Historical Seismicity in the South San Francisco Bay Region

Locations (intensity centers) and moment magnitude M for 24 earthquakes (1858-1911) in the southern San Francisco Bay area are estimated from Modified Mercalli intensity (MMI) assignments. The uncertainties in location and M are generally large because there are few MMI assignments available. Preferred locations are selected to be consistent with a simple model for seismic activity on the central and southern Calaveras fault. Significant seismic activity can be explained by repetitive failure of the same fault areas in nearly identical earthquakes. Significant earthquake activity occurred on both east- and west-Bay faults in the ten or so years before the 1868 east-
Bay earthquake and before the1906 and 1989 west-Bay earthquakes.