HOLOCENE SLIP RATE AND EARTHQUAKE RECURRENCE OF THE NORTHERN CALAVERAS FAULT AT LEYDEN-CREEK, NORTHERN CALIFORNIA

The northern Calaveras fault traverses a heavily populated area in the eastern San Francisco Bay region and has not had a large earthquake i n more than 130 years. To obtain data on the number, timing, and recur rence of large paleoearthquakes, we conducted paleoseismologic investi gations at Leyden Creek, which crosses the fault in the rugged souther n East Bay Hills. The site is characterized by a prominent west facing scarp and five fluvial terraces on the western (upstream) side of the fault. On the eastern (downstream) side of the fault, the creek flows through a narrow bedrock canyon that constricts the modern valley and has constrained the location of a late Pleistocene paleovalley. The m argin of a buried bedrock valley west of the fault trends nearly perpe ndicular to the fault and is offset 54 (+18, -14) m in a right-lateral sense from the narrow bedrock canyon. Based on radiocarbon ages for a lluvial sediments predating and postdating this paleovalley margin, we estimate an age of 11.5 (+3, -1) ka for the valley margin and a Holoc ene slip rate of 5 +/- 2 mm/yr for the fault at Leyden Creek. Slickens ides exposed in multiple trenches across the fault show that the most recent movement was predominantly lateral with a minor component of do wn-to-the-west slip. Multiple displaced scarp-derived colluvial deposi ts are interpreted as results of five or six surface ruptures within t he past 2500 years. Twenty-one radiocarbon samples from scarp-derived colluvium and interfingered alluvial deposits suggest an average inter val between surface rupture earthquakes of 250 to 850 years.