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Proceedings of the National Academy of Sciences of the United States of America
FIG. 1. Two types of earthquakes—small and large. L is rupture length 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 shocks are not strictly periodic. I think the important questions are how predictable and how chaotic are large shocks and on what time-space scales? In this review I exclude short-term prediction (time scales of hours to months) since very little progress has been made in that area. For lack of space I also exclude the Parkfield prediction experiment and failure of predictions made for that area.
Earthquakes in the San Francisco Bay Area
Several large earthquakes according to the terminology used herein have occurred in the San Francisco Bay area (Fig. 2) since 1836. Of those events, the greatest amount of information is available (6–10) for the great (Mw 7.7) 1906 earthquake that ruptured a 430-km portion of the San Andreas fault (Fig. 2A), the 1989 Loma Prieta shock of Mw 6.9 that broke a 40-km segment of that fault (Figs. 2C and 3), and the 1868 event (Fig 2D).
FIG. 2. Distribution of earthquakes of magnitude (M) 5 or larger in San Francisco Bay area for four time intervals (6). Major active faults are shown. Solid circles, epicenters of 1906, 1989, and 1868 earthquakes; heavy solid lines, rupture zones of those three large shocks; dashed lines enclose events taken to be within their precursory areas. Arrows in A denote sense of strike-slip motion along San Andreas fault. Note very low activity near most of 1906 rupture zone from 1920 to 1954 and higher activity in periods before large three events.
on the Hayward fault of Mw 6.8. Not as much is known 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 frequency of moderate-size shocks, herein taken to be events of 5 ≤ M < 7, where M is earthquake magnitude, has varied by as much as a factor of 20 in the Bay area during the past 150 years (6, 12, 13). From 1882 until the great 1906 shock, activity was very high along faults in the area out to about 75 km from those segments of the San Andreas fault that ruptured subsequently in 1906 (Fig. 2A). Those moderate-size events are well enough located based on intensity reports that most, and perhaps all, occurred on faults other than the San Andreas. The northernmost event in Fig. 2A, however, is not well enough located to ascertain on which fault it occurred. Moderate activity dropped off dramatically after 1906 and remained low until about 1955 (Fig. 2B).
Sykes and Nishenko (8) remarked in 1984 that moderate activity increased to the southeast of San Francisco from 1955 to 1982 but in a smaller region than in the 25 years preceding the 1906 earthquake. They concluded that that pattern might represent a long-term precursor to a future event of M=7.0 along 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 the San Andreas fault in the San Francisco Bay area since 1906, was centered along that fault segment (Figs. 2C and 3). A similar pattern of moderate activity occurred from 1855 to 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 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 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 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 determination of M near the lower cutoff used can affect N at about a factor of 1.5. Since the number of small earthquakes in a large region follows the relationship
log N=A−bM 
and b is close to 1.0, about half of the cumulative numbers of events are found between M and M+0.3. Most of the seismic moment (Mo) released in a region, however, is contained in the few largest earthquakes. The cumulative moment release as a function of time, ΣMo, is not very sensitive to the lower cutoff