A foreshock-aftershock sequence associated with an earthquake of magnitude 4.9 on September 10, 1965, in the San Francisco Bay Region is analyzed. The sequence was monitored by nearby Berkeley network stations and by one temporary station in the epicentral region. Precise hypocenters were determined for 29 of the shocks and these clustered in a small focal region with dimensions on the order of a few kilometers and at an average depth of 12 km. Within this focal region a N-S trend was observed with shocks apparently migrating down and then back up the trend.
(First motion radiation patterns were uniform throughout the sequence, and a P-wave fault plane solution for the main shock yielded as one possible source, consistent with observed S-wave polarization, a N-S trending fault with right-lateral motion. The log cumulative frequency of shocks versus magnitude plot yields a slope of b = −0.78. The similarities between this sequence and the 1964 Corralitos sequence are discussed and are shown to be at variance with the 1963 Salinas-Watsonville sequence and, from preliminary indications, from the 1966 Parkfield sequence. As a result, two characteristically different types of earthquake sequences in the magnitude 5.0-5.5 range are defined for the Central California region.