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 Ranch 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 of about 22 mm/yr at Melendy Ranch is 12₋₅⁺⁷ mm/yr less than the long‐term rate found along the San Andreas fault south of the creeping segment, and it is greater than the minimum long‐term rate of about 12 mm/yr found along the fault north of the creeping segment on the San Francisco Peninsula. Thus the long‐term slip‐rate data suggest a ∼20 mm/yr northwestward decrease in slip rate along the San Andreas fault. The northwestward decrease in long‐term slip rate is attributed to a two‐part transfer of slip, south of Hollister, from the San Andreas fault to the Paicines and Calaveras faults. If the Paicines and Calaveras faults south of Hollister have a combined long‐term slip rate of about ∼20 mm/yr, then a similar rate may also represent slip across the Calaveras and Hayward faults north of Hollister.