Egill Hauksson, Crustal structure and seismicity distribution adjacent to the Pacific and North American plate boundary in southern California,  J. Geophys. Res., 105, 13,875-13,905, 2000.

These are the shear velocities. The initial view is approximately to the north.  The most prominent features are the higher upper mantle velocity beneath theContinental Borderland.  The red  (low-velocity) regions are the surficial (1km depth) expressions of the southern San Joaquin Valley, the Los Angeles and Ventura Basins, and the Imperial Valley  The velocities are linearly interpolated in depth onto a 1 km grid.  The vertical exaggeration factor is about 5.