Seismotectonic patterns across a part of the Central California Coast Ranges

The seismotectonic pattern determined along a 65-km-wide corridor across the central California Coast Ranges (Dehlinger and Bolt, 1987), from the San Andreas fault to seaward of the Hosgri fault, is used to identify associated structures in the upper crust. The seismogenic zone is approximately 12 km thick in the corridor, and forms a 90-km-wide border zone of the upper Pacific lithospheric plate. This border includes three provinces (from northeast to southwest): the San Andreas fault zone, an adjacent 40- to 50-km-wide seismically quiescent province, and a 40- to 50-km-wide compressive province along the coastline. These provinces are characterized by distinct focal parameters and distinct rock types, and transition boundaries between these provinces are relatively narrow. The upper crust in the quiescent province consists of high-strength granites of the Salinian block; in both the San Andreas and the contractional provinces, this part of the crust consists of low-strength Franciscan rocks. We conclude that differential strengths of the upper crustal rocks in the corridor have modified the broader, more regional stress fields acting across the North American-Pacific plates to produce the observed seismicity. The extent to which the earthquake data in the corridor corroborate the existence of a proposed deep detachment surface is examined. The detachment model has been suggested to account for the crustal shortening observed across the Coast Ranges (Crouch et al., 1984; Eaton, 1985), where lower crustal materials are being recycled into the mantle. The set of earthquake data analyzed here does not imply the presence of a detachment within the seismogenic zone; neither, although less directly, does it imply such a detachment at greater depth. If a deep detachment due to horizontal shortening does exist, it would be restricted to the southwest half of the corridor, as strike-slip, not horizontal shortening, predominates in the northeast half of the corridor. An alternate model, in which deformation beneath the seismogenic zone occurs by creep and flow over an extensive depth range, can be made to conform with upper crustal shortening in the southwest part of the corridor and with horizontal slip in the quiescent and the San Andreas provinces. Such types of deeper deformation are more consistent with the earthquake focal parameters in the corridor than is a detachment at or below the base of the seismogenic zone.