (18) estimated a 30-year probability of 47-83% for a

45-km segment of Fig. 3 rupturing in an event of M 6.5. Intalking to him and other U.S. Geological Survey scientistsprior to the 1989 earthquake, it was clear to me that theybelieved that segment had ruptured in an event ofM 6.5 in 1865and the 1838 shock had not broken the southernmost 50 km or

so of the 1906 rupture zone. Sykes and Nishenko (8) argued,however, that the 1838 shock ruptured that segment and usedboth the time interval 1906-1838 and estimates of slip alongthat segment in 1906 to calculate a high probability of rupturein an event ofMw 7.0 for the period 1983-2003. They indicateda large uncertainty, however, in their probability estimates. Asubsequent comparison of felt areas for the 1865 and 1989shocks indicates that the former did not occur along the SanAndreas fault (7).

Considerable debate ensued from 1984 until the LomaPrieta earthquake in October 1989 about the amount ofdisplacement in 1906 along the southernmost 75 km of the1906 rupture zone. The likelihood of a large event was debatedat meetings of NEPEC and in the literature (21, 22). Scholz(21) argued that the segment had a more east-west trend (i.e.,was a transpressional feature) and slipped only 1-1.4 m in 1906compared to the 2.5-4 m typical of rupture on the Peninsularsegment to the northwest. By using geodetic data from beforeand after the 1906 shock, Thatcher and Lisowski (22) arguedthat slip in 1906 along the entire southernmost 90-km segmentof Fig. 3 was 2.6 ± 0.3 m and its 30-year probability of rupture,while high compared to fault segments north of San Francisco,was low for the remainder of the 20th century.

During my chairmanship, NEPEC reviewed the long-termpotential of major faults in California. In 1987 I asked mem-bers of NEPEC to rate fault segments and areas considered tohave a relatively high potential of being sites of large earth-quakes in terms of priority for further instrumentation andstudy (23). I and other members of NEPEC were concernedthat a dense monitoring network consisting of a variety ofinstruments was deployed in the United States only at Park-field and that such monitoring need to be carried out in severalareas to have a reasonable chance of observing precursors toa large earthquake within a few decades. NEPEC reports itsfindings about the scientific validity of earthquake predictions

made by others to the director of the U.S. Geological Survey.He appointed a "working group" of scientists to assess theprobabilities of large and damaging earthquakes in Californiaand asked NEPEC to review its report prior to publication in1988 (9). That study was updated for the Bay area in 1990 (10)and for southern California in 1995 (17). Each study assigned30-year probabilities to each fault segment considered.For the southernmost 90 km of the 1906 fault break, the 1988

Working Group adopted a compromise position between theresults obtained from surface displacements and geodeticdata. Relatively little attention was paid to the question ofwhether either the 1838 or 1865 shocks broke the southern partof that zone. They assigned a probability of 0.2 for the entire90-km segment breaking in anM 7 event. They made a separatecalculation, however, for the southernmost 35-km segment ofthat zone (denoted SSCM in Fig. 3) for which they assigned a0.3 probability of its being the site of an M 6.5 earthquake.

Evaluation of predictions. Of the various long-term predic-tions, the prediction of Lindh (18) comes closest to forecastingthe length of ruptureL for the 1989 earthquake and its location(Fig. 3) and in assigning a relative high 30-year probability. Hispredicted magnitude of 6.5, however, was significantly smallerthan the Mw 6.9 of the event itself. The predicted Mw of 7.0 ofSykes and Nishenko (8) was more accurate; their average20-year probability was relatively large but their predicted L,75 km, was too large. The latter discrepancy is reducedsomewhat if the 12-km rupture zone of the 1990 Chittendenearthquake (Fig. 3), which extended the 1989 rupture to thesoutheast, is added to that of 40 km for the 1989 shock, asdetermined from geodetic data and the distribution of earlyaftershocks.While refs. 9 and 22 forecast an event ofM 7, their 30-year

