SEISMIC CHARACTERISTICS OF EARTHQUAKES ALONG THE OFFSHORE EXTENSION OF THE WESTERN TRANSVERSE RANGES, CALIFORNIA

Abstract

The short- and long-period body waves of five moderate (M_L 5 to 6) earthquakes that took place in the nearshore region of southcentral California between 1969 and 1981 are modeled to obtain the source parameters. Both regional Pnl waveforms and teleseismic P waveforms are used in this analysis. Four of the events are thrust earthquakes with strikes that rotate southwestward as the epicenters move southward; the fifth is a strike-slip event occurring on a fault subparallel to the San Andreas fault. The focal mechanisms for all five events are consistent with known faults in the source localities and suggest that the regional stress field is compressional in a north-south direction. At least three events have complex source time histories, suggesting that complex sources are common even for events of this size. The most exotic event started with a low stress drop subevent, ruptured bilaterally, and concluded with strong asperity ruptures at each end. Since Pnl waveforms for thrust events are particularly sensitive to crustal thickness, regionalized crustal models were developed. The crustal thickness appears to increase southward but may be a function of the distance of the event from shore rather than of latitude. Basin and range paths yield the thinnest crustal models, while paths crossing the Colorado Plateau and Rockies yield the thickest. We also examined amplitude variations with station and path. In general, the amplitudes seem to be controlled by the station site rather than the path.