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Seismic activity on the southern Calaveras Fault in central California

U.S. GEOLOGICAL SURVEY OFFICE OF EARTHQUAKE STUDIES, 345 MIDDLEFIELD ROAD, MENLO PARK, CALIFORNIA 94025

Abstract

Cumulative seismic moment M₀ for earthquake on 50-km-long creeping section of the Calaveras Fault from near Mount Hamilton southeast to San Felipe Lake correlates with mapped fault-trace characteristics. In general, M₀ is lower at the left-stepping offset in the trace at the south end of Anderson Lake and along linear segments of the fault than near right-stepping offsets and bends in the trace and intersections of the Calaveras with other faults. Rupture expansion for the August 6, 1979 Coyote Lake sequence main shock, 10 km NNE of Gilroy, California, was unilateral to the southeast (Archuleta, 1979) away from the right-stepping offset in the fault trace near its epicenter. Rupture expansion for the two felt shocks (M_L 4.2 and M_L 3.9) on August 29, 1978 located km apart near Halls Valley east of San Jose was unilateral for each away from the other, suggesting the existence of a rupture-expansion blocking discontinuity between them. The correlations of seismic activity and fault-trace characteristics are similar to those for shocks along the creeping section of the San Andreas Fault in central California and suggest that the specific "stuck" and "creeping" patch model of Bakun et al. (1980) developed for the San Andreas is applicable to the creeping Calaveras Fault as well.

Cumulative seismic moment (January 1, 1969 to August 6, 1979) within the 16-km-long 1979 Coyote Lake sequence aftershock zone was less than that near the fault-trace discontinuities at its ends. Microearthquakes along the Calaveras Fault near the Coyote Lake aftershock zone increased before the sequence beginning with a cluster on June 22, 1978 near the southeast end of the aftershock zone. A similar seismicity pattern preceded the August 29, 1978 shocks and the M_L 4.5 May 8, 1979 shock near Halls Valley.

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