Direct observation of faulting by means of a rotary shear test under X-ray micro-computed tomography

Friction and fault surface evolution are critical aspects in earthquake studies. We present the preliminary result from a novel experimental approach that combines rotary shear testing with X-ray micro-computed tomography (${\mu}$CT) technology. An artificial fault was sheared at small incremental rotational steps under the normal stress of 2.5 MPa. During shearing, mechanical data including normal force and torque were measured and used to calculate the friction coefficient. After each rotation increment, a ${\mu}$CT scan was conducted to observe the sample structure. The careful and quantitative ${\mu}$CT image analysis allowed for direct and continuous observation of the fault evolution. We observed that fracturing due to asperity interlocking and breakage dominated the initial phase of slipping. The frictional behavior stabilized after ~1 mm slip distance, which inferred the critical slip distance. We developed a novel approach to estimate the real contact area on the fault surface by means of ${\mu}$CT image analysis. Real contact area varied with increased shear distances as the contacts between asperities changed, and it eventually stabilized at approximately 12% of the nominal fault area. The dimension of the largest contact patch on the surface was close to observed critical slip distance, suggesting that the frictional behavior may be controlled by contacting large asperities. These observations improved our understanding of fault evolution and associated friction variation. Moreover, this work demonstrates that the ${\mu}$CT technology is a powerful tool for the study of earthquake physics.