LATE QUATERNARY RATE OF SLIP ALONG THE SAN-JACINTO FAULT ZONE NEAR ANZA, SOUTHERN CALIFORNIA

The Anza section of the San Jacinto Fault, southern California, has not experienced a ground-breaking earthquake since at least 1918 and probably since before 1899, leading previous workers to designate this 20-km-long fault segment as a seismic gap. Study of displaced Pleistocene and Holocene alluvial fan and fluvial deposits across the San Jacinto Fault near Anza, California, dated using 14C and soil development age control, indicates a minimum right-lateral slip rate of 9.2 ± 2 mm/yr since 9.5 ka, and a slip rate of 11 +9 -5 mm/yr since 14 ka, 12 +9 -5 mm/yr since 17 ka, and 13 -6 +10 mm/yr since 50 ka. These estimates agree with a previously determined 700 ka to present minimum slip rate of 10 ± 2 mm/yr. Based on an estimated average slip rate of 12 mm/yr from the longer-term slip rate estimates, about 0.8 m of potential slip has accumulated if the 1918 earthquake broke the Anza segment; over 1.1 m of potential slip has accumulated if the last earthquake occurred prior to 1899. These data attest to the potential for an earthquake ≥ M6.5 for this section of the fault. -Authors