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

Pieter Ewald Share, Petr Tábořík, Petra Štěpančíková, Jakub Stemberk, Thomas K. Rockwell, Adam Wade, J. Ramon Arrowsmith, Andrea Donnellan, Frank L. Vernon, Yehuda Ben-Zion

Research output: Contribution to journalArticle

1 Scopus citations

Abstract

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 (<100 m) fault zone at this structurally complex site. Geological 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 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 <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 >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 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 >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.

Original languageEnglish (US)
Pages (from-to)781-794
Number of pages14
JournalGeophysical Journal International
Volume222
Issue number2
DOIs
StatePublished - May 13 2020

Keywords

  • Earthquake hazards
  • Electrical resistivity tomography (ERT)
  • Fractures, faults, and high strain deformation zones
  • Image processing
  • Seismic tomography
  • Structure of the Earth

ASJC Scopus subject areas

  • Geophysics
  • Geochemistry and Petrology

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