Late Holocene Rupture History of the South-Central San Andreas Fault at Van Matre Ranch, California

Nick J. Inserra, & Sinan O. Akciz

Published August 15, 2019, SCEC Contribution #9832, 2019 SCEC Annual Meeting Poster #108

Characterizing long-term rupture patterns for active faults is integral to understanding fault dynamics and evaluating seismic hazard. Even for the south-central San Andreas Fault (SAF), sites with well-constrained past earthquakes evidence are few and geographically widely spaced. Recent paleoseismic data from Bidart Fan and Frazier Mountain sites, in the Carrizo Plain and Big Bend sections of the SAF, 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 an additional constraint on the timing of the Carrizo-only events. Two connected fault-perpendicular trenches were excavated across a linear fault scarp from the active portion of the SAF. The trenches revealed 5 earthquake evidences, contained within 4 distinct stratigraphic packages. These lithological packages are separated by >0.5 m thick bioturbated intervals. Time lost in these zones corresponds to hundreds or thousands of years. Evidences of the 1857 and penultimate earthquakes are preserved within the uppermost package. We used a combination of 10 radiocarbon and 5 OSL ages to define the chronology of the depositional phases and surface ruptures at VMR. Oxcal modeled ages of the earthquakes 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 event to the south at Frazier Mountain (1733 – 1854 A.D.). Assuming the Bidart Fan, Frazier Mountain, and VMR penultimate evidences are from a single continuous rupture, the extent of the penultimate event was as much as 200 km, and is constrained to 1733 – 1796 A.D. The ~100 yr between the most recent earthquakes, assuming elastic slip, should produce ~4 m of slip for 1857 in the Carrizo. Additionally, the calculated slip value for the penultimate earthquake, using Bidart Fan 3rd event age and VMR penultimate earthquake age, is 5.8 ± 2.17 m.

Key Words
Paleoseismology, San Andreas Fault, Earthquakes

Citation
Inserra, N. J., & Akciz, S. O. (2019, 08). Late Holocene Rupture History of the South-Central San Andreas Fault at Van Matre Ranch, California. Poster Presentation at 2019 SCEC Annual Meeting.

Related Projects & Working Groups
Earthquake Geology