probabilities were low and their forecast of 90 km forL was toolarge. I take those forecasts to be incorrect. Likewise, therupture zone predicted by the 1988 Working Group (9) for theSSCM segment only overlaps half of that of the 1989 shock; itspredictedMwas too small, and the 30-year probability was only0.3. I agree with Savage (24) that the latter prediction is ofdoubtful validity in terms of forecasting the 1989 event. He isincorrect, however, in calling his own paper "Criticisms ofSome Forecasts of the National Earthquake Prediction Coun-cil" (24). NEPEC did review the report of the working group(9) in terms of its general scientific validity but NEPEC itselfdoes not make predictions.While the title of the summary article in Science on the 1989

earthquake by staff of the U.S. Geological Survey (25) refersto it as "an anticipated event," the predictions of the twoearliest papers (8, 18) were more accurate than the consensusestimates of the 1988 Working Group. Prior to the eventresponsible agencies of the federal and state governmentsinstalled little additional monitoring equipment and took fewmeasures to mitigate the effects of a large earthquake.Improvements in understanding in hindsight. All of the long-

term predictions made prior to the 1989 event assumed thesame value of W (Fig. 1) for various parts of the San Andreasfault in the Bay area. It is clear that the 1989 shock rupturedto a greater depth and hence a greater W than was assumed inthose calculations. That could, in fact, have been anticipatedfrom the greater depths of small earthquakes close to theLoma Prieta rupture from 1969 to 1989 (Fig. 3). Likewise, itshould be expected that large future earthquakes along sec-tions of the San Andreas fault with deeper than normalactivity, such as near San Francisco (Fig. 3) and in southernCalifornia near San Gorgonio Pass, will release greater thannormal Mo per unit length along strike.

Likewise, the 1990 report (10) increased the slip rateassigned to the Peninsular and Loma Prieta sections of the SanAndreas fault, leading to smaller calculated repeat times forlarge shocks by using the time-predictable model. The poten-tial slip accumulated as strain between 1906 and 1989 [i.e., 83.5

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years x (19 ± 4 mm/year) = 1.6 + 0.3 m]. A reexaminationof the amount of displacement in 1906 across the fault inWright tunnel (km 51 in Fig. 3), the only place where slip wasmeasured at depth along the southern 75 km of the 1906rupture zone, gives 1.7-1.8 m (26). Assuming the Loma Prietasegment ruptured in large earthquakes in 1838, 1906, and 1989gives a mean repeat time of 76 ± 11 years.

Inferences from geodetic data. The Loma Prieta benchmarkis the only one that was remeasured in the 1880s after the 1865and 1868 earthquakes and again soon after the 1906 shock thatwas close to the fault segment that broke in 1989 (27). Whileits average displacement in 1906 was 1.2 m, the 95% confi-dence limits are 0.35 and 2.0 m (27). Simple dislocation modelsassuming slip on a vertical San Andreas fault about 3 km fromthat benchmark give about 2.5 m of slip on that fault segmentwhen rupture is assumed to extend from 0 to 10 km (27) andabout 2.3 m when it extends to 18 km, the maximum depth ofrupture in 1989. Slip deduced in 1906 depends critically uponwhat fault(s) is (are) assumed to have ruptured, uncertaintiesin the sparse geodetic data and W. Dislocation models usingdata from the much denser horizontal geodetic network thatexisted in 1989 (28) yield a displacement for the Loma Prietabenchmark that differs from the observed (7) by a factor of 1.4.Thus, it is clear in retrospect that not as much weight shouldhave been given to geodetic data in estimating long-termprobabilities.Was the 1989 earthquake the event predicted? Many geosci-

entists were surprised that the 1989 shock did not produce aclear primary break at the earth's surface. That expectationarose from widely published photographs of fences and roadsthat were offset in 1906 along those portions of the fault thattraverse more level ground farther north and evidence of offsetat 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 majorsinistral (i.e., transpressional) fault offset. Since surface area isnot conserved as fault displacement accumulates in many largeevents, it is the site of considerably tectonic complexity andvertical deformation. Primary faulting at the surface along thatsegment appears to have been as rare in 1906 as in 1989 (29).

