Abstract

The westernmost North American plate margin in Central California includes major faults with both onshore and offshore segments. Prior to deployment during 1997 and 1998 of a suite of ocean-bottom seismometers within Monterey Bay, the distribution and focal mechanisms of offshore earthquakes were poorly constrained. The use of ocean-bottom seismic stations improves the accuracy of locations for events in Monterey Bay, especially for those far offshore, and permits more robust focal mechanism solutions by reducing uncertainties in strike, dip, and rake. These initial results, using U. S. Geological Survey crustal velocity models and location parameters, reduce the apparent scatter of seismic events and focal mechanisms, and the more consistent solutions provide valuable information on fault characteristics for the San Gregorio fault (SGF) and Monterey Bay fault zone (MBFZ). Most seismic events are relocated to the northern section of the SGF defining a zone of compressional deformation and relatively high microseismicity. The MBFZ is the source for fewer, but important and well-located events. Surprisingly, no events were observed on the southern SGF offshore of Carmel and Pebble Beach, despite the well-defined trace of the San Gregorio along Carmel Canyon.

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