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A tectonic interpretation of earthquake focal mechanisms and hypocenters in Ridge Basin, southern California

UNITED STATES GEOLOGICAL SURVEY ALBUQUERQUE SEISMOLOGICAL LABORATORY, ALBUQUERQUE, NEW MEXICO 87115

Abstract

Ridge Basin is in the territory of the current uplift in southern California. The basin is bounded by the San Gabriel and San Andreas faults. During a seismic experiment conducted there in 1972 to 1973, 42 small earthquakes were located in the region of the basin. Approximately one-third of the epicenters lie near the San Gabriel fault for a distance of more than 10 km. Their composite focal mechansim plot does not show uniquely defined nodes. However, the data permit a solution that agrees with known parameters of the San Gabriel fault. Namely, a vertical fault that strikes northwest, with a right-lateral strike-slip displacement. A composite fault-plane solution for most of the remaining events in the basin gives two well-constrained nodes; one dips southeast nearly parallel with the dip of the hypocenter zone and, therefore, was assumed to be the fault plane. It strikes northeast almost parallel to the basin hinge line of late Pliocene-early Pleistocene age, and the solution indicates reverse right-oblique displacement. The history of the basin suggests that reverse faulting should be expected to accompany subsidence, and subsidence is indeed suggested in the northwest part of the basin where the valleys appear inundated with sediments. Whereas the dip-slip component of faulting corresponds to basin formation, the strike-slip component of the right-oblique movement may relate to the tectonic process that is locking the San Andreas fault. Since the Pliocene was a time of formation of the basin, and the mid- and late Pleistocene mainly was a time of folding of lower Pleistocene and older rocks and erosion of them, the proposed model seems more analogous to the Pliocene tectonic cycle than to the Pleistocene one. Both Pyramid Dam and Castaic Dam are situated in the basin. Their location with respect to the faults of the earthquakes is not resolved.