Source Parameters of Eastern California and Western Nevada Earthquakes from Regional Moment Tensor Inversion

Abstract

The moment tensors and centroid depths are estimated for eastern California and western Nevada earthquakes from regionally recorded long-period seismograms using the moment-tensor inversion method. We compiled the moment tensor solutions into a catalog complete for earthquakes with M W > 4 since 1990. The earthquakes in this study are mostly located within the aftershock zones of Eureka Valley, Double Spring Flat, Coso, Ridgecrest, Fish Lake Valley, and Scotty's Junction, with the remaining earthquakes distributed across the region. We validated the moment tensor solutions by a comparison with Harvard centroid moment tensor (CMT) and P -wave first-motion focal mechanism solutions. The mean difference in strike, dip, and rake between our moment tensors and either Harvard CMT solutions or first-motion focal mechanisms were less than approximately 15° with a standard deviation less than 10°. We also examine the solution mean and variance by inverting resampled datasets using the delete- j Jackknife resampling method. Based on two well-recorded sample events, with more than 11 and 14 associated recording stations, the P - and T -axis trend and plunge can be determined to within ±10° when at least three or four well-distributed stations are used in the inversion. We separated the T -axis trends based on the approximate boundaries for three tectonic regions: Sierra Nevada, Walker Lane-eastern California shear zone, and Basin and Range. Relative to the mean T -axis trend for the Sierra Nevada, the mean T -axis trend for the Walker Lane region is rotated clockwise by 25° and the mean T -axis trend for the Basin and Range is rotated clockwise by 40°. We separated the earthquakes into strike-slip and normal-slip focal mechanisms based on a simple rake angle criterion. About 70% of modern earthquakes and 73% of the 11 large historical earthquakes since 1860 are in the strike-slip category. The fraction of seismic moment released as strike-slip earthquakes is approximately 50% for modern earthquakes and 75% for historical earthquakes. The difference is well within the variability expected from short catalogs. These results emphasize the importance of the component of right-lateral shear within the region and are qualitatively consistent with recent geodetic results. Manuscript received 11 January 2002.