The effect of shear displacement and wear on fault stability. Laboratory constraints

Faults are composed of a damage zone and a fault core with wear material that has accumulated during sliding. Fault zone width and structural complexity have been suggested as key factors that dictate the stability of fault slip and the transition from stable to unstable motion. Here we address the role of fault rock wear rate and the presence of gouge on the stability of frictional sliding. We performed biaxial shear experiments on quartz-rich bare rock surfaces having different resistances to abrasion (i.e., wear production with slip), as well as simulated quartz gouge. To characterize the influence of shear displacement on fault stability, constant velocity and velocity step experiments were performed to displacements up to 30 mm. Mechanical data are analyzed within the rate-and-state framework, and integrated with post-mortem microscopic analyses of the samples. For initially bare surfaces a shear displacement threshold is required to transition from stable to unstable sliding. Stick-slip events (laboratory earthquakes) evolve systematically as a function of fault slip. Shear displacement has a clear effect on the rate-and-state parameters (a-b) and D-c. For all our tests, (a-b) decreases with increasing shear displacement, implying enhanced velocity weakening and potential unstable behavior for larger fault slip. For high wear rates and simulated gouge, D-c decreases with increasing slip. However, for low wear rate faults, changes in D-c are negligible. Our results demonstrate that, fault stability varies systematically with fault slip and in particular that shear displacement and strain localization are dominant parameters controlling fault stability.