Abstract

High‐resolution 3D low‐energy seismic‐reflection profile data were used to map Quaternary geologic structures and buried geomorphology within the uppermost 20–80 m stratigraphic section of San Luis Obispo Bay, California, and to illustrate the geometry of, and late Pleistocene displacement along, the Shoreline fault zone (SFZ) and Oceano fault zone. The SFZ is clearly imaged within the 3D survey area as a distinct 6.5‐km‐long throughgoing structure. The right‐lateral, strike‐slip SFZ intersects the reverse‐slip Oceano fault zone near a bathymetric high known as Souza Rock and continues southward toward Point Sal with a strike of N25°W. Interpretation of new 2D and 3D seismic‐reflection profile data shows an increase in the length of the originally estimated 23‐km‐long SFZ (mapped between Point Buchon to Point San Luis) to 45 km long (from Point Buchon to Point Sal), which increases the maximum magnitude for ruptures limited to the SFZ from M 6.5 to 6.7. Seismic imaging of offset late Pleistocene paleoshorelines and paleochannels that cross the southern strand of the SFZ in San Luis Obispo Bay indicate a dextral slip rate of 0.050.09  mm/yr within a 0.010.4  mm/yr range. Vertical offset of the marine isotope stage (MIS) 1–2 transgressive surface and older paleochannels by the Oceano fault zone indicate a preferred vertical slip rate between 0.01 to 0.07  mm/yr within a 0.01 to 0.2  mm/yr range. Although both the SFZ and the Oceano fault zone are shown to be active Quaternary faults, evidence of more recent (middle‐to‐late Holocene, <10  ka) fault rupture is equivocal in this low‐slip‐rate environment, based on the 2–3 m acoustic resolution of the 3D seismic‐reflection volume.

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