Assessing the role of roughness on the frictional strength of faults and frictional weakening by thermal pressurization
Monica R. Barbery, & Terry E. TullisSubmitted September 7, 2025, SCEC Contribution #14883, 2025 SCEC Annual Meeting Poster #TBD
Characterizing the frictional behavior of faults at earthquake conditions remains an important, critical problem. Fault roughness is well established over a wide range of length scales, but the effects of roughness on the frictional strength of faults, particularly at sliding velocities approaching seismic slip rates, are not well constrained. Fault roughness may play a significant role in suppressing dynamic weakening by thermal pressurization (TP). On undrained faults sliding at seismic slip rates, frictionally heated pore fluids expand leading to a decrease in the effective normal stress on a fault. On rough faults, offset results in the opening of new pore space along the sliding surface which may be able to accommodate this increase in pore volume, preventing any reduction in effective stress. In this work, we conducted velocity step experiments to investigate the efficacy of TP on rough faults. Experiments were performed on flat and rough samples of Frederick diabase using a rotary shear apparatus at Brown University. Rough surfaces were prepared with a surface grinder and incorporated 6 overlapping wavelengths, 154 mm to 4.8 mm in wavelength, with amplitude-to-wavelength ratios of 0.001 (mimicking the lower end of natural fault roughness). Experiments were performed at room temperature with an effective normal stress of 20 MPa, and either 0 or 25 MPa pore pressure. We conducted velocity step experiments (VSE), oscillating the sliding velocity from 1 µm/s to 6-10 mm/s for 0.3 mm and 100 mm of slip, respectively, for up to 3 meters of total slip. In flat experiments, reductions in the apparent friction coefficient (µ) occur in 86% of VSE. In rough experiments, weakening occurs in only 45% of VSE during the first m of slip. The frequency increases with cumulative slip, reaching 62% after 2-3 m of slip for rough faults. The magnitude of weakening is similar between rough and flat experiments, with a typical change in µ of 0.15-0.2 under experimental conditions. Both static and dynamic friction coefficients decrease with cumulative slip. The timing of weakening on rough surfaces coincides with a transition from dilatant to compressive phases, mapped using the initial surface geometry, suggesting TP is linked with roughness-induced changes in pore space. These results suggest that thermal pressurization may be significantly delayed or inhibited along rough or immature faults.
Citation
Barbery, M. R., & Tullis, T. E. (2025, 09). Assessing the role of roughness on the frictional strength of faults and frictional weakening by thermal pressurization. Poster Presentation at 2025 SCEC Annual Meeting.
Related Projects & Working Groups
Fault and Rupture Mechanics (FARM)
