Pores and cracks have an important role in the evolution of fault rocks because they strongly influence the behavior of the fluids that promote rock alteration and trigger the mechanical instability of faults. We used rock physics model inversion of measured elastic wave velocity and porosity to estimate the grain elastic moduli and crack aspect ratios of a range of fault rocks (intact rocks, fractured rocks, transition rocks, and fault gouge) from the Median Tectonic Line in southwest Japan. Our results show distinct gaps in the evolutionary trends of crack aspect ratios and grain elastic moduli from intact rocks to fault rocks. Crack aspect ratios show a nonlinear trend from intact rock to fault gouge, and then these values in fault gouge were considerably higher than in fractured rock and transition rock. In contrast, grain elastic moduli decreased as fracture evolved with the development and subsequent extinction of shear planes and then increased markedly with the formation of fault gouge. Our results show that crack aspect ratios and grain elastic moduli are clearly related to the evolution of shear fabrics in faults. Therefore, they might be useful indicators of fault activity and maturity. •Elastic wave velocity and porosity are closely related to fault rock evolution.•There is a strong relationship between crack aspect ratio and shear fabric.•Crack aspect ratios and grain elastic moduli are indicators of fault activity and maturity.