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Shoreline and Oceano Fault Zones’ Geometry and Slip Rate Constraints, San Luis Obispo Bay, Offshore South Central Coastal California

As part of the Central Coastal California Seismic Imaging Project, high-resolution 3D low energy marine seismic-reflection data were acquired within San Luis Obispo Bay in 2011 and 2012.

23 Apr 2015
Phil Hogan, H. Gary Greene, Stuart Nishenko, and Bryan Bergkamp


Mapping of the sediment-buried bedrock surface using 2D and 3D data reveals that the trace of the Shoreline fault zone extends southeast across San Luis Obispo Bay to the coast near the Santa Maria River mouth, for a total length of ~45 km. The fault zone bifurcates at Souza Rock, where one strand trends toward the east-southeast, connecting with the Oceano fault zone onshore.
Both of these fault strands are crossed by Pleistocene low-stand paleochannels eroded into bedrock, and are buried by marine and non-marine sediment. Two paleoshorelines developed in Quaternary strata also cross the Shoreline fault, providing additional high-quality piercing points for measuring offsets along the fault zone. Possible ages of these features are estimated from Quaternary sea level curves and Pleistocene sequence stratigraphy.
The slip rate of the Shoreline fault zone as calculated using the paleochannel piercing points is estimated to be ~0.05–0.12 mm/yr. The estimated slip rate generated from the paleoshoreline piercing points is ~0.06 mm/yr. The highest-confidence slip rate estimate for the Shoreline fault zone is derived from the two paleostrandline piercing points. While no evidence for Holocene surface rupture of the Shoreline fault was identified, higher resolution (e.g. Chirp) data may be required to image recent offsets associated with a low slip rate fault.
The estimated slip rate for the Oceano fault (~0.1 mm/yr.) falls within published slip rate estimates for the Oceano fault onshore (0.01–0.20 mm/yr), and is comparable to the estimated slip rate of the Shoreline fault southeast of Souza Rock. The uncertainty in seismic hazard for the Central California Coast Region has now been reduced by improved estimates of slip rates on the Shoreline and Oceano fault zones.


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