Joint analysis of seismic, geologic, resistivity and topographic data collected within the San Jacinto fault zone trifurcation area near Anza, California

We present results from complementary seismic, geologic, DC resistivity and topographic surveys at the Sage Brush Flat site along the Clark fault (CF) in the San Jacinto fault zone trifurcation area southeast of Anza, CA. Joint interpretation of these datasets, each with unique spatiotemporal characteristics, allow us to better understand the properties of the shallow (mostly <100 m depth) fault zone at this structurally complex site. Mapping at the surface shows the CF consisting of three main subparallel strands within a <100 m zone with varying degrees of damage along them. These strands intersect units of banded gneiss, tonalitic rocks, Plio-Pleistocene Bautista Beds, and late Quaternary sediments. Seismic properties are derived from data of a spatially dense rectangular array with 1108 sensors spaced 10-30 m apart over a 600 m by 600 m area. Shallow P wave velocities are obtained by inversion of travel times associated with 33 Betsy gunshot sources. S wave velocities are extracted from a recent ambient noise-based tomography model. Electrical resistivities are inverted using data from a 3D survey consisting of 7 parallel profiles 20 m apart spanning all three CF strands and orientated roughly SW to NE. Each profile has 64 electrodes with 4 m spacing between electrodes. Topography with resolution of several cm is obtained using Structure from Motion analysis of multiple images collected by a small UAV.

Initial comparisons between the imaged properties reveal some noteworthy correlations. At shallow depths (<50 m) P and S wave velocity extremes correspond to lithology (gneisses generally low; tonalities generally high) and not fault related damage. In contrast, the three CF strands have low electrical resistivities (40-100 ohm.m) compared to the surrounding rock with the central strand having lowest resistivity. A shallow trench reveals a well-defined fault core a few m wide along this strand. The employed seismic imaging tools probably have too low resolution to adequately resolve this fault feature. The low resistivity and localized damage structure point to increased strain conditions within the fault core, which correlates with a nearby topographic high (linear ridge). These and other features will now be analyzed and compared further to gain a better understanding of fault properties and processes along the CF.