Characterizing the uppermost 100 m structure of the San Jacinto fault zone southeast of Anza, California, through joint analysis of geological, topographic, seismic and resistivity data

Abstract

<jats:title>SUMMARY</jats:title> <jats:p>We present results from complementary geological, topographic, 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 southeast of Anza, California. Joint interpretation of these data sets, each with unique spatiotemporal sensitivities, allow us to better characterize the shallow (&amp;lt;100 m) fault zone at this structurally complex site. Geological mapping at the surface shows the CF has three main subparallel strands within a &amp;lt;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 southwest have relatively high VP. The low-lying flat sedimentary basins around the two southwestern-most CF strands and elevated damaged gneiss to the northeast have lowest VP &amp;lt;500 m s–1. The high relief of the northeast 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 &amp;gt;1300 Ohm.m) contrasted against the compacted fine-grained (potentially wet) materials within the CF core and the Bautista Formation (minimum &amp;lt;40 Ohm.m), which is slightly elevated above the flat basins. The inverse relationship between VP (increases) and resistivity (decreases) in the uppermost ∼15 m can be characterized as log–log linear with slopes of –2.6 to –4. At depths &amp;gt;30 m, the velocity heterogeneity near the surface merges into larger-scale structures that are generally slower on the northeast side of the CF core compared to the southwest side (as much as ∼40 per cent reduction in average VP). A previous study revealed a 20–37 per cent 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 northeast damaged gneiss unit. Preliminary analysis of time-dependent topography mostly shows effects of changing vegetation and anthropogenic activity.</jats:p>

A Distortion Matrix Framework for High-Resolution Passive Seismic 3-D Imaging: Application to the San Jacinto Fault Zone, California

“…This value actually dictates the maximum spatial frequency exhibited by body waves induced at the Earth surface. However, the chosen value c 0 = 1500 m/s seems in agreement with recent measurements of the P-wave velocity in SJFZ [44]. The other explanation is that the heterogeneous subsoil acts as a lens between the geophones and the focal plane either by scattering or wave guiding.…”

“…Assuming the V p /V s ratio to be a linear function of depth, the following averaged value was found for the P-wave velocity over the top 800 m: V p ∼1500 m/s. More recently, the P-wave velocity distribution in the 100 m-thick shallow layer has also been inverted using travel time data associated with active shots [44]. Low-velocity structures were detected, associated with a shallow sedimentary basin [42,44,45] and a fault zone trapping structure [41,46].…”

“…More recently, the P-wave velocity distribution in the 100 m-thick shallow layer has also been inverted using travel time data associated with active shots [44]. Low-velocity structures were detected, associated with a shallow sedimentary basin [42,44,45] and a fault zone trapping structure [41,46].…”

Peak Ground Velocity Spatial Variability Revealed by Dense Seismic Array in Southern California

“…Large‐N arrays allow the shallow velocity structure of an area to be imaged on a subkilometer scale (Hillers et al, 2016; Mordret et al, 2019; Share et al, 2020). We find that P and S wave velocities can deviate by up to 40% within our study region, and that they weakly correlate with the PGV variations (Figure 11).…”

Distribution of seismic scatterers in the San Jacinto Fault Zone, southeast of Anza, California, based on passive matrix imaging

“…1a). The locations of the surface fault traces are derived from recent detailed studies of the surface geological mapping and shallow geophysical imaging [39,40]. The basic goal of this paper is to interpret the obtained scattering images that exhibit features with higher lateral and vertical resolutions than conventional seismic investigations.…”