Grain‐Scale Heterogeneity in Fault Veins Influences Shear Rupture, Damage Distribution, and Microseismicity

Abstract Fault veins, mineral‐filled fractures within fault zones, exhibit substantial grain‐scale structural variation, yet, the mechanical consequences of this heterogeneity on fault deformation and seismicity remain poorly constrained. Here, we use the Discrete Element Method to investigate how grain‐scale heterogeneity in quartz veins, which are common in crustal fault systems, affects shear rupture, frictional evolution, and microseismic activity. Two end‐member vein models are constructed: a homogeneous vein with uniform mineral properties, and a heterogeneous vein containing coarse, irregular grains and weak grain‐boundary contacts, each embedded within a representative host‐rock domain. Direct‐shearing simulations are performed to capture deformation from the initial cohesive phase through frictional sliding. Results show that the heterogeneous vein, relative to homogeneous veins, exhibits lower shear strength and friction during the cohesive‐influenced phase, but develops slightly higher and more stable friction in the subsequent friction‐influenced phase. Grain‐scale heterogeneity promotes pervasive cracking not only within the vein but also in the adjacent host rock and along vein‐host interfaces. This leads to more frequent, larger‐magnitude microseismic events. These events form broad microseismicity clouds, splay‐like off‐fault fractures, and elevated off‐fault b‐values, whereas the homogeneous vein localizes deformation near the vein‐host boundary. Our results show that grain‐scale heterogeneity in fault veins strongly influences rupture style, distributed damage, and seismicity. Importantly, they show that shear failure of cohesive, mineralized veins can generate both near‐field and far‐field off‐fault fracturing, a mechanism largely overlooked in laboratory and numerical studies focused on weak or granular gouges, with implications for rupture complexity and damage evolution in fault systems.