Transition from locked to creeping behavior on the San Andreas Fault, San Juan Bautista, CA

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

Submitted September 10, 2023, SCEC Contribution #13286, 2023 SCEC Annual Meeting Poster #128

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. Samples taken from the southern transition from locked to creeping during the San Andreas Fault Observatory Drilling (SAFOD) project were found to be both frictionally weak and incapable of healing, pointing towards a lithological control on creep behavior. However, the lithology in the northern transition region, near San Juan Bautista, has not been extensively studied.

Here we combine structural analysis of exposed faults, microstructural analysis of fault rocks, and subsurface data from a recent drilling project to map fault strands in the northern transition region and assess the spatial distribution of creep along these strands. We report a structurally complex network of faults with multiple SW-dipping strands in a laterally extensive gabbro unit. The northernmost creeping strands of the SAF identified to date are located approximately three km north of San Juan Bautista, however other strands in the area show no evidence of active creep (e.g. offset curbs). In addition, the non-creeping strands show microstructural evidence of past seismic slip in the form of clay clast aggregates and multiple generations of gouge formation. These observations indicate that the transition from locked to creeping behavior can happen piecemeal along faults. Although all fault strands in this study region are located within the same gabbro host rock, it is still possible lithology plays a role here. Because clays are exceptionally frictionally weak, minor differences in clay composition could significantly affect the fault strength and stability of individual strands. To test this, we perform XRD analysis and friction experiments on gouge samples from both locked and creeping strands in this region and report both steady-state friction and frictional healing rates.

Krogh, J. E., Williams, J., Savage, H. M., Brodsky, E. E., Ulrich, C., & Guglielmi, Y. (2023, 09). Transition from locked to creeping behavior on the San Andreas Fault, San Juan Bautista, CA. Poster Presentation at 2023 SCEC Annual Meeting.

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