Grain-based modeling of shear instability in rough granite joints: Coupling Shear Stress Drop and Acoustic Emission

Understanding the transition of rough rock fractures from a stable sliding state to a dynamically unstable state is crucial for the assessment of geological hazards. However, the relationship between macroscopic stress release and microscopic damage remains unclear. This study investigates the shear response of granite fractures using a grain-based model that combines acoustic emission (AE) analysis with moment tensor inversion. Direct shear simulations were performed for rock fractures with varying roughness ( JRC = 0.42–18.96) under normal stresses ranging from 4 to 20 MPa. The results indicate that high confining pressure (particularly ≥ 15 MPa) inhibits crack opening and promotes brittle failure, abrupt stress release, and the enhancement of implosion sources associated with grain fragmentation and chipping. A dual-index criterion combining stress drop (Δ τ ) and AE b-value is further proposed to assess shear instability. The stable region (Δ τ < 1.25 MPa, b > 1.0) is characterized by stable wear, distributed surface microdamage, low-amplitude acoustic emission activity, and a relatively small shear stress drop. The unstable region (Δ τ ≥ 1.25 MPa, b ≤ 1.0) appears only in rough joint surfaces under high confining pressure conditions, particularly when JRC ≥ 11.12 and ≥ 15 MPa, reflecting rapid failure of the asperities and rapid crack propagation. Segmentation statistics further indicate that as roughness and normal stress increase, the proportion of the stable region decreases, while the proportion of the unstable region increases. These findings establish a directly measurable stress drop– AE b-value framework for assessing shear instability and slip risk in fractured rock masses.