EVOLUTION OF MODERATE SEISMICITY IN THE SAN-FRANCISCO BAY-REGION, 1850 TO 1993 - SEISMICITY CHANGES RELATED TO THE OCCURRENCE OF LARGE AND GREAT EARTHQUAKES

The rate of seismic activity of moderate-size (M > 5.5) earthquakes in the San Francisco Bay (SFB) region has varied considerably during the past 150 years. As measured by the rate of seismic moment release, se ismic activity in the SFB region is observed to accelerate prior to M > 7.0 earthquakes in 1868, 1906, and 1989 and then to decelerate follo wing them. We examine these seismicity changes in the context of the e volution of the stress field in the SFB region as a result of strain a ccumulation and release using a model of dislocations in an elastic ha lfspace. We use a Coulomb failure function (CFF) to take into account changes in both shear and normal stresses on potential failure planes of varying strike and dip in the SFB region. We find that the occurren ce of a large or great earthquake creates a ''stress shadow'': a regio n where the stress driving earthquake deformation is decreased. Inters eismic strain accumulation acts to reverse this process, gradually bri nging faults in the SFB region out of the stress shadow of a previous large or great earthquake and back into a state where earthquake failu re is possible. As the stress shadow generated by a large or great ear thquake disappears, it migrates inward toward the fault associated wit h that large or great event. The observed changes in the rate of occur rence of moderate earthquakes in the SFB region are broadly consistent with this model. In detail, the decrease in seismicity throughout mos t of the SFB region and a localized increase in the Monterey Bay regio n following the great 1906 earthquake is consistent with our predicted stress changes. The timing and location of moderate-size earthquakes when the rate of seismicity increases again in the 1950s is consistent with areas in which the 1906 stress shadow had been eliminated by str ain accumulation in the SFB region. Those earthquakes that are most in consistent with our stress evolution model, including the 1911 earthqu ake southeast of San Jose, are found to occur in regions where dip-sli p faulting is common in addition to strike-slip. The 1906 earthquake b rought that zone of dip-slip faulting closer to failure, suggesting th at the 1911 event may have been a reverse faulting earthquake rather t han a strike-slip one similar to the 1984 Morgan Hill earthquake. The occurrence of activity on faults very close to the San Andreas, such a s the Lake Elsman earthquakes of 1988 and 1989, appear to be associate d with the last disappearence of the stress shadow on the Loma Prieta segment of the San Andreas fault. Thus events of that type may represe nt an intermediate-term precursor to a large earthquakes such as the 1 989 Loma Prieta event. Much of the moderate-size earthquake activity i n the SFB region appears to be modulated in time by the buildup and re lease of stress in large and great earthquakes. A tensorial approach t o earthquake prediction, i.e., taking into account changes in the comp onents of the stress tensor, has several advantages over examining sca lar changes such as those in seismic activity and moment release rates . This tensorial approach allows for both activation and quiescence (b ut in different subregions) prior to as well. as after large earthquak es.