Late Quaternary slip rate on the Oak Ridge fault, Transverse Ranges, California: Implications for seismic risk

Abstract

The slip rate on the Oak Ridge fault at South Mountain in the densely populated lower Santa Clara Valley, California, is estimated as 5.9–12.5 mm/yr since the end of Saugus deposition at 0.4–0.2 Ma. A displacement of 3 m per event, assumed on the basis of surface rupture on the 1952 Kern County, 1971 San Fernando, and 1978 Tabas‐e‐Golshan earthquakes, gives an average recurrence interval of 250–500 years, with the major uncertainty being the age of the top of the Saugus. Displacement of 2375–2490 m of the top of the Saugus at South Mountain includes piercing‐point displacement and distributed displacement from drag folds near the fault; both are the near‐surface expression of faulting at potential main shock depths beneath well control. Slip rates for the nearby San Cayetano and Red Mountain faults are less well constrained because post‐Miocene strata are largely absent in their hanging‐wall blocks, but available evidence suggests that their slip rates are in the same range as that for the Oak Ridge fault. The lower Santa Clara Valley has not had a large, damaging earthquake in 200 years of record keeping, although the December 21, 1812, tsunami‐producing earthquake could have occurred on the offshore continuation of the Oak Ridge or Red Mountain fault. A trench on the Harmon alluvial fan near the Ventura County Government Center revealed evidence of cracks filled with sediments from below, suggesting liquefaction during a Holocene earthquake. Earthquake repeat times measured in hundreds of years have been reported by others for the 1971 San Fernando fault and a small fault near the Red Mountain fault, and comparable repeat times are suggested for a normal fault within the hanging‐wall block of the Oak Ridge fault. These are consistent with the average recurrence interval calculated for the Oak Ridge fault, suggesting that a destructive earthquake may strike the lower Santa Clara Valley in the near future.