Abstract

A paleoseismic study conducted along the Cholame segment of the San Andreas fault provides evidence for three earthquakes and the amount of lateral offset for the most recent event (1857 Fort Tejon earthquake). Excavations at the Las Yeguas (LY4) site include five fault-perpendicular, two parallel, and several hand-dug trenches. Abruptly truncated sand and silt layers that are not correlative across the fault zone constrain the timing of the penultimate event (L2) between cal. A.D. 1030-1300 and 1390-1460. Vertical offset, shearing, and fracturing of silty sand and gravel units that overlie L2 and historical artifacts that bracket the timing of the MRE (L1) provide evidence that the most recent ground-rupturing event, L1, occurred between cal. A.D. 1390-1460 and ∼1865. L1 is likely the 1857 Fort Tejon earthquake. Tectonic silt-filled fractures that dissect historic gray-tan silt and sand suggest a ground shaking or a triggered slip event (L0), which occurred after L1. Three-dimensional excavation of an alluvial fan edge (unit 4) indicates that 3.0 ± 0.70 m of near-fault brittle slip occurred during the 1857 earthquake at this site.

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