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Observations of the Coyote Lake, California earthquake sequence of August 6, 1979

SEISMOGRAPHIC STATION DEPARTMENT OF GEOLOGY AND GEOPHYSICS UNIVERSITY OF CALIFORNIA, BERKELEY, CALIFORNIA 94720

Abstract

At 1705 UTC on August 6, 1979, a strong earthquake (M_L = 5.9) occurred along the Calaveras fault zone south of Coyote Lake about 110 km southeast of San Francisco. This strong earthquake had an aftershock sequence of 31 events (2.4 M_L 4.4) during August 1979. No foreshocks (M_L 1.5) were observed in the 3 months prior to the main shock.

The local magnitude (M_L = 5.9) and the seismic moment (M_o = 6 x 10²⁴ dyne-cm from the SH pulse) for the main shock were determined from the 100 x torsion and 3-component ultra-long period seismographs located at Berkeley. Local magnitudes are determined for the aftershocks from the maximum trace amplitudes on the Wood-Anderson torsion seismograms recorded at Berkeley ( 110 km).

Temporal and spatial characteristics of the aftershock sequence are presented and discussed. Some key observations are: (1) the first six aftershocks (M_L 2.4) proceed along the fault zone progressively to the south of the main shock; (2) all of the aftershocks (M_L 2.4) to the south of the largest aftershock (M_L = 4.4) have a different focal mechanism than the aftershocks to the north; (3) no aftershocks (M_L 2.4) were observed significantly to the north of the main shock for the first 5 days of the sequence; and (4) the b-value (0.70 ± 0.17) for the aftershock sequence is not significantly different from the average b-value (0.88 ± 0.08) calculated for the Calaveras fault zone from 16 yr of data.

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