Crustal and uppermost mantle structure under southern California

Abstract

The travel times of Pg and Pn arrivals are used together as a new approach to estimate the station delays, lateral velocity variations in the upper crust and upper mantle, crustal thickness variations, and Pn anisotropy in the upper mantle under southern California. The main advantage of this approach is that crustal effects are estimated directly from the data and therefore do not contaminate the results from Pn arrivals. Significant negative station delays are found in the Colorado desert, San Jacinto Mountains, San Gabriel Mountains, central Mojave desert and from the southern Ventura Basin to Santa Cruz Island in addition to the positive delays associated with well-known sedimentary basins. Lateral velocity variations in the upper crust exceed a few percent only in the Coso ranges. These variations show no clear association with surface faulting; they also show little resemblance to the upper mantle velocity variations, which have fast velocity under the Mojave and near the offshore area and slow velocity in most other areas. The southern California Moho shows large variations. Large crustal thinning occurs near the Salton Sea, northwest of the White Wolf fault, in the Coso Range near San Diego and offshore, while the San Bernardino Mountains, western Granite Mountains, and eastern Mojave (south of the Ludlow fault) are underlain by roots. A significant amount (3%) of Pn anisotropy is present in the uppermost mantle with a fast direction N 75° ± 4°W. This could be explained very well by averaging the Pn anisotropy estimates east and west of the San Andreas.