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Neotectonics of the Offshore Oak Ridge Fault near Ventura, Southern California

¹ U.S. Geological Survey
MS 999, 345 Middlefield Road
Menlo Park, California 94025
mfisher{at}usgs.gov
 (M.A.F., W.R.N., R.W.S.)

² Moss Landing Marine Laboratory
8272 Moss Landing Road
Moss Landing, California 95039
greene{at}mlml.calstate.edu
 (H.G.G.)

The Oak Ridge fault is a large-offset, south-dipping reverse fault that forms the south boundary of the Ventura Basin in southern California. Previous research indicates that the Oak Ridge fault south of the town of Ventura has been inactive since 200–400 ka ago and that the fault tip is buried by 1 km of Quaternary sediment. However, very high-resolution and medium-resolution seismic reflection data presented here show a south-dipping fault, on strike with the Oak Ridge fault, that is truncated at 80 m depth by an unconformity that is probably at the base of late Pleistocene and Holocene sediment. Furthermore, if vertically aligned features in seismic reflection data are eroded remnants of fault scarps, then a subsidiary fault within the Oak Ridge system deforms the shallowest imaged sediment layers. We propose that this subsidiary fault has mainly left-slip offset. These observations of Holocene slip on the Oak Ridge fault system suggest that revision of the earthquake hazard for the densely populated Santa Clara River valley and the Oxnard coastal plain may be needed.