Joint Analysis of Seismic, Geologic, Resistivity and Topographic Data Collected Within the San Jacinto Fault Zone Trifurcation Area Near Anza, California

Date: 4/26/2019

Time: 06:00 PM

Room: Grand Ballroom

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 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 and tonalite, and various sedimentary units. Seismic properties are derived from data of a spatially dense rectangular array with 1108 sensors spaced 10-30 m apart over a 0.36 km² area. Shallow P-wave velocities (V_P) are obtained by inversion of travel times associated with 24 Betsy Gun shot sources. S-wave velocities (V_S) are extracted from a recent ambient noise-based tomography model. Subsurface electrical resistivities are obtained through inversion of apparent resistivity data from a 3D survey consisting of 7 ~250-m-long parallel profiles (4 m electrode spacing) that were ~20 m apart spanning all three CF strands and orientated roughly SW to NE. Topography with resolution of several cm is obtained from images collected with a small UAV.

At <100 m depths, V_S properties correspond mostly to rock type (gneiss=generally low; tonalite=generally high). The V_P model in addition contains its lowest velocities within the shallow sedimentary basins. In contrast, the three CF strands have anomalously low electrical resistivities (40-100 ohm.m), with the two SW-most strands having the lowest values. A shallow trench reveals a well-defined fault core a few m wide along these strands. 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 local topographic high.

Presenting Author: Pieter-Ewald Share

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