Effect of normal stress dynamic perturbation on frictional properties of simulated granite gouges and implications for fault reactivation mechanism

Tectonic movements and human engineering activities often cause disturbances in the stress of strata. However，it is still unclear how the frictional properties of fault gouge evolve under the disturbance of normal stress，and whether this will lead to the reactivation of faults and trigger earthquakes. This paper documents 20 direct shear experiments on simulated granite gouges(grain size<0.25 mm) collected from Heyuan fault zone，China，to investigate frictional properties under dynamic normal stresses. All experiments were conducted，using a conventional direct shear equipment，at room temperature with a fixed shear rate of 1 μm/s. Dynamic loading was applied by using square and triangular waves(initial normal stress of 10 Pa，amplitudes of 1–9.5 MPa and frequencies of 0.003 3–0.001 0 Hz). The results show that：(1) Shear stress exhibited a clear hysteresis effect with dynamic normal stress loading. (2) The apparent friction coefficient linearly decreases from 0.63 to 0.14 with increasing amplitude during loading and increases linearly from 0.63 to 0.91 during unloading. Microstructure analysis revealed that samples subjected to higher stress perturbation amplitudes were more intact and exhibited fewer tensile fracture zones. These hysteresis effects and the evolution of the apparent frictional coefficient align with the micromechanical model of grain contact proposed by Wang and Scholz. Additionally，a relationship between the apparent friction coefficient of fault gouge and the amplitude of normal stress perturbations was established based on experimental results. Using the Heyuan fault zone as a case study，we analyze the impact of dynamic perturbation in principal stress and pore water pressure on fault stability based on the Coulomb strength theory. It was found that increasing the disturbance amplitude of the maximum principal stress by 1 MPa reduces the fault shear strength by 0.18 MPa. This could reactivate faults，potentially causing seismic events with magnitude above 6 if the fault exceeds 10 km.