Abstract

Crustal velocity structure for the region near Coalinga, California, has been derived from both earthquake and explosion seismic phase data recorded along a NW-SE seismic-refraction profile on the western flank of the Great Valley east of the Diablo Range. Comparison of the two data sets reveals P-wave phases in common which can be correlated with changes in the velocity structure below the earthquake hypocenters. In addition, the earthquake records reveal secondary phases at station ranges of less than 20 km that could be the result of S- to P-wave conversions at velocity interfaces above the earthquake hypocenters. Two-dimensional ray-trace modeling of the P-wave travel times resulted in a P-wave velocity model for the western flank of the Great Valley comprised of: (a) a 7- to 9-km-thick section of sedimentary strata with velocities similar to those found elsewhere in the Great Valley (1.6 to 5.2 km/sec); (b) a middle crust extending to about 14 km depth with velocities comparable to those reported for the Franciscan assemblage in the Diablo Range (5.6 to 5.9 km/sec); and (c) a 13- to 14-km-thick lower crust with velocities similar to those reported beneath the Diablo Range and the Great Valley (6.5 to 7.30 km/sec). This lower crust may have been derived from subducted oceanic crust that was thickened by accretionary underplating or crustal shortening.

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