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).