Unraveling coseismic kinematics of frictional melts in extensive pseudotachylyte networks of the Santa Rosa Mountains, California

Eric C. Ferre, Haley M. Benoit, Nina Zamani, & John Geissman

Submitted September 7, 2025, SCEC Contribution #14792, 2025 SCEC Annual Meeting Poster #TBD

Fault pseudotachylytes form from frictional melting and are critical markers of deformation at seismic velocities (>0.1 m/s). Nonetheless, a significant gap exists between the extensive rupture lengths inferred for Mw > 7 earthquakes, often hundreds of kilometers, and the typically short lengths of exposed fault zones associated with continuous pseudotachylyte veins, rarely exceeding a few hundred meters. This raises a multiple questions: 1) do these pseudotachylytes reflect actual seismic slip, or do they rather represent localized friction anomalies? 2) how does one obtain reliable information from frictional melts about seismic slip direction and sense considering how dark and aphanitic these materials are? To explore these issues, we investigated a pseudotachylyte vein in the Santa Rosa Mountains in California. This site offers excellent exposures of frictional melt veins up to 200 meters long, making it an ideal natural scale laboratory, less than 15 km away from the San Andreas Fault. Our research is currently based on 10 oriented pseudotachylyte samples from a 120 m-long continuous vein. We used the anisotropy of magnetic susceptibility (AMS) method and downsized it to sets of 12-40 cubic samples of 3.5 mm size, a method dubbed as mini-AMS. This method provides quantitative determination of earthquake parameters, including slip plane, slip direction, slip sense, and displacement for a single, large magnitude event. We show that the AMS fabric is consistent at the specimen scale and varies along the fault length, reflecting changes between releasing and restraining bends of the slip plane. Additionally, these results show that beyond providing the flow direction and sense of frictional melts this method constitute an underestimated paleoseismic tool that provides information otherwise not accessible on earthquake slip dynamics. This research also enhances our understanding of fault friction and slip partitioning at moderate depths (~10 km), contributing to better seismic hazard assessments and risk mitigation strategies.

Key Words
pseudotachylyte, frictional melt, fault plane slip

Citation
Ferre, E. C., Benoit, H. M., Zamani, N., & Geissman, J. (2025, 09). Unraveling coseismic kinematics of frictional melts in extensive pseudotachylyte networks of the Santa Rosa Mountains, California. Poster Presentation at 2025 SCEC Annual Meeting.

Related Projects & Working Groups
Earthquake Geology