LATE HOLOCENE RUPTURE HISTORY OF THE SOUTH-CENTRAL SAN ANDREAS FAULT AT VAN MATRE RANCH, CALIFORNIA

Characterizing long-term rupture patterns for active faults is integral to understanding fault dynamics and evaluating seismic hazard. Paleoseismic data from the Bidart Fan site and the Carrizo Plain suggest frequent but variable magnitude earthquakes occurring every 88 ± 41 yr. The new paleoseismic trenches at the Van Matre Ranch (VMR) site, located in between the Bidart and Frazier Mountain sites, were opened to confirm the Bidart-Frazier Mountain event correlations and put additional constraints on the timing of these events. Two fault-perpendicular trenches excavated across a linear fault scarp revealed evidence for five earthquakes, contained within four distinct stratigraphic packages. These lithological packages are separated by >0.5 m thick zone of bioturbation. We used a combination of 10 radiocarbon and 5 OSL ages to constrain the chronology of the surface ruptures at VMR site. Earthquake event ages modeled using Oxcal are Event A (1857 A.D.), Event B (1681 – 1796 A.D.), Event C (1223 – 1447 A.D.), and Event D and E (both occurring between 7570 – 713 B.C.). Our new chronological data for the penultimate earthquake (1681 – 1796 A.D.) provides a tighter constraint than the data from the Bidart Fan site (1631-1823 A.D.). This new data also falls within the age constraints of the penultimate earthquake at the Frazier Mountain site (1733 – 1854 A.D.). Assuming that the penultimate earthquake evidence at Bidart Fan, Frazier Mountain, VMR, as well as the sites along the Cholame section are the result of a single continuous rupture, the extent of the penultimate earthquake was as much as 200 km and is constrained to 1710 – 1794 A.D. as modeled by Oxcal. Assuming elastic rebound behavior, the calculated slip estimate for the penultimate earthquake is 6.1 ± 2.8 m. The magnitude of this slip estimate suggests that the penultimate earthquake was also a large magnitude, 1857-like event.