Abstract: Seismotectonics of the San Francisco Bay Block

Mary Lou Zoback, Jean Olson

San Francisco Bay is characterized by diffuse low-level microseismicity. Through-going fault zones within the Bay block were suggested a decade ago on the basis of prominent linear N50-60°W trending aeromagnetic anomalies aligned with mapped onshore shear zones within the Franciscan Formation (Brabb and Hanna, 1982, USGS Map GP-932). Both gravity data and recent high resolution seismic-reflection profiling of Holocene bay mud rule out significant vertical offsets across these proposed fault zones, despite their similarity in strike to thrust faults in the southern San Francisco Peninsula. In contrast to earlier reports of extensive Holocene strike-slip faulting (Mann et al., 1993, EOS, v. 74, p. 693), new high resolution seismic reflection data also appear to preclud any major Holocene strike-slip-related disruption along these fault zones (M. Marlow, USGS, personal communication, 1994).

Relocation of this microseismicity indicates that seismic activity within the Bay block generally decreases with increasing distance from the two bounding strike-slip faults, the San Andreas and the Hayward faults. Most events occur between 6 and 12 km depth (similar to the depth range of the background seismicity along the bounding strike-slip faults). A persistent active cluster of seismicity occurs beneath San Francisco airport (SFO) at about 10 km depth. Reliable fault-plane solutions for 11 of the largest events (2.0<=M<=3.0) within the Bay block indicate oblique strike-slip mechanisms with both normal and reverse components of slip and a distinct contrast in deformation axes between the North and South Bay. South Bay events have NE-trending P-axes consistent with "fault-normal" compression observed in seismic deformation adjacent to the San Andreas fault on the southern San Francisco Peninsula. In contrast, North Bay events have "fault parallel" NW-trending P-axes, again consistent with deformation along the adjacent San Andreas fault in the region offshore from the Golden Gate where many normal faulting focal mechanisms (and some right-lateral mechanisms consistent with San Andreas slip) are observed. This normal faulting occurs on planes trending N-to-NNW and results in a component of approximately fault normal extension in a region in which projection of onshore fault traces indicate a roughly 1-3 km right step of the main San Andreas Fault.

Neither the hypocenters nor the focal mechanisms for the south San Francisco Bay seismicity suggest any clear or convincing association of these earthquakes with the postulated N50-60°W trending transbay fault zones; nodal planes for the cluster of seismic deformation beneath SFO are nearly 45° oblique to the trend of the proposed nearby transbay fault zone.

AAPG Search and Discovery Article #90958©1995 AAPG Pacific Section Meeting, San Francisco, California