Frictional properties of locked and creeping strands in the northern transition region of the San Andreas Fault

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

Published September 8, 2024, SCEC Contribution #14003, 2024 SCEC Annual Meeting Poster #155

The San Andreas Fault is functionally 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 a weak, velocity-strengthening lithology. Samples taken from the southern portion of the creeping section during the San Andreas Fault Observatory at Depth (SAFOD) project were found to be both frictionally weak and incapable of healing. However, the northern portion of the creeping section and the transition region into the locked section have not been extensively studied.

Recent work in the northern transition region has uncovered a structurally complex network of faults with multiple SW-dipping strands in a laterally extensive gabbro unit. Creeping strands have been identified by road offsets and are located adjacent to other strands that are not actively creeping. We collected fault gouge from these creeping and potentially locked strands to determine if there are lithological distinctions that could contribute to their disparate behavior, such as variations in clay type or fraction. The composition of each sample has been determined with XRD analysis on both the bulk samples as well as the isolated clay fraction for each fault strand to determine any differences in lithology. 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. As effective stress increases, the host rock and the locked strand samples become less velocity strengthening, while the creeping strand samples are consistently velocity-strengthening, showing no change as a function of effective stress. Healing rates are negligible at low normal stress and increase with greater effective stress, and the number of samples displaying healing with logarithmic time dependence also increases. These results indicate that slip behavior in this region is governed by a combination of effective stress and fault strand lithology, with gouge from the creeping strands more likely to remain stable within the upper 2 km of the crust.H

Citation
Krogh, J. E., Savage, H. M., Brodsky, E. E., Ulrich, C., & Guglielmi, Y. (2024, 09). Frictional properties of locked and creeping strands in the northern transition region of the San Andreas Fault. Poster Presentation at 2024 SCEC Annual Meeting.

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