Abstract: Seismicity of the Hayward Fault

Peggy A. Johnson, T. V. McEvilly

The Hayward fault has taken a central role in the seismic hazard of the San Francisco Bay Area. In 1836 and 1868, two moderate (M~ 6.8) earthquakes ruptured the northern and southern segments of the Hayward fault. Uncertainties in fault loading, slip and creep rates render return times that are poorly known, but estimates as short as 120 years are possible, depending on values accepted.

New models for the plate boundary geometry in the San Francisco Bay region call for pervasive compressive deformation and a mid-crustal detachment. The base of seismicity, or brittle-ductile transition, occurs at 10-12 km beneath the Hayward fault and deepens toward to east. Along the Hayward fault, patterns of hypocenters exhibit clustering into several pockets of activity separated by gaps. There are similar 'multiplet' events within the seismicity, with implications for the distribution and size of asperities on the fault and promise for monitoring fault-zone properties. There appear to be events with non-strike-slip mechanisms throughout the clusters. We are using the data recorded on a new modern High-Resolution Digital Network along the northern segment of the Hayward Fault to i vestigate the seismicity of the Hayward fault, improving the hypocentral uncertainties and focusing on the rupture process of the earthquakes and implications for plate boundary deformation.

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