Frictional Properties and Seismic Implications of Pelona–Orocopia–Rand Schists in Southern California: Insights from High-Pressure, High-Temperature Experiments

Sezim E. Guvercin, Sylvain D. Barbot, Lei Zhang, Zkang Yang, John P. Platt, Caroline Seyler, Baoning Wu, & Hao Zhang

Published September 8, 2024, SCEC Contribution #13862, 2024 SCEC Annual Meeting Poster #232

Understanding the frictional properties of faults in their natural lithological and hydrothermal conditions is essential for determining fault dynamics and seismic hazards. Although recent work has explored the frictional properties of subduction megathrusts, continental strike-slip faults have received less attention. Here, we seek to characterize the strength of faults within the San Andreas Fault system to better understand the controls on the depth of the seismogenic zone and the brittle-to-flow transition. We focus on the frictional characteristics of the Pelona–Orocopia–Rand (POR) schists that were emplaced in the middle and lower crust by subduction underplating during the Cretaceous and are now cut by the San Andreas and other faults in Southern California. We examine the mechanical properties of natural gouges from the Pelona, Portal, and Rand Mountain schists under varying pressure and temperature conditions using a triaxial apparatus. We conduct systematic velocity-step experiments (0.04 μm/s - 1 μm/s) under wet conditions from room temperature to 500°C with effective normal stress from 100 to 300 MPa. We interpret the mechanical data with a physical model that explains the brittle, semi-brittle, and ductile behaviors consistently. The resulting models show different deformation mechanisms activated under varying temperature and normal stress conditions. The stability of schist depends on lithology. Pelona schist, with 80% clinochlore and hornblende, displays three distinct mechanical regimes with a strong dependency on temperature and slip-rate, exhibiting velocity-weakening between 100ºC and 300ºC at low slip-rate and between 200º and 400ºC at high slip-rate. Portal schist, which is more quartz-rich with a contribution of clinochlore, muscovite, and albite, displays velocity-weakening behavior between 300ºC and 450ºC, but only at low slip-rates. The Rand Mountain schist, which consists mainly of hornblende, clinochlore, and albite, is velocity-strengthening in all conditions considered. Further experiments on Pelona schist from 100 MPa to 300MPa effective normal stress reveal that velocity-strengthening behavior is favored by high normal stress. The distribution of POR schists, modulated by the ambient temperature and pore-fluid pressure, exerts a strong control on seismic activity in southern California.

Citation
Guvercin, S. E., Barbot, S. D., Zhang, L., Yang, Z., Platt, J. P., Seyler, C., Wu, B., & Zhang, H. (2024, 09). Frictional Properties and Seismic Implications of Pelona–Orocopia–Rand Schists in Southern California: Insights from High-Pressure, High-Temperature Experiments. Poster Presentation at 2024 SCEC Annual Meeting.

Related Projects & Working Groups
Community Earth Models (CEM)