Characterizing the Uppermost 100 M Structure of the San Jacinto Fault Zone Southeast of Anza, California through Joint Analysis of Geologic, Topographic, Seismic and Resistivity Data

Session: Seismic Imaging of Fault Zones
Type: Oral
Date: 4/29/2020
Time: 02:30 PM
Room: 215 + 220
Description: 

We present results from complementary geologic, geodetic, seismic and electrical resistivity surveys at the Sagebrush Flat (SGB) site along the Clark fault (CF) strand of the San Jacinto fault zone trifurcation area SE of Anza, CA. Joint interpretation of these datasets, each with unique spatiotemporal sensitivities, allow us to better characterize the shallow (<100 m) fault zone at this structurally complex site. Geologic mapping at the surface shows the CF has three main subparallel strands within a <100 m zone with varying degrees of rock damage. These strands intersect units of banded gneiss and tonalite and various sedimentary units. Near the surface, the weathered but more intact tonalite and gneiss to the SW have relatively high V_P. The low-lying flat sedimentary basins around the two southwestern-most CF strands and elevated damaged gneiss to the NE have lowest V_P<500 m/s. The high relief of the NE gneiss unit may in part be explained by its extensive damage and inferred increased relative rock uplift. Resistivity imaging shows the unconsolidated dry basin sediments (maximum >1300 Ohm.m) contrasted against the compacted fine-grained (potentially wet) materials within the CF core and the Bautista Formation (minimum <40 Ohm.m), which is slightly elevated above the flat basins. The inverse relationship between V_P(increases) and resistivity (decreases) with increasing depth can be characterized as log-log linear with slopes of -2.6 to -4 down to a depth of ~15 m. At depths >30 m, the velocity heterogeneity near the surface merges into larger-scale structures that are generally slower on the NE side of the CF core compared to the SW side (as much as ~40% reduction in average V_P). A previous study revealed a 20-37% variability in peak ground velocities across the SGB site from local earthquakes. The upper end of that range is associated with the near-surface unconsolidated sedimentary basins and NE damaged gneiss unit. Preliminary analysis of time-dependent topography mostly shows effects of changing vegetation and anthropogenic activity.

Presenting Author: Pieter-Ewald Share

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