From fragmentation to fault zone architecture as a structural framework for fault weakening

Earthquake nucleation is governed by the evolution of fault zone structure, yet the physical mechanisms linking grain-scale processes to macroscopic weakening remain elusive. Here we use discrete element simulations of quartz-rich gouge in which angular grains fragment dynamically, revealing a robust strain–damage law in which damage accumulates as grains are consumed. Grain size dictates the geometry and timing of localization, while fragmentation enhances compaction and promotes the emergence of a dominant Principal Slip Zone (PSZ). As comminution approaches a saturation state (i.e. minimum grain size), localization can migrate to competing bands, providing a mechanical explanation for overprinted shear fabrics in experiments and natural faults. By linking fragmentation, porosity evolution, and the resulting shear localization, our results provide a local fragmentation state variable for constitutive laws that couple grain-size evolution to fault weakening and the conditions for earthquake nucleation. This study combines simulations and laboratory data to show how grains breaking inside fault gouge reshape fault-zone architecture, focus slip into narrow bands and help faults weaken during deformation.