A seismicity study of the Monterey Bay
region using a temporary deployment of three-component digital ocean-bottom and land
Michael Begnaud, Ph.D.
Monterey Bay Aquarium Research Institute
Wednesday, September 29, 1999
3:00 p.m.Pacific Forum
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.
variation between Lion's Mane jellyfishes from Australian coastal waters
Last updated: December 19, 2000