P-wave tomography of crust and upper mantle under Southern California: Influence of topography of Moho discontinuity

Abstract

We use 146 422 P-wave arrival times from 6 347 local earthquakes recorded by the Southern California Seismic Network to determine a detailed three-dimensional P-wave velocity structure at 0–35 km depth. We have taken into account the Moho depth variations, which were obtained by seismological methods. Checkerboard tests suggest that our inversion results are reliable. Our models provide new information on regional geological structures of Southern California. At shallow depths P-wave velocity structure correlates with surface geological features and expresses well variations of surface topography of the mountains and basins. The velocity structure at each layer is characterized by block structures bounded by large faults. Ventura Basin, Los Angeles Basin, Mojave Desert, Peninsular Ranges, San Joaquin Valley, Sierra Nevada, and Salton Trough show respectively all-round block. San Andreas Fault becomes an obvious boundary of the region. To its southwest, the velocity is higher, and there are strong heterogeneity and deeper seismicity; but to its northeast, the velocity is lower and shows less variation than to the southwest, the seismicity is shallower. To investigate the effect of the Moho geometry we conducted inversions for two cases: one for flat Moho geometry, another for a Moho with lateral depth variations. We found that the topography of the Moho greatly affects the velocity structure of the middle and lower crust. When the Moho topography is considered, a more reasonable tomographic result can be obtained and the resulting 3-D velocity model fits the data better.