Inversion of various seismological data sets for the 1989earthquake led to models that differ in the amount and senseof slip as a function ofL and W (Fig. 1). Most authors assumed,however, the same best-fitting planar rupture surface that wasdeduced from geodetic data soon after the earthquake (11)and varied only the rake and slip as a function of L and W, notthe strike and dip. All four probably vary in the transpressionaloffset, and slip probably occurred on more than one fault asjudged from aftershocks and vertical displacements in 1989(14, 30, 31). While one of the inversions of seismic dataindicates slip was negligible at depths shallower than 8 km,others do not. I put greater reliance on the loci of aftershocksand the inversion of geodetic data (14, 28, 30-32), whichindicate that significant slip in 1989 extended to a shallowdepth 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 0to 10 km, and the latter, a nearby steeply dipping fault from 10to 18 km (14, 27). Evidence that rupture in 1989 was as shallowas 2-5 km indicates that a small to negligible Wis still availablefor generating a sizable event at a shallower depth on a steeplydipping fault. An event ofMw 6.5 still could take place in theupper 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 event, focal mechanisms, evidence of geological defor-mation in the last few million years, and balanced cross sectionsto derive models of fault and fold structure at depth in theLoma Prieta zone. They conclude that fault strike and dipchange from southeast to northwest as the restraining

(transpressional) part of the Loma Prieta zone is encountered.They suggest that the orientation of the slip vector in the 1989event, parallel to the line of intersection of the two faultsegments, was not fortuitous and that it permits slip to occuron the two faults without opening subsurface voids. Whenisostacy is taken into account, they conclude that observeduplift rates are consistent with long-term slip on this section ofthe San Andreas fault occurring in 1989-type events. Thus, thedisplacement field of the 1989 earthquake does not appear tobe anomalous for the geometry of the restraining bend.

Large Events as a Quasi-Periodic Process

Deficit of Small Shocks Along San Andreas Fault. It hasbecome increasingly clear in the last decade that an extrapo-lation of Eq. 1 as determined from small earthquakes along agiven fault segment seriously underestimates the rate of oc-currence of large events along the same feature (19, 33, 34).Thus, large events account for nearly all of the strain energyand seismic moment release along that fault segment. This isdemonstrated very clearly for earthquakes along most seg-ments of the San Andreas fault. For the several decades forwhich a complete record is available, the rate of seismic activityat the M - 3 level has been at a very low level for thosesegments that ruptured in the great historic earthquakes of1812 and 1857 in southern California and for its southernmostsegment (Fig. 4), which last broke in a great event about 1690

34K ^\S^~A

36'

34'

32°

FIG. 4. Earthquakes of M > 5 in southern California from Cali-fornia Institute of Technology catalog. (A) 1977 through 1985. (B)1985 through 1994. Thin solid lines, active faults; heavier line (dashedwhere multibranched and poorly delineated in San Gorgonio Pass),San Andreas fault.

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(9, 20). Similarly, such activity has remained at a very low levelfor that portion of the 1906 rupture zone that did not break in1989 (Fig. 2) and for many decades before 1989 for the LomaPrieta segment (10, 13, 14).From 1907 to 1995, no earthquakes ofM > 6 have occurred

along the San Andreas fault itself for the entire 430-km lengthof the rupture zone of the 1906 shock with but one exception,the 1989 earthquake ofMw 6.9. If Eq. 1 were correct, about 8events ofM > 6 would be expected to have occurred during theinterval from 1906 to 1989 for the Loma Prieta segment forreasonable values of the slope, b (i.e., those close to 1.0), andabout 80 events ofM > 5. During that period the Loma Prietasegment experienced one complete cycle of large shocks.Low activity along 1989 rupture zone. From 1910, when the

