Dynamic slip behavior and off-fault damage of tension-induced fractures constrained by different grain sizes in granite

The mineralogy and texture of granite have been found to have a pronounced effect on its mechanical behavior. However, the precise manner in which the texture of granite affects the shear behavior of fractures remains enigmatic. In this study, fine-grained granite (FG) and coarse-grained granite (CG) were used to create tensile fractures with surface roughness (i.e. joint roughness coefficient (JRC)) within the range of 5.48–8.34 and 12.68–16.5, respectively. The pre-fractured specimens were then subjected to direct shear tests under normal stresses of 1–30 MPa. The results reveal that shear strengths are smaller and stick-slip behaviors are more intense for FG fractures than for CG fractures, which is attributed to the different conditions of the shear surface constrained by the grain size. The smaller grain size in FG contributes to the smoother fracture surface and lower shear strength. The negative friction rate parameter a – b for both CG and FG fractures and the larger shear stiffness for FG than for CG fractures can account for the more intense stick-slip behaviors in FG fractures. The relative crack density for the post-shear CG fractures is greater than that of the FG fractures under the same normal stress, both of which decrease with the distance away from the shear surface following the power law. Moreover, the damage of CG fracture extends to a larger extent beneath the surface compared with the FG fracture. Our findings demonstrate that the grain size of the host rock exerts a significant influence on the fracture roughness, and thus should be incorporated into the assessment of fault slip behavior to better understand the role of mineralogy and texture in seismic activities.