Constraining continental margin seismicity by extending on-shore seismograph stations to critical off-shore sites

The westernmost North American plate margin in Central California includes major faults with both onshore and offshore segments. Prior to deployment d uring 1997 and 1998 of a suite of ocean-bottom seismometers within Monterey Bay, the distribution and focal mechanisms of offshore earthquakes were po orly constrained. The use of ocean-bottom seismic stations improves the acc uracy of locations for events in Monterey Bay, especially for those far off shore, and permits more robust focal mechanism solutions by reducing uncert ainties in strike, dip, and rake. These initial results, using U.S. Geologi cal Survey crustal velocity models and location parameters, reduce the appa rent scatter of seismic events and focal mechanisms, and the more consisten t solutions provide valuable information on fault characteristics for the S an Gregorio fault (SGF) and Monterey Bay fault zone (MBFZ). Most seismic ev ents are relocated to the northern section of the SGF defining a zone of co mpressional deformation and relatively high microseismicity. The MBFZ is th e source for fewer, but important and well-located events. Surprisingly, no events were observed on the southern SGF offshore of Carmel and Pebble Bea ch, despite the well-defined trace of the San Gregorio along Carmel Canyon.