On-fault damage along the southern San Andreas fault: material characterization from natural and experimental clay-rich faults

Alexandra A. DiMonte, Alexis K. Ault, Greg Hirth, & Cameron D. Meyers

Published September 10, 2023, SCEC Contribution #13233, 2023 SCEC Annual Meeting Poster #144

The last major earthquake on the southern San Andreas fault (SSAF) occurred ~300 years ago. Triggered and episodic shallow creep events occurring on this segment accommodate some of the slip, but estimating the potential near-surface distribution of and mechanical response to seismic and aseismic slip requires characterization of the fault materials.

Indurated clay gouge is exposed along the volumetric SSAF in the Mecca Hills, CA. We target fault gouge and friable sandstone from the main NW-striking SSAF and subsidiary E-W-striking faults that converge into the main structure, as well as sediment from the adjacent Pleistocene Upper Palm Springs Formation along a NW-SE transect from Painted to Box canyons. Field observations, petrography, and scanning electron microscopy (SEM) reveal the SSAF gouge comprises networks of waxy fault surfaces locally adorned with Fe- and Mn-oxide, with a scaly and crenulated fabric, and microscale sandstone phacoids. Strike-slip slickenlines on these surfaces are parallel to sub-vertical fold axes. X-ray diffraction indicates SSAF gouge is primarily illite with some smectite and kaolinite. Ongoing work applies K-Ar and (U-Th)/He chronology to characterize the provenance and shallow tecto-thermal history of SSAF gouge.

We conducted deformation experiments on SSAF gouge sampled from the mouth of Painted Canyon using an Instron rotary shear apparatus at room temperature and room or 100% humidity at 5 to 10 MPa and 1 μm/s-1 mm/s. Additional experiments using the triaxial Paterson apparatus were conducted using the inclined piston technique at 20 MPa confining pressure, 20 and 60 °C, and ambient humidity at rates of 1 to 85 μm/s. Experiments indicate dry SAF gouge has a coefficient of friction (μ) of 0.62 ± 0.08 and is velocity-strengthening to velocity-neutral; wet gouge has a μ of 0.36 ± 0.08 and is velocity-neutral to slightly velocity-weakening. Instron slide-hold-slide experiments suggest wet gouge may heal faster, but ongoing assessment of healing properties aims to refine these preliminary observations. SEM shows experimental slip surfaces resemble the source fault material.

Our collective observations imply that, during dry periods, shallow faults may hinder earthquake propagation. Below the groundwater table and above it during wet periods, the mechanical influence of water and coupled pore pressure effects may prime the shallow SSAF gouge network to localize and propagate earthquake ruptures to the surface.

Key Words
clay gouge, San Andreas fault, friction, brittle

Citation
DiMonte, A. A., Ault, A. K., Hirth, G., & Meyers, C. D. (2023, 09). On-fault damage along the southern San Andreas fault: material characterization from natural and experimental clay-rich faults. Poster Presentation at 2023 SCEC Annual Meeting.

Related Projects & Working Groups
Fault and Rupture Mechanics (FARM)