Earthquake locations in the inner Continental Borderland, offshore southern California

The inner Continental Borderland region, offshore southern California, is tectonically active and contains several faults that are potential seismic hazards to nearby cities. However, fault geometries in this complex region are often poorly constrained due to a lack of surface observations and uncertainties in earthquake locations and focal mechanisms. To improve the accuracy of event locations in this area, we apply new location methods to 4312 offshore seismic events that occurred between 1981 and 1997 in seven different regions within the Borderland. The regions are defined by either temporal or spatial clustering of seismic activity in the Southern California Seismic Network (SCSN) catalog. Obtaining accurate locations for these events is difficult, due to the lack of nearby stations, the limited azimuthal coverage, and uncertainties in the velocity structure for this area. Our location procedure is based on the L-l norm, grid search, waveform cross-correlation method of Shearer (1997), except that we use a nearest neighbor approach (Astiz et al., 2000) to identify suitable event pairs for waveform cross-correlation and we explore the effect of different velocity models on the locations and associated station terms. In general, our relocated events have small estimated relative location errors and the events are more clustered than the SCSN catalog locations. A quarry on the south tip of Catalina Island provides a test of our location accuracy and suggests that, under ideal conditions, offshore events can be located to within 1 to 2 km of their true locations. Our final locations for most clusters are well correlated with known local tectonic features. We relate the 1981 Santa Barbara Island (M-L = 5.3) earthquake with the Santa Cruz fault, the 13 July 1986 Oceanside (M-L = 5.3) sequence with the San Diego Trough fault zone, and events near San Clemente Island with the known trace of the San Clemente fault zone. Over 3000 of the offshore events during this time period are associated with the 1986 Oceanside earthquake and its extended aftershock sequence. Our locations define a northeast-dipping fault plane for the Oceanside sequence, but in cross-section the events are scattered over a broad zone (about 4-km thick). This could either be an expression of fault complexity or location errors due to unaccounted for variations in the velocity structure. Events that occur near Coronado Bank in the SCSN catalog are relocated closer to the San Diego coast and suggest a shallow-angle, northeast-dipping fault plane at 10 to 15 km depth.