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Seismicity of South-Central Coastal California: October 1987 through January 1997

Geosciences Department
Pacific Gas and Electric Company
MC N4C, P.O. Box 770000
San Francisco, California 94177
mkm2{at}pge.com

Earthquakes recorded along south-central coastal California from October 1987 through January 1997 by the Pacific Gas and Electric Company's 20-station Central Coast Seismic Network and the U.S. Geological Survey's Northern and Southern California Networks are used to study their relation to previous seismicity, mapped Quaternary faults, and regional tectonic features. We developed P- and S-wave velocity models, station corrections, and duration magnitude calibrations to process 1184 reliable earthquake locations and 212 well-constrained focal mechanisms. Seismicity generally coincides with mapped Quaternary faults and areas of tectonic uplift, including the southern offshore Santa Maria Basin and areas to the west, and the uplifted blocks within the Los Osos tectonic domain. Comparison of our data to previous microearthquake locations (1969–1987) and larger (M 5) historical earthquakes shows that these patterns have been persistent. Seismically quiescent areas have few or no mapped Quaternary faults and typically are structural lows. The base of the seismogenic zone is about 12 km in depth, above the relic oceanic crust.

Microearthquake locations and strike-slip focal mechanisms document the strike-slip nature of the Hosgri fault zone, and its step-over to the San Simeon fault, along the western boundary of the Los Osos domain. Uplifting blocks within the Los Osos domain are most seismically active toward their western ends along the Hosgri fault zone, and they exhibit reverse and oblique faulting consistent with block uplift and interaction with the Hosgri fault zone. The observed patterns and character of microseismicity and macroseismicity clearly show transpressional deformation associated with the Hosgri fault zone overprinted by rotation and westward migration of the Western Transverse Ranges. Within this tectonic setting, clockwise rotation of the Western Transverse Ranges causes internal deformation of the Los Osos domain, and westward migration of the ranges appears to deflect the Hosgri fault zone westward and causes east-west-directed crustal shortening in the southern offshore Santa Maria Basin.

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