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Paleoseismic Investigation of Van Matre Ranch Site Along the Carrizo Plain Section of the San Andreas Fault

Nick J. Inserra, Clayton Nelems, Radwan Muthala, Jeremy Torres, & Sinan O. Akciz

Published August 15, 2017, SCEC Contribution #7859, 2017 SCEC Annual Meeting Poster #154

Correlation of paleoseismic data from both Bidart Fan and Frazier Mountain sites indicate recurrence intervals of ~100 yrs between large surface rupturing earthquakes in the Carrizo Plain and Big Bend sections of the San Andreas Fault during the past 800 years. Three new trenches were opened at the Van Matre Ranch paleoseismic site, located ~20 km SE of Bidart Fan and ~80 km NW of Frazier Mountain sites, to confirm and expand the earthquake record. Two fault perpendicular trenches (T3 and T4), and a connector trench at their eastern ends (T2), were opened across the SAF scarp. The main fault zone in T3 and T4 contained no capable stratigraphic units due to focused bioturbation and shearing. East of the fault zone there are several layers of upward-fining fine sand-to-silt with no evidence of bioturbation or erosion. West of the fault zone, several distinct lithological packages separated by zones of bioturbated massive sand give a clearer picture of deformational history. In trenches T3 and T4 we observed evidence for the last two paleoearthquakes in sequence and another two earlier events that are potentially out of sequence. Apparent vertical offsets, filled fissures, upward terminations and fault-bounded stratigraphic units were used to identify these earthquakes. Evidences for all four earthquakes are contained within 3 distinct lithological packages. The upper package is a series of upward-fining sediments ranging from well-cemented fine sand to silty-clay. It is overlain by massive sand and silty layers near the surface. Evidences for the most recent (event A) and the penultimate (event B) earthquakes effect this upper package. The middle package is much coarser, with medium to coarse sand below clast and matrix supported pebble units. Thin, discontinuous carbonate coatings on the underside of most pebbles, and complete coating of some, were observed. Event C affects this middle package, but is overlain by the upper package. The oldest lithological package is a series of upward-fining sands and silts with an overlying coarse pebble unit without any pedogenic carbonate cement. Event D affects this lower package but is capped by the middle package. Over 60 detrital charcoal, 10 Optically-Stimulated Luminescence samples and 9 bulk samples of paleosols were collected from layers within and surrounding the sedimentary packages. These different samples cover almost all continuous units and collectively will allow us to constrain our earthquake ages.

Key Words
Paleoseimology, San Andreas Fault, Carrizo Plain

Inserra, N. J., Nelems, C., Muthala, R., Torres, J., & Akciz, S. O. (2017, 08). Paleoseismic Investigation of Van Matre Ranch Site Along the Carrizo Plain Section of the San Andreas Fault. Poster Presentation at 2017 SCEC Annual Meeting.

Related Projects & Working Groups
San Andreas Fault System (SAFS)