Strength and fluid transmissivity evolution during shearing of laboratory faults under hydrothermal conditions

Tamara N. Jeppson, David A. Lockner, & Diane Moore

Published August 12, 2021, SCEC Contribution #11305, 2021 SCEC Annual Meeting Poster #139

There is significant evidence for healing in natural fault systems, including changes in fluid transport properties and earthquake recurrence. Healing results from coupled thermal, hydraulic, mechanical, and chemical interactions but most laboratory experiments focus either solely on how fluid transport properties evolve in stationary fractures at elevated temperatures or on how strength evolves in shear faults at room temperature. Studies that examine the combined evolution of these properties, especially under hydrothermal conditions, are limited. Laboratory restrengthening tests, referred to as slide-hold-slide (SHS), measure the difference between the steady sliding strength and the peak shear strength immediately following a hold period at constant stress or displacement. We present the results of SHS experiments on Westerly granite bare surfaces. These experiments are combined with in-plane flow-through tests and conducted at an effective confining pressure of 20 MPa and temperatures of 22 or 200 ˚C. Consistent with previous studies, we observe a greater reduction in fluid transmissivity at 200 ˚C than at 22 ˚C and, generally, strength increasing with log(hold duration). However, there are deviations from the expected restrengthening behavior that likely relate to mineralogy, pore fluid chemical equilibrium, and flow rate. For example, when a set volume of fluid is allowed to equilibrate with the rock, initially time-dependent strengthening is observed at a rate of 0.015 per decade increase in hold duration for holds < 10^4 s. For longer hold durations time-dependent weakening occurs at a rate of 0.03 per decade. If new fluid is continuously introduced to the sample the weakening occurs at shorter hold periods (>10^3 s) and at a slower rate of 0.01 per decade. But this behavior appears to only occur at low displacments while the flow rate is still high (10^-4 to 10^-3 ml/s). This suggests the process underlying the weakening behavior is diffusion limited and related to chemical equilibrium. The mechanisms behind the complex healing behavior are not fully understood but it is apparent that fault zone restrengthening under hydrothermal conditions is the result of multiple interacting processes.

Key Words
healing, friction, fluids

Citation
Jeppson, T. N., Lockner, D. A., & Moore, D. (2021, 08). Strength and fluid transmissivity evolution during shearing of laboratory faults under hydrothermal conditions. Poster Presentation at 2021 SCEC Annual Meeting.

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