Experimental Study of Thermal Pressurization Weakening and the Role of Fault Roughness

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

Published August 15, 2017, SCEC Contribution #7712, 2017 SCEC Annual Meeting Poster #191 (PDF)

Dynamic weakening due to pore fluid thermal pressurization is controlled by the hydraulic properties of the host rock, including the permeability (k) and the available pore space. Rough sliding interfaces dilate and contract during slip, in accordance with the surface geometry, and thus effectively change the available pore space during slip events. Thermal pressurization weakening has been studied under elevated confining pressure in the laboratory, using a rotary-shear apparatus having a sample with independent pore pressure and confining pressure systems. Frederick Diabase with two different permeabilities was used and the introduction of water for the two permeabilities was done differently. (1) The natural impermeable rock (k<10-22 m2) was used in two experiments, both having fault surfaces with fine roughness produced by sandblasting; water was applied to the fault before closing up the sample. One fault was ground flat, whereas the other had rough, mated surfaces, with a topography having an amplitude to wavelength ratio of 1.4×10-3. (2) A thermally cracked sample with a permeability of 10-20 m2 and flat ground surfaces, finished with #100 grit, had water introduced using the flow-through pore pressure system. Results indicate a frictional resistance decrease of 20% in flat interfaces and 50% in rough surfaces. However, it is not clear if these weakening events result from thermal pressurization weakening. Moreover, the actual dilation and contraction in the rough-surface runs were much smaller than first predicted according to the surface roughness had the rock behaved rigidly, and the coefficient of friction was anomalously low (0.6) for these experiments - in contrast to previously measured values for this rock (0.8-9). In comparison, an experiment on a water-soaked, heat-treated sample showed clear thermal pressurization weakening. Dynamic weakening up to 40% was observed on a sequence of fast slip events (2.5 mm/s) with a characteristic friction decay curve as predicted by the thermal pressurization model. However, after a sequence of three high-speed sliding events, the magnitude of weakening diminishes progressively from 40% to 15%. The decrease in the weakening with progressive fast-slip events suggest that the hydraulic diffusivity may increase locally near the sliding interface during thermal pressurization-enhanced slip, perhaps due to stress- or thermally-induced damage to the host rock.

Key Words
Thermal pressurization, roughness, permeability

Citation
Badt, N. Z., Tullis, T. E., & Hirth, G. (2017, 08). Experimental Study of Thermal Pressurization Weakening and the Role of Fault Roughness. Poster Presentation at 2017 SCEC Annual Meeting.

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