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Slip Rates of the Hosgri and Shoreline Fault Zones Offshore Point Buchon, Central California – Results of High-Resolution 3D Seismic Reflection

​Recent marine high-resolution, low-energy seismic surveys (LESS) were undertaken as part of the Pacific Gas and Electric Company’s seismic hazards analyses offshore of the Diablo Canyon Power Plant in central California to understand fault activity within the region and improve seismic hazards analyses for the plant.

23 Apr 2015
G. Green, S. Nishenko, P. Hogan, H AbramsonWard, R. Dame


These data, along with multibeam echoesounder, PG&E legacy, USGS and other geophysical data revealed surface and shallowly buried (10-240 m) structural and geomorphic features not observed before. Processed 2D and 3D seismic-reflection profiles and 3D volume attributes provide extremely detailed images of faults, stratigraphic horizons and geomorphologic features that are used to estimate offsets and slip-rates along the Hosgri and Shoreline fault zones. Geomorphic piercing points consisting primarily of buried paleo-stream channels, submarine slope channels, and paleo-shorelines (strandlines) were used to assess offsets. Sequence stratigraphic units and global sea level curves were used for age control to calculate slip-rates. The results of the LESS studies indicate that the Hosgri Fault Zone exhibits areas of transpression and transtension and locally defines the continental shelf edge in the San Simeon to Point Sal offshore region. A buried submarine channel in the Point Sal area is offset by the fault and provides the best constrained estimated long term dextral slip-rate of ~1-3 mm/yr. In addition, the relationship of the Shoreline fault zone, including the Point Buchon fault segments, to the Shoreline seismicity lineament and its total offshore length was mapped. The Shoreline fault zone’s total length is mapped at ~45 km and bifurcates in San Luis Obispo Bay to trend east where it joins the Oceano fault zone and southeast as the main strand to intercept the coast near the Santa Maria River mouth. The slip rate for this fault zone is estimated to be less than 0.1 mm/yr., an order of magnitude less than that for the Hosgri Fault Zone.


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