COCORP seismic reflection profiles collected in 1977 on the west side of the San Joaquin Valley near Coalinga in southern California provide information on the Cenozoic and Mesozoic structures in the subsurface of the western Great Valley. The data show evidence of significant normal faulting during the Cretaceous, when this part of California was the site of a forearc basin. Neogene compression, probably associated with transform motion on the nearby San Andreas fault system, may be reactivating these pre-existing faults in a reverse sense and causing the active folding of the southern Coast Ranges. The recent Coalinga earthquake sequence that began on May 2, 1983, appears to correspond to movement on the high-angle reverse fault within the basement that is inferred from the COCORP lines.

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