catalog ofM > 5 becomes complete, until the 1989 shock, only10 events of 5 <M < 6 occurred along or near the southeastern75 km (0-75 km in Fig. 3) of the 1906 rupture zone (14, 15).Epicentral locations more precise than a few kilometers onlybecome available starting in 1969 (14). Of the 3 events from1969 until the 1989 mainshock, the 2 Lake Elsman earthquakesof 1988 and 1989 are well enough located that they clearlyoccurred on a nearby fault, one of steep but opposite dip(northeast) to the one that ruptured in the Loma Prieta shock.The other, in 1974, occurred well to the east of the SanAndreas on the Busch fault. Prior to 1960 epicentral locationsfor that area are more uncertain than 10 km (14). Four of theremaining 7 events occurred during that period; the otherthree occurred in 1963, 1964, and 1967. A special study of the1963 shock (35) indicates that it occurred close to the SanAndreas fault but southeast of Parajo gap (15 km in Fig. 3)beyond the 1989 rupture zone and along that part of the SanAndreas where fault creep takes place at the surface and smallto moderate shocks have been more numerous historically(14). The 1964 event was well enough located (36) to ascertainthat it occurred east of the 1989 rupture zone. The 1967 shockofM 5.6 occurred close to the 1989 rupture zone but no specialstudy of its aftershocks or mechanism was published.

Thus, of the 10 events of M - 5 from 1910 until the 1989mainshock, none occurred on the coming rupture zone itselfduring the 20 years for which precise locations are available,the 1967 shock may have been on it, and large uncertaintiesexist in the locations of four events between 1910 and 1959.Hence, 0-5 shocks ofM > 5 occurred along the Loma Prietarupture zone itself during almost a complete earthquake cycleas opposed to 80 predicted from Eq. 1. Also, the rupture zoneof the 1989 shock appears to have been very quiet even at thelevel of the smallest earthquakes detected from 1969 until the1989 mainshock (14, 31).Peninsular segment. Likewise, for the Peninsular segment of

the San Andreas fault (60-120 km in Fig. 3), an extrapolationof Eq. 1 predicts about 30 events ofM > 5.5 and 10 ofM >6 between 1838 and 1906, the dates that segment ruptured inlarge events (Mw > 7). The historic record probably is com-plete for that region forM > 5.5 since about 1850 but not forsmaller shocks (13, 15). Only 3 events of 5.5 - M < 6 and noneofM > 6 occurred along that segment from 1850 until the 1906earthquake (6, 13, 15). Only a single event ofM > 5 occurrednear that segment since 1910 (Fig. 2). Its mechanism, involvingmainly dip-slip motion (15), suggests that it was not located onthe San Andreas fault. Thus, the record of M > 6 for the145-year period since 1850 and that of M > 5 since 1910 areat least a factor of 10 lower than rates predicted from shocksof Mw > 7 by using Eq. 1.

Southern California. Fig. 4 shows events ofM> 5 in southernCalifornia for a recent 18-year period. Activity was very low(i.e., a single event) for the San Andreas fault itself. Thehistoric record of the last 100 years indicates similar low levelsof activity for the San Andreas even though the Mojavesegment to the north of Los Angeles ruptures in large shocksabout every 130 years (9, 20).

Large Earthquakes Are Not a Clustered Process. Davisonand Scholz (34) examined the frequency of moderate-sizeearthquakes for segments of the Alaska-Aleutian plate bound-ary and found that the Mo of large segment-rupturing eventswas much higher than predicted from an extrapolation ofsmaller events using Eq. 1. Kagan (5) states that their result wasbiased by uncertainties in b value, saturation of the magnitudeused, and poor knowledge of repeat times of large events. Theabsence of events of M > 6 and the very small number ofshocks ofM > 5 along the San Andreas from 1906 to 1989,however, cannot be attributed to those uncertainties.A possibility is the 1906 and 1989 events broke either

