Breaking down Byerlee: The effect of normal stress on the frictional strength of monomineralic fault gouge

Otis Wickenhaeuser, Kristina K. Okamoto, Heather M. Savage, & Annia Fayon

Published September 8, 2024, SCEC Contribution #13924, 2024 SCEC Annual Meeting Poster #145

Deformation in fault gouges can occur by several processes - including gouge layer dilation, grain crushing, and plastic deformation at asperity contacts - that ultimately give rise to the shear strength (coefficient of friction) and the frictional stability of the material. However, under different pressure and temperature conditions, we expect that one or more of these processes may dominate and a change in frictional behavior may be apparent when a transition in process takes place. We present findings of a normal stress dependence on the frictional behavior of gouge experiments on quartz, hematite, apatite, and calcite. We performed friction experiments under triaxial stress conditions at a range of normal stresses (10-150 MPa) in a single direct-shear configuration. Steady-state friction was determined from the constant velocity (1μm/s) run in, before imposing velocity steps ranging from 0.05-10 μm/s. The post-experiment gouges were preserved for microstructural analysis.

We show that all minerals have a high steady-state friction coefficient (0.75-1) at low normal stresses. Each mineral gradually transitions to a lower friction coefficient (0.6-0.7) with increasing normal stress. Hematite, calcite, quartz, and apatite reach a final friction at normal stresses at approximately 50, 50, 75, and 100 MPa respectively. A similar transition in frictional strength from ~0.85 to 0.6 was noted by Byerlee (1978), however that dataset was polymineralic and the transition stress was proposed to be ~200 MPa. Microstructural analyses show little grain crushing or development of localized slip or fabric at low normal stresses. At intermediate stresses, grain crushing is apparent and slip localizes within Reidel and boundary shears. At the highest normal stresses, we see increased dislocation density in apatite and twin density in calcite. We propose that gouge layer dilation is the dominant process at lower normal stresses. Once stresses are high enough to break grains, friction begins to decrease. At the highest stresses, plastic deformation becomes more viable and/or slip localization reduces the stress available to fracture. Because different minerals transition at different stresses, material properties such as yield strength and fracture toughness may determine what stresses these transitions occur.

Key Words
friction, experiment, gouge, rate and state

Citation
Wickenhaeuser, O., Okamoto, K. K., Savage, H. M., & Fayon, A. (2024, 09). Breaking down Byerlee: The effect of normal stress on the frictional strength of monomineralic fault gouge. Poster Presentation at 2024 SCEC Annual Meeting.

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