Geologic and Structural Characterization of The Rock Volume Imaged by the Dense Nodal Seismic Array Along the San Jacinto Fault at Sage Brush Flat, Southern California

Adam Wade

Submitted August 14, 2018, SCEC Contribution #8486, 2018 SCEC Annual Meeting Poster #229

Characterization of fault zones is critical to understanding earthquake mechanics and seismic hazard evaluations. Investigations of fault zones often involve detailed surface mapping, and/or subsurface imaging through seismic waves and other geophysical data. Direct comparison of these different data sets are often difficult due to their differing scales and depth ranges. This study composites detailed mapping and local morphology, along with a previous study’s 3D seismic shear wave tomography from a dense nodal array, to evaluate shallow (<1 km depth) properties of the San Jacinto fault zone (SJFZ). Previous mapping of the SJFZ at Sage Brush Flat (SGB) near Anza, CA, traces multiple fault strands into a single fault along the metamorphic-plutonic contact. We’ve revised both the fault trace and geologic mapping using high resolution topography. Previous unmapped geologic units, include a subunit within the metamorphic rock, and the Plio-Quaternary Bautista Beds, which both help define the fault zone via rock deformation extents and displacement. Topographic profiles reveal a general asymmetry of basin bounding gully widths and spur/ridge slopes. Spatial distribution of rock damage within SGB and Alkali Wash are seemingly asymmetric with pulverization dominantly between fault strands or in the NE fault block. 3D shear wave tomography illuminates up to three low velocity troughs in the upper 700 m depth. Southwest of SGB the fault is not isolated to a single strand along the main geologic boundary, but remains bifurcated with the second strand NE of the main trace. Slow shear wave velocity troughs are attributed to both active and older fault structures, traced at the surface, and illustrated in our 3D fault zone model. Asymmetric morphology of the SGB basin is partially attributed to structural growth and fault zone damage. Geologic models from both 3D shear wave tomography, and geologic mapping, indicate surface observations and shallow seismic data compare well. Theoretical models of earthquake ruptures along a bi-material interface result in asymmetric damage from a preferred rupture direction. Asymmetric rock damage in outcrops appear similar to those from seismic wave interpretations at SGB, and findings elsewhere along the SJFZ, suggesting a preferred NW rupture propagation. This study highlights the utility of a dense seismic array paired with detailed surface observation in evaluating shallow fault zone depths to capture complete 3D structure.

Key Words
San Jacinto Fault, high resolution topography, shallow geophysics, dense seismic array, structural geology, Sage Brush Flat

Wade, A. (2018, 08). Geologic and Structural Characterization of The Rock Volume Imaged by the Dense Nodal Seismic Array Along the San Jacinto Fault at Sage Brush Flat, Southern California. Poster Presentation at 2018 SCEC Annual Meeting.

Related Projects & Working Groups
Earthquake Geology