SOURCE PARAMETERS OF THE SIERRA-MADRE EARTHQUAKE FROM REGIONAL AND LOCAL BODY WAVES

We inverted the three-component, long-period data recorded by the TERRAscope array for the June 28, 1991 Sierra Madre, California event to determine the seismic moment and source orientation of the mainshock (M(L) = 5.8). Remarkably four of the six stations were located 159.2 +/- 0.7 km from the epicenter. Variations in absolute traveltime were smaller than 2 % and 3 % for P(n) and S(n), whereas the surface waves displayed greater variation. Similarities in the waveforms and the small variance in traveltime suggest that a common Green's function can be used to invert the data. We used the whole waveforms excluding the fundamental surface waves at the more distant stations to invert for source parameters. The results of the inversion indicated that this event was predominately a thrust type earthquake, where the strike, rake and dip were determined to be 235-degrees, 74-degrees and 50-degrees, respectively. The seismic moment was determined to be 2.5 x 10(24) dyne-cm. The source duration was found to be about 1.0 seconds by direct measurement of the S-wave recorded at the Pasadena station (located 21 km from the epicenter). Considering circular rupture we obtain a stress drop of 460 bars.