Abstract

Strong-motion records were used to determine the seismic moments, source dimensions, and stress drops for two earthquakes near the proposed Anza seismic gap (Thatcher et al., 1975) along the San Jacinto fault: the 25 February 1980 (ML 5.5) event and the shock of 15 June 1982 (ML 4.8). Seismic moments determined for the two events were 2.5 × 1023 and 1.1 × 1023 dyne cm, respectively. The source radii were estimated from the displacement pulse widths using the source model of Madariaga (1976). Static stress drops for the 1980 and 1982 earthquakes were calculated as 110 and 100 bars, respectively. Stress drops determined from the rms accelerations using the method of Hanks and McGuire (1981) were 220 bars for the 1980 earthquake and 180 bars for the 1982 event. The long-period seismic moments for these events, derived from regional recordings of 30- to 40-sec Love waves, were a factor of two to three greater than the moments estimated from the strong-motion records.

As a comparison to the Anza events, source parameters were also determined from strong-motion records for an immediate aftershock (ML 5.0) of the 1979 Imperial Valley earthquake. The seismic moment of this aftershock was calculated as 4.2 × 1023 dyne-cm, and its static stress drop was 130 bars. Static stress drops calculated from the displacement pulse shapes for the three events studied in this paper, as well as for an aftershock of the Oroville earthquake reported by Fletcher et al. (1980), exhibit a relatively narrow range between about 100 and 180 bars. This result indicates that the stress drops of the two main shocks near Anza were not unusually high, relative to those of certain aftershocks in other tectonic settings in California.

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