Thermal pressurization weakening in laboratory experiments

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

Submitted October 8, 2019, SCEC Contribution #9941

Thermal pressurization (TP) is expected to be a dominant frictional weakening process during earthquakes. However, most of our understanding of TP relies on theoretical studies. Our unique experimental setup allows us to test the mechanical response of experimental faults under elevated confining and pore pressures in the laboratory, using a rotary-shear apparatus. We observe dynamic weakening by TP of 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. Dynamic weakening was investigated after both low and high total fault displacements. For low displacement samples: (1) the magnitude 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. High displacement samples show similar-magnitude weakening as low displacement samples, but with much slower weakening rates. The evolution of weakening in the high displacement samples may reflect changes in pore space compressibility in the fault zone, which would have a significant effect on the frictional response during TP.

Citation
Badt, N. Z., Tullis, T. E., & Hirth, G. (2019). Thermal pressurization weakening in laboratory experiments. Journal of Geophysical Research, (submitted).


Related Projects & Working Groups
Fault and Rupture Mechanics