Filling the paleoseismic gap between Bidart and Frazier Mountain: Exploration of Van Matre Ranch (VMR) paleoseismic site in the Carrizo Plain

Sinan O. Akciz, Joe Marino, Juliet O. Olsen, Eui-jo Marquez, James B. Salisbury, Alana M. Williams, Thomas K. Rockwell, Ramon R. Arrowsmith, & Lisa Grant Ludwig

Published 2015, SCEC Contribution #6061

In August 2015, we opened one exploration trench at the Van Matre Ranch (VMR) paleoseismic site (35.146321°, -119.688535°) to confirm and expand the Holocene earthquake history of the Carrizo section of the San Andreas Fault (SAF). Trench 1 is located across the simple and subtle 1857 scarp of the San Andreas Fault in an area which appears to receive alluvial fan units including well-laminated fine sand and silts originating from the Temblor Range. In trench 1, a 3 m deep, 1 m wide and 30 m long trench, we observed evidence for the last two or three paleoearthquakes in sequence and another one other earlier, but potentially out of sequence, event. Apparent vertical offsets, filled fissures, upward terminations and fault-bounded were used to identify these earthquakes. In Trench 1, we found the surface scarp to coincide with a featureless ~3m-wide fault zone. No major bioturbation zones were observed. Detrital charcoal pieces were not as abundant as the Bidart Fan site, ~ 20 km to the NE, but 34 individual charcoal pieces and 1 bulk sample from an organic-rich paleosol layer were collected. In addition, samples from six different quartz-rich fine- to coarse-sand layers near earthquake event horizons were collected for Infra-red stimulated luminescence (IRSL) dating. With the presence of good, undisturbed stratigraphy with abundant datable layers and good access on private property, VMR has the potential to be developed into a high quality paleoseismic site, providing much needed earthquake event data between the Bidart Fan and Frazier Mountain sites.

Citation
Akciz, S. O., Marino, J., Olsen, J. O., Marquez, E., Salisbury, J. B., Williams, A. M., Rockwell, T. K., Arrowsmith, R. R., & Grant Ludwig, L. (2015). Filling the paleoseismic gap between Bidart and Frazier Mountain: Exploration of Van Matre Ranch (VMR) paleoseismic site in the Carrizo Plain. Poster Presentation at 2015 SCEC Annual Meeting.

Related Projects & Working Groups
Earthquake Forecasting and Predictability