Evidence of thermal pressurization in high-velocity friction experiments on dolerite under elevated pore pressure

Lu Yao, Shengli Ma, & Toshihiko Shimamoto

Published August 14, 2019, SCEC Contribution #9546, 2019 SCEC Annual Meeting Poster #159

Dynamic weakening of faults plays a key role in aiding earthquake propagation. Several mechanisms, such as frictional melting, thermal pressurization and flash heating have been suggested as major weakening mechanisms. Originally proposed through theoretical analyses, frictional melting and flash heating have been corroborated by high-velocity friction experiments, yet there is rare experimental evidence to support the occurrence of thermal pressurization. In this study, high-velocity (2.0 m/s) friction experiments were performed on dolerite under pore pressure up to 25 MPa, with effective normal stresses ranging from 3 to 10 MPa. Slip surfaces of dolerite specimens were roughened with 80# SiC, and some of them were further pitted with small holes to vary the amount of pore water on fault planes (referred to as flat and pitted slip surfaces, respectively).

Two kinds of friction evolution were observed in the experiments. One is typical slip weakening of exponential-decay type (type-1), the other one is slip weakening superposed by drastic weakening and then strengthening in early stages (type-2). For all the experiments showing the type-2 friction evolution, the initial drastic weakening temporarily leads to friction coefficient as low as 0.01–0.05 and, more importantly, the weakening is always contemporaneous with bulk pressure rise (though it could be much lower than the pressure rise on the fault surface). The type-2 frictional behavior can be observed occasionally in experiments with flat slip surfaces but consistently with pitted slip surfaces, which is reasonable as more pore water exists on the pitted slip surfaces. Microstructural observation reveals that the deformation is dominated by comminution and only local melting can be observed on the slip surface at the displacement where the drastic weakening is contemporaneous with bulk pressure rise. The continuous molten layers formed at the late stage of experiments. All the data suggest that thermal pressurization accounts for the initial weakening observed in the experiments showing the type-2 friction evolution. Considering the complexity of natural faults in terms of roughness and geometry, the experiments with pitted slip surfaces are more meaningful. The experimental results suggest that thermal pressurization weakening of cohesive fault rocks could occur and thus postpone frictional melting during earthquakes, and the bulk melting may be prohibited if seismic slip is not large enough.

Key Words
Thermal pressurization, Dynamic weakening of faults, Frictional melting, Elevated pore pressure

Citation
Yao, L., Ma, S., & Shimamoto, T. (2019, 08). Evidence of thermal pressurization in high-velocity friction experiments on dolerite under elevated pore pressure. Poster Presentation at 2019 SCEC Annual Meeting.

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