Direct Observation of Faulting by Means of Rotary Shear Tests Under X-Ray Micro-Computed Tomography

Friction and fault evolution are critical aspects in earthquake studies as they directly influence the nucleation, propagation, and arrest of earthquake ruptures. We present the results of a recently developed experimental approach that investigates these important aspects using a combination of rotary shear testing and X-ray micro-computed tomography technology. Two sets of experiments at normal stresses (σ_n) of 2.5 and 1.8 MPa were conducted on synthetic laboratory faults. We identified real contact areas (A_c) on the fault surfaces and estimated sizes of contact patches by means of micro-computed tomography image analysis. The number of contact patches and their sizes showed positive correlations with σ_n, and contact patch size distributions followed power law relations. The total number of contact patches decreased with increasing slip distance, and large contact patches endured longer slip distance than small ones. Secondary off-fault fractures created by interlocking and breakdown of large contact patches were closely related to the sudden drops of frictional resistance, suggesting the dominant role of surface roughness on shear behavior especially at low stress.