Earthquake Locations in the Inner Continental Borderland, Offshore Southern California

Luciana Astiz, & Peter M. Shearer

Published April 2000, SCEC Contribution #490

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 not well constrained due to a lack of surface observations and uncertainties in earthquake locations and focal mechanisms. To improve the accuracy of earthquake locations in this area, we apply new location methods to 4,312 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-1 norm, grid search, waveform cross-correlation method of Shearer (1997), except that we use a nearest neighbor approach (Astiz et al., 1999) 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. In addition, they exhibit good correlation with known local tectonic features, relating the 1981 Santa Barbara Island (ML=5.3) earthquake with the Santa Cruz fault, the July 13, 1986 Oceanside (ML=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. Locations for events in two regions do not appear to be related to known submarine escarpments: (1) events near the southern tip of Catalina Island are located near a quarry on this island, and (2) some events that appear near the Coronado Bank escarpment in the SCSN catalog are located closer to shore, although they are aligned parallel to this bathymetric feature. Over 3000 of the offshore events during this time period are associated with the 1986 Oceanside earthquake and its extended aftershock sequence. Our final locations clearly define a northwesterly dipping fault plane for the Oceanside event, but in cross-section they 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 in this region.

Key Words
United States, East Pacific, focal mechanism, geologic hazards, continental borderland, offshore, magnitude, California, spatial distribution, errors, Southern California, fault planes, seismicity, marine methods, Pacific Ocean, risk assessment, epicenters, seismic networks, active faults, earthquakes, accuracy, faults, fault zones

Citation
Astiz, L., & Shearer, P. M. (2000). Earthquake Locations in the Inner Continental Borderland, Offshore Southern California. Bulletin of the Seismological Society of America, 90(2), 425-449. doi: 10.1785/0119990022.