LATE HOLOCENE RUPTURE HISTORY OF THE SOUTH-CENTRAL SAN ANDREAS FAULT AT VAN MATRE RANCH SITE, CARRIZO PLAIN NATIONAL MONUMENT, CALIFORNIA

Characterizing long-term rupture patterns for active faults is integral to understanding fault dynamics and evaluating seismic hazard. The Carrizo Plain has long been regarded as a site of world-class examples of strike-slip faulting and provides significant insights on past earthquake activity along the South-Central San Andreas Fault (SAF). Recent 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 SAF during the past 800 years. Paleoseismic data from a site in-between is necessary to provide new timing constraints to the correlation of earthquake events between the two sites. Two connected fault-perpendicular trenches, both 3.5 m deep and 30 m long, were excavated at the new Van Matre Ranch (VMR) paleoseismic site, located ~20 km SE of Bidart Fan and ~80 km NW of Frazier Mountain. These trenches were excavated across a single, linear fault scarp from the active portion of the SAF. The primary fault zone in the trenches is 5 m wide and located below a prominent, linear fault scarp observed at the surface. This featureless fault zone juxtaposes the distal sediments of the VMR fan to the east against deposits of a more proximal fan to the west. Trenches revealed distinct stratigraphic packages separated by >0.5 thick bioturbated zones, suggesting that the earthquake record is incomplete. Several different evidence types including fissures, upward termination of faults, apparent vertical offset, and lateral unit thickness changes were used to identify as many as five surface-rupturing earthquakes. Detrital charcoal radiocarbon ages indicate that four of the five earthquakes recognized in the two trench walls occurred out of sequence during the last 9000 years. The penultimate earthquake is identified by evidence including mole track, truncations, and apparent vertical offsets, and has occurred sometime after 360±20 b.p. (uncalibrated C14 age). Focused deformation across the 5m-wide fault zone and discontinuous deposition within the last 1000 years limit the potential of the VMR site to be further developed to correlate paleoearthquake records between Bidart Fan and Frasier Mountain sites.