A Crustal Velocity Model for Locating Earthquakes in Monterey Bay, California

Abstract

The seismicity of the Monterey Bay displays a sparse distribution of events with a majority in northern Monterey Bay, on the San Gregorio fault. The paucity of near-shore and offshore seismic recording instruments 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 on major faults that crosscut the Bay. New three-component seismic data were acquired during 1997–1999 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 the University of California, Santa Cruz. We have analyzed phases from earthquakes in the Monterey Bay vicinity located by these instruments and used them to supplement the adjacent coastal stations of the permanent Northern California Seismic Network. A new one-dimensional velocity model for the region requires slow velocities from 2 to 6 km that we attribute to sheared granites observed in the Salinian Block in the center of Monterey Bay. Velocities in the 10–16 km layer are consistent with continental crustal velocities. A sharp increase in velocity at ∼16 km suggests a boundary that results from underplating of oceanic crust. This underplated zone appears to extend to a depth of ∼27–30 km where we observe normal upper mantle velocities near 8.0 km/sec. New details from the ocean-bottom corehole and coastal RefTek 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 interpretation.