A seismicity study of the Monterey Bay region using a temporary deployment of three-component digital ocean-bottom and land seismographs

Michael Begnaud, Ph.D.
Monterey Bay Aquarium Research Institute

Wednesday, September 29, 1999
3:00 p.m.—Pacific Forum

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The San Gregorio fault (SGF) is currently listed as Class A by the California Division of Mines and Geology, with the potential for a significant magnitude (>M6) earthquake. Since the Monterey Bay area is a rapidly developing economic and population center, it is important to evaluate the hazards and mechanics of the local coastal margin faults as well as the crustal velocity structure and active tectonics. The historic lack of sufficient seismic recording instruments in the vicinity of Monterey Bay and the use of velocity models from inland regions for determining earthquake hypocenters and focal mechanisms have led to uncertainties and inaccuracies in the local seismic data base. New three-component seismic data have been derived from the Monterey Bay Aquarium Research Institute's (MBARI) "Margin Seismology Project" to develop a new crustal velocity model and to more accurately map the seismicity and active Monterey Bay faults. MBARI's instrumentation includes ocean-bottom digital seismometers precisely-placed into boreholes and on sediment using MBARI's remotely-operated vehicle Ventana in 1996, 1997, and 1998.

The deployments in 1997-1998 were supplemented by ocean-bottom seismometers/hydrophones from Scripps Institution of Oceanography, Institut de Physique du Globe, Paris, and the University of California, Davis. 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. The 1998 MBARI offshore instruments were complemented by a temporary deployment of nine digital RefTek instruments obtained from IRIS-PASSCAL by UC Santa Cruz. Using these new seismic data, we developed a crustal velocity model for the Monterey region which requires slow velocities from 2-6 km that we attribute to sheared granites known to exist in the Salinian Block in the center of Monterey Bay. A sharp increase in velocity at 16 km suggests a boundary that results from either asthenospheric upwelling or underplating of oceanic crust. New details from the ocean-bottom and coastal 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.

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