Post-1857 fracturing and deflection of an apparent offset channel along the San Andreas Fault in the Carrizo Plain

Sinan O. Akciz, Lisa B. Grant Ludwig, Olaf Zielke, & J Ramon R. Arrowsmith

Under Review 2013, SCEC Contribution #1544

Topographic maps produced from Light Detection and Imaging (LiDAR) data are becoming indispensable for paleoseismic and neotectonic research because they provide unprecedented representation of faulting-related surface features. While increase in the spatial resolution of topographic datasets enable more subtle or vegetation covered possible piercing lines to be measured more precisely (repeatable and reproducible), its accuracy improvement over traditional field methods is still debated. Offset measurements of geomorphic features, made in the field or on a computer screen, commonly assume a linear or smooth, undeflected pre-earthquake geometry. Here we present the results from 25 field-reviewed, hand dug, ~30 cm deep, linear pits surrounding a ~20 cm deep channel which was initially interpreted to be tectonically offset by ~4.7 m at the Bidart site along the San Andreas Fault in the Carrizo Plain, California. Our observations show: 1) a sharp channel bend coincides with the trace of the 1857 A.D. fault rupture. 2) Channel swale is only within a gravely sheet flow deposit which has deposited since the Fort Tejon earthquake. 3) Surficial gravel and underlying mudflow deposits show no evidence of intense deformation compared to the underlying units which show strong evidence (moletracks, fissures, apparent offsets, unit thickness changes, etc.) of rupture in the 1857 earthquake. 4) A sharp bend in the channel, clearly observed in field and LiDAR images, is younger than the 1857 offset and is now interpreted as a deflection. 5) Subtle fractures within the surface gravel unit containing the deflected channel are similar to fractures previously documented at the Phelan Fan and LY4 paleoseismic sites (3 km and 35 km NW of Bidart Fan site), which may collectively indicate a post-1857 moderate magnitude Cholame/Carrizo earthquake. These new observations do not lessen the tectonic significance of >40, 5.3 ±1.4 m offset channel measurements elsewhere in the Carrizo Plain, but they do emphasize the importance of field validation of remote measurements, especially for seismic hazard assessment. The new observations also provide a possible explanation for mismatches between LiDAR-based measurements and results of 3-D excavations at Wallace Creek and Phelan fan.

Akciz, S. O., Grant Ludwig, L. B., Zielke, O., & Arrowsmith, J. R. (2013). Post-1857 fracturing and deflection of an apparent offset channel along the San Andreas Fault in the Carrizo Plain. Bulletin of the Seismological Society of America, (under review).