Locations and magnitudes of a swarm of earthquakes in the hills east of San Francisco Bay have been estimated. The largest (ML = 4.3 ± 0.13 S.E.M.) event had an origin time of 09h38m07.5s ± 0.11s, January 8, 1977 UTC, a latitude of 37°54.31′ ± 0.69′N and longitude 122°10.97′ ± 0.50′W and a focal depth of 9.5 ± 0.8 km. The P fault-plane solution for earthquakes in the swarm was consistent with right-lateral slip on a plane with strike N28°W and dip 65°SW. No surface rupture was observed; but the mechanism is consistent with present regional deformation. Displacement seismograms recorded at Berkeley show a close similarity of wave form but the P/S amplitude ratios for the foreshocks are systematically greater than for the main shock and subsequent events. The calculated relative seismic energy suggests a very limited region of successive slip surfaces.

This content is PDF only. Please click on the PDF icon to access.

First Page Preview

First page PDF preview
You do not have access to this content, please speak to your institutional administrator if you feel you should have access.