Structure and Earthquake Offsets on the San Andreas Fault at the Wrightwood, California, Paleoseismic Site

Abstract

Thirty-eight trenches and natural exposures across the San Andreas fault zone, four km northwest of Wrightwood, California, reveal the structure and stratigraphy of this paleoseismic site. A 25-m-thick stratigraphic section of late Holocene peat, debris flows, and fluvial sediments have accumulated syntectonically and are deformed in a complicated flower, or flake, structure within the fault zone. Whereas slip is everywhere parallel to the trend of the San Andreas fault, layering within the fan-shaped section guides the surface faulting, and transtensive portions of the structure are connected to transpressive portions by bedding-parallel detachment faults. The structure has grown through time, widely distributing the evidence for individual earthquakes. The broad zone of deformation and rapid sedimentation permit the characterization of an unusually large number of prehistoric earthquakes. The presence of at least four geomorphically similar sites within 10 km along the fault zone suggests that this kind of site is common and other sites should be exploited. To date, the upper 12 m of section, deposited during the past ∼1600 yr, has been completely characterized and dated to show evidence of 14 prehistoric earthquakes. The A.D. 1857 earthquake produced only 1-2 m of slip here, commensurate with the site9s proximity to the southern end of the rupture. Six to 10 m of slip accumulated during the past three earthquakes, and 30-60 m of cumulative slip occurred during the past 14 events, suggesting a mean displacement of 2-4 m per event at the site. Coupled with the mean recurrence interval of slightly more than 100 yr, this implies a slip rate of 2-4 cm/yr for the San Andreas fault north of its junction with the San Jacinto fault.