Abstract

The slip rate on the Oak Ridge fault at South Mountain in the densely-populated lower Santa Clara Valley, California is estimated as 6.15 to 12.3 mm/yr since the end of Saugus deposition at 0.4 to 0.2 Ma. Slightly lower slip-rate estimates result from a different projection of the top of the Saugus in the hanging-wall block of the fault. An assumed displacement of 3 m per event gives an average recurrence interval of 250 to 500 yr, with the major uncertainty being the age of the top of the Saugus. Displacement of 2460 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 mainshock depths beneath well control. Slip rates for the nearby San Cayetano and Red Mountain faults are not well constrained because post-Miocene strata are absent in their hanging-wall blocks, but available evidence suggests that their slip rates are about the same as that for the Oak Ridge fault. Earthquake return times measured in hundreds of years have been shown by others for the 1971 San Fernando fault and a small fault near the Red Mountain fault, and comparable return times are suggested for a normal fault in the hanging-wall block of the Oak Ridge 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 an earthquake. The lower Santa Clara Valley has not had a large, damaging earthquake in 200 years of record-keeping, but one is expected there in the near future. The December 21, 1812 earthquake may have occurred on the offshore continuation of the Oak Ridge or Red Mountain fault.