different faults separated by a few kilometers or differentdepth ranges for the same fault. I argued earlier that both areunlikely. Even if each event occurred on a different nearbyfault, both involved substantial strike-slip motion and releasedshear strain energy, not from a fault surface, but from a volumeof rock that extends outward about 75 km from each rupturezone. Hence, the drop in strain energy associated with strike-slip motion on northwest-trending faults in the Loma Prietaregion is similar for most of that volume of rock.The hypothesis that large and small events differ in many

of their properties is supported by simple dynamical models offaults (37, 38) that can be run on computers for thousands ofcycles of large events and by observations of the frequencyof occurrence of avalanches of various sizes on a large sandpile(39). In both cases small events follow a distribution like Eq.1 but large events occur much more often than an extrapola-tion from small events predicts.A catalog of global shallow earthquakes ofMw > 7 from 1900

to 1990 indicates a change in b value in Eq. 1 from 1.5 for eventsofMw 2 7.5 to 1.0 for smaller shocks (1). In that study N wasa cumulative count-i.e., the number of events greater than orequal to Mw. While such a cumulative number, of course,cannot decrease as Mw is reduced, the number per magnitude(or moment) interval does decrease as b changes from 1.5 to1.0. The interval distribution for the global catalog exhibits amaximum at Mw 7.5 followed by a minimum at a somewhatsmaller Mw. Since interplate thrust events dominate the globalcatalog for Mw > 7, this behavior is appropriate to those typesof events. The fact that b is about 1.5 for shocks ofMw> 7.5does not mean that large thrust events are rare but merely thatthe distribution of large earthquakes, when summed (1) overmany different segments of plate boundaries, fits Eq. 1 but witha different slope than small events. (It does mean that shocksofM 9.5 are rare.) The maximum and minimum in the globaldistribution are not as extreme as for a single fault segmentsince W Mw, and Mo usually differ among segments, resultingin the two extrema being smeared out when summed overmany fault segments.Kagan and Jackson (3) concluded that shocks remaining in

several earthquake catalogs (after removal of events involvedin short-term clustering like aftershocks) are characterized byclustering, not quasi-periodic behavior. They examined theHarvard catalog for events of Mw > 6.5, claiming Mw 6.5 islarge enough to be a plate rupturing (i.e., a large) shock. Sincethat catalog is dominated by thrust events at convergent plateboundaries, however,Mw 7.5 is an appropriate lower bound forlarge shocks (1). That and the other catalogs of shallow eventsthey examined are dominated by small, not large, events. Thus,the clustering properties that they find are pertinent to theformer, but not the latter.Some examples of the clustering of large events do exist.

Adjacent segments of a major fault often rupture in largeevents separated by days to years. The 1984 earthquake on theCalaveras fault and the 1989 Loma Prieta event 30 km from itmay be considered clustered events but not on the same fault.Individual segments of major faults, however, rarely, if ever,rerupture in large events within a short time. Kagan andJackson (4) state "earthquakes in the near future will be

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similar in location and mechanism to those of the recent past."That proposition, however, is pertinent to small earthquakes.Most, and perhaps all, large events along a given fault segmentoccur quasi-periodically in time. The fault segments thatrupture in large events that I examined are parts of very activefaults, the main loci of plate motion. Whether large shocks inareas of complex multibranched faulting, as in Asia, occurquasi-periodically is yet to be ascertained.

Recent Buildup of Activity in Southern California

Fig. 4 shows earthquakes ofM - 5 in southern California forthe periods 1977-1985 and 1986-1994. In the first 9-yearperiod, no shocks of that size occurred on or close to the SanAndreas fault, while in the second interval, activity occurredon both sides of the San Andreas fault along a 200-km-longzone in the transverse ranges and the northern Los Angelesbasin. That pattern of activity, especially the occurrence ofseveral earthquakes of M > 6, resembles that in the 25 yearsbefore the 1906 earthquake (Fig. 2A). It was not centered nearthe Landers earthquake.The possibility that the recent pattern of activity is a

