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The Briones Hills earthquake swarm of January 8, 1977, Contra Costa County, California

SEISMOGRAPHIC STATION UNIVERSITY OF CALIFORNIA, BERKELEY, CALIFORNIA 94720

Abstract

Locations and magnitudes of a swarm of earthquakes in the hills east of San Francisco Bay have been estimated. The largest (M_L = 4.3 ± 0.13 S.E.M.) event had an origin time of 09^h38^m07.5^s ± 0.11^s, 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.

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