Thermal pressurization in experimental faults

Nir Z. Badt, Terry E. Tullis, & Greg Hirth

Published August 14, 2019, SCEC Contribution #9567, 2019 SCEC Annual Meeting Poster #160

Thermal pressurization (TP) is expected to be a dominant process during earthquake rupture, however most of our understanding on TP relies on theoretical studies. Our unique experimental setup allows us to test the mechanical response of experimental faults under elevated confining pressure in the laboratory, using a rotary-shear apparatus with independent confining pressure and pore fluid pressure systems. We observe dynamic weakening by TP in water-saturated faults in Frederick diabase, sliding at sub-seismic slip rates of 2.5-5 mm/s, under effective normal stress of 25-50 MPa and effective confining pressure of 20-49 MPa. No other weakening mechanism is activated at these conditions in dry samples. Permeability of the samples is varied by heat-treatment before each experiment, while frictional response is tested by velocity step experiments. Our results show that: (1) the magnitude of the stress drops and rate of weakening increase as sample permeability decreases; (2) the rate of re-strengthening of the faults, after the fault resumes sliding at the slow slip rate, is greater for high permeability samples; and (3) total frictional heating is limited during sliding. These observations are all consistent with the expected hydro-mechanical behavior of TP from theory. This study represents the first direct evidence that weakening due to TP is observed in experiments, opening the door for future research focusing on the effects of fault geometry, velocity and fault zone structure.

Key Words
Thermal pressurization, experimental, permeability

Citation
Badt, N. Z., Tullis, T. E., & Hirth, G. (2019, 08). Thermal pressurization in experimental faults. Poster Presentation at 2019 SCEC Annual Meeting.

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