Non-SAF type focal mechanisms adjacent to the SAF, Mojave Segment: Implications for Blind Thrust Beneath the San Gabriel Mountains, Southern California

Abstract

Focal mechanisms of earthquakes with 2.5 ≤ M ≤ 4.1 from 1978 to 1990 were analyzed within a 15 km-wide belt of the San Andreas fault (SAF) zone, in the Mojave segment, southern California. Of the 29 events, 41% are strike-slip, 28% oblique--slip, 24% thrust, and 7% are normal fault types. Most of the thrust events are located in the central section of the studied zone, where the fault geometry is relatively simple. In contrast, most of the strike-slip events arc at the intersection between the SAF and the San Jacinto fault. Both stress tensor inverted from slip vectors and strain tensor calculated from earthquake moment tensors produce similar directions of maximum principal compressional axes that arc oriented in the direction between 351° and 5°, which is at an angle of about 60° to the strike of the SAF. These non-SAF type faulting events obviously can not be explained by simple shear motion on the SAF. Instead, they accrued on the structures that were activated in response to the regional N compression at the latitude of the Transverse Ranges. For the thrust events, we interpret that a back thrust (northward thrust) exists beneath the northern side of the San Gabriel Mountains. This back thrust is located at the depth of 8-13 km, within or beneath the Pelona schist. It is apparently associated with aseismic slip and uplifts the San Gabriel Mountains on the northern side.

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