Experimental and modeling study of the effect of fault roughness on dynamic frictional sliding

Friction governs many processes in nature from the motion of glaciers and landslides to tremors and earthquakes. The roughness of the sliding interfaces presumably affects their dynamics, in particular the condition for stable or unstable sliding. We investigate this issue by modeling numerically tri-axial stick-slip experiments on Westerly granite samples with different levels of roughness. The model accounts for surface roughness at the millimeter scale and assumes rate and state friction with enhanced dynamic weakening. It reproduces key characteristics observed in the experiments, such as sequences of fast stick-slip events for smooth surface and sequence of slow slip events with small stress drop followed by a large-stress drop unstable slip event for the rough interface. The increase in loading stiffness due to the additional resistance from roughness can explain the initial slow slip. The fast-unstable event may be the result of the higher critical stiffness due to the increase of local normal tractions with slip, as well as of the reduction of the loading stiffness due to contacts losses. The study allows examining numerically the relationship between the frictional behavior on the interface and the apparent friction estimated from external loads. We find that, because of dynamic overshoot, the reduction in apparent friction is larger than that of the actual friction coefficient on the interface.