The near - Source Ground Motion of the 6 August 1979 Coyote Lake , California , Earthquake

A finite fault striking N24°W and extending to a depth of 10 km is proposed to explain the strong ground motion data for the 6 August 1979 Coyote Lake, California, earthquake (M, = 5.9). Our source model suggests that right-lateral faulting initiated at a depth of 8 km and ruptured toward the south with a velocity of 2.8 km/sec. This unilateral rupture can explain the large displacement recorded south of the epicenter. However, the waveform coherency across an array south and southwest of the epicenter suggests that the rupture length is less than 6 km. The maximum dislocation is about 120 cm in a small area near the hypocenter, and the total moment is estimated to be 3.5 x 1024 dyne-cm. An abrupt stopping phase which corresponds to a deceleration of right-lateral motion can explain the high peak acceleration recorded at array station 6. The stress drop in the hypocentral area is about 140 bars; the average stress drop over the entire rupture surface is 30 bars. The preferred finite-source model can predict the Pnl waveforms and the beginning features in the teleseismic seismograms. No clear arrivals can be observed in the near-source data for the possible second and third smaller events suggested by Nabelek (personal communication). INTRODUCTION The strong ground motions recorded at the Gilroy array and San-Martin CoyoteCreek (SMCC) station from the 6 August 1979 Coyote Lake, California, earthquake (ML = 5.9) provide a good opportunity to investigate the faulting process along the Calaveras fault zone. From a seismological point of view, the near-in data reflects the most detailed information available about the faulting process, especially that portion of the fault which radiates the high-frequency energy. However, interpreting these details of the source process necessitates constructing source models with a large number of free parameters (e.g., rupture velocity, rupture direction, dislocation size, and distribution). Data from an array of near-source instruments are essential to reliably constrain these unknowns. A detailed analysis of the strong ground motion data will be presented in this paper. In particular, we will demonstrate how to interpret the data in terms of fault size, rupture speed, rupture direction, and fault dislocation heterogeneity. We will then use our near-source model to interpret regional and teleseismic seismograms. SOURCE LOCATION AND MECHANISM The epicenter of the Coyote Lake earthquake was located about 13 km northeast of Gilroy by the University of California, Berkeley (Uhrhammer, 1980). This location is about 3 km to the northeast of the location obtained by the U.S. Geological Survey as shown in Figure 1 (Lee et al., 1979). The Gilroy stations (GA-1, GA-2, GA-3, GA-4, and GA-6) and SMCC range from 2 to 16 km from the epicenter (Brady et al., 1979). The first P-motion distribution indicates a nearly pure strike-slip mechanism with strike (N24°W) parallel to the Calaveras fault (Lee et al., 1979). The teleseismic and regional first P motions suggest a plane dipping 80 ° to the northeast (see Figure 2). 201 202 HSUI-LIN LIU AND DONALD V. H E L M B E R G E R No clear surface faulting was found near the epicentral region. Right-lateral surface breakage of about 0.5 cm is reported near the junction of Highway 152 and the Calaveras fault and is marked as the s h a d e d area in Figure 1 (Herd et al., 1979). STRONG GROUND MOTION DATA The recorded strong ground accelerations were corrected and integrated to velocities and displacements (Brady et al., 1979). We rotate the horizontal velocities into the radial and transverse components defined relative to source BK (shown in Figure 3). The arrows in Figure 3 indicate the S arrivals and all traces start at the trigger time. The major signal is less than 2 sec in duration at all stations. In general, the waveforms are very coherent across the Gilroy array although broader waveforms are observed on the tangential components southwest of the epicenter. Table 1 lists the station information and their peak amplitudes. S e n ~ Merlin \ A i Anderson Reservoir SMCC %*Gs ~I'BK ole Reservoir