June 3, 2010 A new catalog of earthquake locations and focal mechanisms for the California Central Coast underscores the fault complexity of the region and identifies newly observed features offshore near San Luis Obispo.
The Central Coast is bounded on the east by the San Andreas Fault, the major plate boundary fault, and lies between the greater San Francisco and Los Angeles areas. This coastal region is not as densely instrumented or as tectonically well understood as the San Andreas Fault or the major urban areas.
The identification of new faults, and the reinterpretation of known faults, suggests that further work is necessary to better constrain the seismic hazards of the Central Coast. While the locations and focal mechanisms (the direction of slip in an earthquake and the orientation of the fault on which it occurs) for aftershocks of the 2003 M6.5 San Simeon and 2004 M6.0 Parkfield earthquakes are similar to those found in previous aftershock studies, the seismicity features in the offshore region near San Luis Obispo are sharpened considerably by this study.
The most prominent newly-observed feature is the Shoreline Fault, a ~25 km-long vertical strike-slip fault running parallel to the coastline just offshore of Point Buchon. Several smaller strike-slip seismicity lineations are also observed in Estero Bay, along with a deep reverse structure at the depth of the top of the remnant subducted slab. Strike-slip faulting is observed along the Hosgri-San Simeon Fault system, up to ~10-15 km inland from the Hosgri Fault in Estero Bay and near Point Buchon, and on the onshore Rinconada and West Huasna Faults.
The Shoreline Fault in particular requires further study to better constrain its geometry, how it may connect to the Hosgri Fault or other faults to its east, its slip rate and whether it has produced large earthquakes in the past.
This research was recently published in the Bulletin of the Seismological Society of America (BSSA)
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- Hardebeck et al. Seismotectonics and Fault Structure of the California Central Coast. Bulletin of the Seismological Society of America, 2010; 100 (3): 1031 DOI: 10.1785/0120090307
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