Rate Effect on the Direct Shear Behavior of Granite Rock Bridges at Low to Subseismic Shear Rates

Natural faults are often heterogeneous with spatially variable cohesion caused by healing processes. Using rock bridges as laboratory analogs, we investigate how cohesion-healed patches rupture at different shear rates. We performed direct shear experiments on granite rock bridges at shear rates from 1 mu m/s to 10 mm/s (low to subseismic shear rates), and quantitatively characterized the failure surface morphology by integrating laser scanning with ArcGIS. The results show a notable dependence of both mechanical and morphological characteristics on shear rate. The failure mode transitions from tensile failure at low shear rates to shear failure at subseismic shear rates. At low shear rates, tensile failure forms curved coalescence patterns and undulating failure surfaces with significant roughness and morphological anisotropy. At subseismic shear rates, straighter shear-formed coalescence patterns are observed. In addition, the peak shear strength, the roughness and morphological anisotropy increase with increasing shear rate within the subseismic rate regime. We explain the observed rate-dependent behaviors by the energy and geometric features of micro-cracking mechanisms. We suggest through the variations in 3D morphological parameters that rate effects should be incorporated into models of fault morphology. In addition, the failure surfaces at all shear rates are asymmetric, with asperities more inclined to the direction of the shear load. We find that this asymmetry results from both the fracture of rock bridges and the further shearing after failure, and it can be used to determine the sense of fault displacement.