SEISMICITY PATTERNS AND THE EARTHQUAKE CYCLE [c5, p144]

What changes in spatial-temporal patterns of earthquake occurrence might we expect to see as the next great earthquake on the San Andreas fault approaches? Both historical and instrumental seismicity records indicate that the spatial distribution of earthquakes in California changes only slowly over periods of decades to centuries, although the intensity of activity within this distribution fluctuates year to year (Ellsworth and others, 1981; Hill and others, in press; Hutton and others, in press). Temporal fluctuations in activity during the interval 1980-86, for example, were dominated by a short-lived aftershock sequence following the 1980 Eureka M=7.2 earthquake and by the long-lived aftershock sequence following the 1983 Coalinga M=6.7 earthquake. The overall spatial distribution of earthquakes in California, however, remained nearly stationary throughout this 7-yr interval. Furthermore, the spatial pattern defined by 1980-86 seismicity is much the same as that outlined by the record of M>5 earthquakes that extends back nearly 200 yr (see chap. 6).

Variations in the historical rate of moderate to large (M>5) earthquakes in central California before and after the 1906 San Francisco earthquake appear to mimic those described by Fedotov (1965) and Mogi (1968) for the earthquake cycle associated with great, subduction-zone earthquakes in Japan, Kamchatka, and the Kurile Islands (see chap. 6; Ellsworth and others, 1981). The history of instrumentally recorded M<5 earthquakes in California is too short, however, to indicate whether we might expect to see distinctive changes in the seismicity pattern a short time (months to years) before the next great earthquake on the San Andreas fault. We have yet to see, for example, whether the quiescent (locked) segments of the San Andreas fault remain aseismic except for the rupture of a great earthquake, or whether these segments become active with small to moderate earthquakes as foreshock activity to great earthquakes.