Lithology or Geometry? Frictional properties of fault gouges in the northern transition region of the San Andreas fault highlight the importance of local effective stress

Julia E. Krogh, Heather M. Savage, Emily E. Brodsky, Craig Ulrich, Yves Guglielmi, & Jacquelyn Williams

Submitted September 7, 2025, SCEC Contribution #14714, 2025 SCEC Annual Meeting Poster #TBD

The San Andreas fault is divided into three regions: the northern and southern locked sections, which are capable of producing large earthquakes, and the middle creeping section. One possible mechanism for stable behavior in the central portion of the fault is the presence of a weak, velocity-strengthening, non-healing lithology, as was found in the San Andreas Fault Observatory at Depth (SAFOD).

Recent work in the northern transition zone uncovered a complex network of faults with multiple SW-dipping strands in a laterally extensive gabbro unit. Three creeping strands identified by road offsets are located adjacent to the mapped main strand of the SAF, which is not actively creeping. A drilling project in the summer of 2022 recovered cuttings from the creeping strands to depths of 180 m, which were identified by clay-size gouge in otherwise gravel and sand-sized gabbro host rock. Fault dips of ~60° were calculated from the changes in depth of gouge layers between the three boreholes. In addition to sampling faults at depth, we also collected gouge in the creeping and potentially locked strands from outcrop.

A semi-quantitative XRD analysis of the samples shows no lithologic difference between the locked and creeping strands. Friction experiments with velocity steps and slide-hold-slide tests were performed in a triaxial deformation apparatus at effective stresses of 15 to 30 MPa, mimicking the conditions in the upper two km of the crust. We find that all samples are frictionally strong and velocity strengthening under the conditions tested. Healing rates are negligible at low normal stress and increase significantly with greater effective stress. These results indicate that lithology is not likely responsible for the disparate slip behavior observed between the fault strands. Instead, local variations in effective stress could play a significant role in reducing healing rate and therefore increasing stability on certain strands. Elevated pore pressures are not likely the explanation; ongoing monitoring of one borehole has only recorded hydrostatic pore pressures. We posit that local fault geometry plays a crucial role at this location, with the creeping strands dipping much less steeply than the main strand.

Citation
Krogh, J. E., Savage, H. M., Brodsky, E. E., Ulrich, C., Guglielmi, Y., & Williams, J. (2025, 09). Lithology or Geometry? Frictional properties of fault gouges in the northern transition region of the San Andreas fault highlight the importance of local effective stress. Poster Presentation at 2025 SCEC Annual Meeting.

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