long-term precursor to a great earthquake along the southernSan Andreas fault deserves serious study and debate. Segmentsof the fault in that region have not ruptured in great earth-quakes since either 1812 or about 1690 (9, 20). Also, changesin stress generated by the Landers sequence of shocks resultedin portions of the fault in San Gorgonio Pass moving closer tofailure by about 10 years (40). The San Andreas fault under-goes a complex compressional left step in San Gorgonio Passthat is much larger than that in the Loma Prieta region. Muchremains to be learned about the distribution of faults at depth,possible changes in seismic activity, the loci of volumes ofweak- and strong-rock, fluid pressures, and the state of stress.An intensified effort is needed to understand that area indetail, which did not happen before the 1989 earthquake.Damage and loss of life may not be greatest in large events suchas one on the southern San Andreas. Moderate-size shocksthat are part of the buildup to a great earthquake but locatedcloser to centers of population may cause the largest catas-trophes. The Northridge event of 1994 may turn out to be suchan example.Discussion and Summary

Time-varying probabilistic estimates of large earthquakes forsegments of several active faults in California are now in theirsecond generation (10, 20) and are generally accepted andwidely used. Debate continues about the width of the proba-bility function to use either in general or for specific segments.Since those predictions are for 30-year periods, however, theprobability gain with respect to a random distribution in timeis only about a factor of 1.5-3. Those long-term forecasts,which have replaced the earlier seismic gap concepts of the1970s, help to focus scarce scientific resources on specific areasand conversely to indicate segments unlikely to rupture in thenext few decades. While progress can be expected over the next20 years in improving those types of forecasts, probability gainslikely will remain less than 5-10.Major changes in the space-time distribution of moderate-

size earthquakes have occurred in the San Francisco Bay areaduring the time intervals between large shocks. Computermodeling of earthquakes (37, 38) and studies of avalanches ona large sandpile (39) show many similarities to those found forshocks in the Bay area. All indicate that large events occurmuch more frequently than predicted by extrapolating rates ofsmall events. In each case the rate of moderate-size eventsincreases before large events and the dimension of the regionof increased activity increases with the size of the coming largeevent. Much, if not all, of the activity that builds up prior to a

large earthquake in the Bay area, however, occurs off itsrupture zone on nearby faults. This is an important lesson forprediction and a factor that needs to be incorporated in futurecomputer modeling.These and three other findings make me moderately opti-

mistic for intermediate- and long-term prediction.(i) Rates of relative plate motion are virtually constant from

a few to a few million years. Plate motion is the driving enginethat leads to the buildup of elastic stresses that are released inearthquakes along plate boundaries. Accurate estimates oflong-term rates of deformation have become available formany active faults by using space geodesy.

(ii) The key nonlinearity in the earthquake process appearsto be associated with the stick-slip frictional force at faultinterfaces (37). Most of that effect is concentrated during orclose to the rupture time of large shocks. Fortunately, sociallyuseful prediction needs to be attempted only for the next largeevent, not several such shocks into the future where nonlineareffects become cumulative. A better understanding of thenature of rupture in the last large event seems crucial tolong-term prediction. Better modeling of fault interactionsshould permit a choice of which fault segments will ruptureeither separately or together in the next large earthquake alongthe San Andreas fault in southern California.

(iii) I foresee progress being made by recognizing thatprecursory processes are tensorial in character, not scalar.Stress (and its evolution with time), which is basic to anunderstanding of the earthquake process, is a second-ordertensor. This would explain why increases as well as decreasesin seismicity have been reported as precursors. While activityin most of the San Francisco Bay area decreased greatly soonafter the great 1906 earthquake, the moderate activity that didtake place in the next few decades was largely concentratedsouth and west of the end of the rupture zone (Fig. 2B) in areasthat are predicted from dislocation models to have, in fact,moved closer to failure (15).

