Abstract

A radio-telemetered network of high-gain seismographs allowed determination of hypocenters for 150 earthquakes with depths of 5 to 30 km in the Cape Mendocino area. Analysis of P- and S-wave arrival times indicates that the region exhibits a Poisson's ratio in excess of 0.3. The pattern of seismicity reveals a diffuse distribution. Composite fault-plane solutions indicate general north-south compression, consistent with generally inferred movements of the Pacific, North American, and Gorda plates. Near the cape, however, the principal compression is oriented roughly northwest-southeast, consistent with Gorda plate motion normal to the ridge. Right-lateral strike-slip movement appears to be the dominant faulting pattern. Thrust-fault mechanisms seen at the continental margin near the cape may reflect late Cenozoic underthrusting at the Gorda plate boundary. East-west cross sections of hypocenters show no indications of an active Benioff zone. These results are consistent with the hypothesis that the San Andreas fault joins the Mendocino fracture zone, which absorbs most of the right-lateral strike-slip motion. The Gorda plate is now deforming near the continental margin, apparently the consequence of transition from thrusting near the cape to right-lateral faulting northward as the mode of accommodation of the relative plate motions.

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