Deep structure of southern California

We used 214,210 P-wave arrival times from 7536 local earthquakes and 16,470 travel-time residuals from 332 teleseismic events recorded by the southern California Seismic Network to determine a detailed three-dimensional (3D) P-wave velocity structure of the crust and mantle down to 600 km depth beneath southern California. In this study, we have taken into account the Moho topography under this region determined by a previous receiver-function study. We found that the undulations of the Moho discontinuity affect considerably the tomographic images of the lower crust and uppermost mantle. When the Moho topography is taken into account, ray paths and travel times can be computed more accurately. The tomographic images show a very heterogeneous structure in the crust and upper mantle under southern California. The velocity structure in the shallow depth correlates well with the surface geological features. Three major anomalies in the upper mantle are revealed clearly beneath the southern Sierra Nevada, Salton Trough and Transverse Ranges. Compared with the previous result, both the shape and size of the three anomalies show some differences. Our result revealed two prominent low-velocity (low-V) anomalies above the high-velocity (high-V) anomalies in the upper mantle under southern Sierra Nevada and the Transverse Ranges. We consider that the upper-mantle anomaly under the Transverse Ranges was formed through convergence and sinking of the entire subcrustal lithosphere. Vertical motions and removal of the dense crust root caused the high-V anomaly under the southern Sierra Nevada. The prominent low-V anomalies above the two high-V anomalies were either caused by the crustal materials pulled down to the uppermost mantle or by upwelling of partial melt in asthenosphere when the sinking of lithosphere occurs. The Salton Trough low is the response to the lithospheric extension when the Pacific plate was rifted away from the North American plate.