Late Holocene movement along the San Andreas Fault at Melendy Ranch: implications for the distribution of fault slip in central California

Abstract

An alluvial deposit of the San Benito River at Melendy Ranch, which is located along the creeping segment of the San Andreas fault in central California, records an 800-year-long average slip rate of 22 -4+6 mm/yr. The rate is calculated from one radiocarbon age and two measures of displacement and is based on various sedimentologic and stratigraphic data. Numerous other radiocarbon ages were determined for detrital charcoal from the alluvium at Melendy Ranch. These radiocarbon ages appear to be at variance with the age of the alluvium on the basis of sedimentologic and stratigraphic data. The 22 mm/yr long-term rate is similar to the 20-23 mm/yr range of slip rates that are recorded by geodetic instruments at Melendy Range and a 22 mm/yr slip rate recorded by an offset corral fence built in 1945. This suggests that the historic rate of movement along the San Andreas fault at Melendy Ranch is near its 800-year average. The long-term slip-rate data suggests a ~20 mm/yr northwestward decrease in slip rate along the fault which is attributed to a two-part transfer of slip, south of Hollister, from the San Andreas fault to the Paicines and Calaveras faults. -from Authors