GEOMORPHIC INDICATORS OF NEOTECTONIC ACTIVITY ALONG THE SANTA YNEZ FAULT, IN SANTA BARBARA COUNTY, CA

With a length of about 80 km, the Santa Ynez Fault (SYF) is the largest continuous fault in the western Transverse Ranges (WTR), located in southern California. Previous research has resulted in many contradictory theories related to the fault’s past kinematic movement. The faults movement has been characterized as a steep, south-dipping reverse fault; a north vergent back thrust; right-lateral slip; and left-lateral slip. These conflicting interpretations are most likely a result of possible reactivation during the fault’s lifetime; furthermore, these interpretations are associated with the SYF’s pre-Quaternary slip history. Documentation of the Quaternary kinematics and recency of surface rupture along the fault zone is lacking, due to a complex kinematic history and the presence of dense chaparral vegetation. In this study, we mapped the Holocene geomorphic expression of tectonic deformation along the Santa Ynez Fault, using a combination of field and digital mapping on a 1m Elevation Product dataset. This data set is a LiDAR-derived DEM created using the USGS 2018 post-fire LiDAR surveys.

Geomorphic indicators along the fault-controlled landforms include fault scarps, gouge, deflected streams, benches, sag ponds, and knickpoints. These morphological features suggest both left-lateral slip and reverse slip, with a dominating component of reverse slip. The SYF appears to demonstrate both a dip-slip and a left-lateral strike slip motion, which indicates that the characterization of the Quaternary fault motion is oblique slip. Evaluation of the DEM reveals that some Quaternary deposits, specifically the fluvial terraces, appear offset while others do not. Continued work is needed to date these terraces at specific sites, which will elucidate the Quaternary kinematics and activity. The result of this investigation fills a gap in the present-day regional tectonics and highlights the importance of improved earthquake hazard estimates of the SYF.