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 of 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. ¿ American Geophysical Union 1988