BEGNAUD, MICHAEL L., MBARI, Moss Landing, CA; KAREN C. McNALLY, UCSC,Santa Cruz, CA;DEBRA S.STAKES, AND VICKY A. GAL-LARDO, MBARI, Moss Landing, CA

Abstract: A New Crustal Velocity Model for the Monterey Bay Coastal Region and its Tectonic Implications

The seismicity of the Monterey Bay displays a sparse distribution of events with a majority on the northern San Gregorio fault (SGF).The paucity of seismic recording instruments in the vicinity of Monterey Bay and the use of velocity models from inland regions for earthquake hypocentral locations and focal mechanisms have led to uncertainties and inaccuracies for seismic events within the bay. New three-component seismic data have been acquired during 1997-1998 by the Monterey Bay Aquarium Research Institute (MBARI) Margin Seismology project using ocean-bottom digital corehole and standard seismometers, and coastal digital RefTek instruments obtained from IRIS-Passcal by UCSC. We have analyzed phases from earthquakes in the Monterey Bay vicinity located by these instruments supplemented by the adjacent coastal stations of the permanent USGS network. New velocity models for the region require slow velocities from 2-10 km that we attribute to sheared granites known to exist in the Salinian Block in the center of Monterey Bay. Velocities in the 10-22 km layer are consistent with continental crustal velocities. A sharp increase in velocity at 22 km suggests a boundary that results from either asthenospheric upwelling or underplating of oceanic crust.This zone of upwelling or underplating appears to extend to a depth of ~30 km where we observe normal upper mantle velocities near 8.0 km/s. New details from MBARI's corehole instruments for events along the SGF and Monterey Bay fault zones hint at local fine-scale structures and have implications for tectonic history and plate reconstruction interpretations.

AAPG Search and Discovery Article #90920©1999 AAPG Pacific Section Meeting, Monterey, California