Recent Rupture History of the San Andreas Fault Southeast of Cholame in the Northern Carrizo Plain, California

Abstract

We conducted a paleoseismic study on the San Andreas fault (SAF) southeast of Cholame, California, to investigate the record of earthquakes along an 80-km paleoseismic data gap between Parkfield and the Carrizo Plain. At the LY4 site, located 37.5 km southeast of Highway 46 along the SAF, we excavated a fault-perpendicular trench on the distal end of an alluvial fan that emanates from the Temblor Mountains to the northeast. We found evidence of three and possibly four ruptures recorded within the stratigraphy. The only age constraints are radiocarbon dates on a paleosol three units (50 cm) below the oldest event horizon and the presence of recently introduced exotic pollen species in an upper unit. The radiocarbon dates indicate there have been at least three surface-rupturing events at the LY4 site since cal. A.D. 1058-1291. Exotic (historic) pollen in the top of a unit possibly cut by the youngest event suggests that an earthquake affected at the LY4 site close to A.D. 1873-1874. Manuscript received 11 January 2000.