Moderate-size shocks are modulated in their occurrence bychanges in the Coulomb failure function (ACFF) at the time oflarge events (6, 15-17, 31, 40). The ratio of the numbers ofsmall events before to that after the 1989 earthquake forindividual nearby fault segments changed in accord withpredictions of ACFF by using dislocation models (16, 17, 31).These changes occurred for fault segments extending out to75-100 km from the 1989 rupture zone. This corresponds toACFF > 0.01 to 0.03 MPa, where 1 MPa = 10 bars. Thosechanges are about a factor of 10 times larger than thosegenerated by earth tides. Thus, changes in rates of smallearthquakes may become more useful for prediction whenindividual fault segments are analyzed separately.Changes in the distribution of moderate-size events in the

Bay area of the past 150 years can be explained in terms of adrop in stress at the time of large earthquakes and the slowbuildup of stress with time. The 1906 shock created a broadarea of reduced shear stress (or CFF) and suppressed mod-erate activity for many decades until stresses were graduallyrestored by plate motion. The southern Calaveras fault was oneof the first to return to the pre-1906 stress level and was the siteof some of the earliest moderate activity prior to 1989 (6, 15,17).The pair of Lake Elsman earthquakes ofM 5.3 and 5.4 that

occurred 16 and 2.5 months before the 1989 mainshock may beinterpreted as an intermediate-term precursor. Both occurredon a fault dipping steeply to the northeast 22 km from the 1989rupture zone. Like the 1989 event, they involved strike slip andreverse slip (14, 31). No other moderate-size events occurredon or near the 1989 rupture zone since the shocks of 1964 and1967. As shear stress was restored in the region, some of thelast places to be returned to the pre-1906 level of CFF werefaults very close to the San Andreas, like the Lake Elsmanfault. Those events resulted in the release of short-term (5 day)

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warnings. While they may have had some value in terms ofpublic preparedness, they were, in fact, false alarms.

If it had been realized that the Lake Elsman events were ona nearby but different fault, an intermediate-term warningwould have been more appropriate. They probably indicatedthe return of stresses in that area to pre-1906 levels rather thanthe initiation of accelerated precursory slip on the San An-dreas fault itself. Another example of an intermediate-termseismic precursor is the northward growth in aftershock ac-tivity in the Joshua Tree earthquake sequence in southernCalifornia between its mainshock on April 23 and the Landersshock of June 28, 1992 (41). While these precursors are subtlein character, they, and other examples, indicate that precursoryphenomena likely exist on time scales of months to a decade.How earthquake prediction is and has been viewed in the

United States has a number of parallels to skepticism aboutcontinental drift and paleomagnetism prior to the late 1960s.Like them, prediction invokes strong views about what prob-lems are "worth working on." Earthquake prediction hassuffered in this regard; only 10-20 scientists in the U.S. arecurrently working on intermediate-term prediction. Work inprediction also has suffered from a general belief that onlyshort-term predictions would have social value. While notpossible now, a well-founded 5-year prediction could be ofgreater value since serious mitigation measures could beundertaken.

Several observations of precursors have turned out uponreexamination to be artifacts of either environmental changesaffecting instruments or changes in earthquake catalogs thatare of human, not natural, origin. A superficial application ofthe ideas of chaos has led some to conclude that earthquakesare not predictable. Several workers active in studying earth-quakes as an example of deterministic chaos, however, aremoderately optimistic about prediction. Long- and interme-diate-term prediction are areas where I think progress ispossible in the next 20 years. Much remains to be done inunderstanding the physics of earthquakes and the role of fluidpressures at depth in fault zone and in deploying densenetworks of a variety of observing instruments.

I thank J. Deng, S. Jaum6, C. Scholz, and B. Shaw for criticalcomments and discussions. This workwas supported by grants from theU.S. Geological Survey, the National Science Foundation, and theSouthern California Earthquake Center (SCEC). This is Lamont-Doherty Earth Observatory Contribution 5486 and SCEC contribu-tion 319.

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