Rock friction under variable normal stress

Brian D. Kilgore, Julian C. Lozos, N. M. Beeler, & David D. Oglesby

Accepted April 21, 2017, SCEC Contribution #7237

Determined the detailed change in fault shear strength and other fault properties due to changes in normal stress at room temperature using dry initial bare rock surfaces of granite at normal stresses between 5 and 7 MPa. Rapid normal stress changes result in gradual, approximately exponential changes in shear resistance with fault slip. The charactistic length of the exponential change is similar for both increases and decreases in normal stress. In contrast fault normal displacement and the amplitude of small high frequency elastic waves transmitted across the surface follow a two stage response consisting of a large immediate and a smaller gradual response with slip. The characteristic slip distance of the small gradual response is significantly smaller than that of shear resistance. The stability of sliding in response to large step decreases in normal stress is well-predicted using the shear resistance slip length observed in step increases. Analysis of the shear resistance and slip-time histories suggest nearly immediate changes in strength occur in response to rapid changes in normal stress; these are manifest as an immediate change in slip speed. These changes in slip speed can be qualitatively accounted for using a rate-independent strength model. Collectively the observations and model show that acceleration or deceleration depends on the size of the stress change, the friction characteristics of the fault and the elastic properties of the loading system.

Kilgore, B. D., Lozos, J. C., Beeler, N. M., & Oglesby, D. D. (2017). Rock friction under variable normal stress. JGR - SE, (accepted).