Structural and Geochemical Characterization of the Elsinore Fault in the Coyote Mountains, Southern California

Victor M. Sanchez, Rafael Almeida, Thomas K. Rockwell, & William A. Griffith

Submitted August 30, 2026, SCEC Contribution #15623, 2026 SCEC Annual Meeting Poster #TBD

Fault damage zones record cumulative deformation produced by fault growth and repeated earthquake rupture. We investigate how lithology, structural position, and fluid–rock interaction influence damage across a 50-m transect through the main strand of the Elsinore Fault in the Coyote Mountains, Southern California. The transect crosses the interpreted fault core and surrounding damage zone, where Plio-Quaternary sedimentary rocks are juxtaposed against crystalline basement. This segment is paleoseismologically well characterized, with a slip rate of 2.4 ± 0.4 mm/y and a mean slip of ~1.5 m over the past 60-70 ka and probably extending to 150 ka in the most recent event (Rockwell et al., 2019).

Detailed field mapping and UAV photogrammetry documented fault-zone structure and juxtaposed lithologies. Outcrop inspection ss that pulverization is more intense in granitoids than in adjacent amphibolite, implying that amphibolite fabric attenuates shattering. Pulverization also decreases away from the fault core, although the detailed pattern remains unresolved. Outcrops reveal structurally controlled fluid flow, including calcite and manganese veins with oxidation, and secondary shear features such as conjugate fractures and secondary faults. Structural analyses of these features will be presented.

Twenty-one samples were collected from sedimentary units south of the fault and from diorite, gneissoid granite, amphibolite, and marble to the north. Many were pulverized and friable, requiring epoxy stabilization for extraction and thin-section preparation. Preliminary microstructural observations of samples 1.7 m and 20 m from the fault core reveal volumetric strain expressed by intragranular opening-mode fractures of variable intensity and shattered grains that retain jigsaw-fit geometry, indicating little shear. Comminution seams record localized brittle shear slip. We will compare deformation styles among lithologies in similar structural positions and quantify changes in damage intensity with distance from the fault using fracture density, factor of increase in perimeter length, and grain-size-distribution measurements.

Citation
Sanchez, V. M., Almeida, R., Rockwell, T. K., & Griffith, W. A. (2026, 08). Structural and Geochemical Characterization of the Elsinore Fault in the Coyote Mountains, Southern California. Poster Presentation at 2026 SCEC Annual Meeting.

Related Projects & Working Groups
Earthquake Geology