Response of an Artificial Filled Fracture with Varied Water Contents to Dynamic Stressing

Field observations indicate that gouge-filled faults can be weakened under dynamic stressing and further trigger earthquakes. Previous studies have qualitatively attributed such fault weakening to elevated pore pressure within fault gouges induced by seismic deformation, which reduces fault shear strength and frictional resistance. However, direct experimental evidence and systematic quantitative analyses clarifying the hydromechanical controls on the dynamic weakening process remain scarce. Here, artificial filled fractures were prepared by sandwiching granular infills with varied water content between two cylindrical aluminum bars to constitute an aluminum split Hopkinson pressure bar (SHPB) system. Benefiting from the rock-like wave impedance of aluminum bars, this setup can better reproduce real in situ dynamic loading conditions. Using this system, we investigated the hydromechanical response of filled fractures under dynamic stressing and clarified the underlying fault weakening mechanism. The experimental results indicate that the filled fractures undergo significant compression under dynamic loading and exhibit limited rebound deformation during unloading, presenting a distinctive trapped-unloading behavior and obvious stiffness enhancement relative to the loading stage. Variations in the water content of fracture infills substantially alter the stiffness, viscosity, and stress relaxation properties of the filled fractures, thereby regulating stress wave propagation and dynamic deformation characteristics. It is inferred that higher water content significantly hinders the dissipation of excess pore water pressure, reduces the effective confining stress, and ultimately intensifies the hydromechanical weakening of filled fractures. These quantitative findings provide novel experimental insights into the hydromechanical weakening mechanism of natural gouge-filled faults under dynamic stressing.