  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.
