Dislocation density as a control on the frictional strength of fault gouge

Kristina K. Okamoto, Amy G. Ryan, Heather M. Savage, Lars H. Hansen, & Amanda M. Dillman

Submitted September 8, 2024, SCEC Contribution #14036, 2024 SCEC Annual Meeting Poster #162

The frictional behavior of fault gouge is frequently modeled with the empirical rate-state frictional law. However, the frictional parameters that are determined by this method are only valid at the exact conditions at which experiments were conducted. Extrapolating laboratory results to spatial and temporal scales of natural systems requires a microphysical understanding of frictional behavior. Recently, a number of microphysical models indicate that asperity creep caused by low-temperature plasticity or pressure solution can explain many experimental observations of changes in friction as a function of velocity, pressure, and temperature (Aharonov & Scholz, 2018; Chen et al., 2021; Ikari et al., 2016; Thom et al., 2022). In addition, investigations into low-temperature plasticity show that backstress, which is proportional to dislocation density, is a major control in the rheological behavior of olivine (Hansen et al. 2019). However, the possible role of backstress in friction has not been experimentally validated.

We investigate the effects of low-temperature plasticity on contact-scale deformation by performing friction experiments on gouges created from San Carlos olivine. In order to test the effect of backstress, we annealed one set of samples to eliminate dislocations and pre-stressed a second set of samples in a Paterson gas-medium apparatus at a differential stress of 270 MPa, temperature of 1250°C, and confining pressure of 300 MPa to generate a high dislocation density. Both beginning and final dislocation densities were quantified through line counting on SEM images of decorated samples. Friction experiments at 500°C and 100 MPa normal stress were performed in torsion in the Paterson apparatus. The steady-state coefficient of friction increases from 0.45 to 0.6 for experiments at 500°C. These observations are in agreement with predictions of asperity deformation in the presence of increased backstress (Thom et al., 2022). We will also present results on friction experiments performed at 150°C and 100 MPa normal stress, as well as the velocity-dependent behavior under all experimental conditions. Our results suggest that the coefficient of friction is dependent on dislocation density, and therefore stress history, of the fault gouge.

Citation
Okamoto, K. K., Ryan, A. G., Savage, H. M., Hansen, L. H., & Dillman, A. M. (2024, 09). Dislocation density as a control on the frictional strength of fault gouge. Poster Presentation at 2024 SCEC Annual Meeting.

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