P-WAVE IMAGE OF THE CRUST AND UPPERMOST MANTLE IN SOUTHERN CALIFORNIA

We have determined a P-wave tomographic image of the crust and uppermost mantle in southern California by using 131,372 P-wave arrival times from 6,437 local earthquakes recorded by the Caltech-USGS Southern California Seismic Network in the past twelve years. The obtained image has a spatial resolution of 25 km in the horizontal direction and 8-11 km in depth. The tomographic image is found to correlate well with major surface geological features. In the shallow crust, sedimentary basins such as the Los Angeles Basin, Ventura Basin and Santa Maria Basin are well imaged as low velocities, while batholiths such as the Peninsular Ranges and San Gabriel Mountains are imaged as high velocities. In the deeper crust, the velocity is low beneath the Mojave Desert, Coso volcanic area and Salton Trough, while it is high beneath the Great Valley, Continental Borderland and the major basins. A high-velocity layer exists in the mid-crust beneath the Salton Trough, in good agreement with a previous study using seismic explosions and gravity data. In the uppermost mantle, the velocity is low beneath southeastern Sierra Nevada and the volcanic areas while it is high beneath the Mojave Desert and along the Pacific coast.