Mechanics-based simulation of aftershock sequences in complex 3D fault networks

Understanding the physical mechanisms governing aftershock patterns and their evolution in fault networks is crucial for interpreting seismic catalogues and improving physics-based seismic hazard assessment. Here, we develop a mechanics-based modeling framework based on the discrete fracture network approach to explicitly simulate mainshock rupture, coseismic stress changes, and aftershock generation in complex 3D fault networks. The fault system that we model comprises a primary strike-slip fault surrounded by a network of thousands of secondary faults with sizes following a power-law distribution. Dynamic rupture nucleates within a localized patch on the primary fault and propagates spontaneously at a sub-Rayleigh speed, producing a M_w 7.6 mainshock. The model captures aftershock triggering driven by radiated seismic waves and/or permanent stress redistribution, and quantifies their combined effect using Coulomb failure stress changes. Fault slip is governed by a linear slip-weakening friction law, where the critical slip distance is varied over orders of magnitude to explore its influence on breakdown-zone size, fracture-energy dissipation, and rupture propensity on secondary faults. The simulations capture key emergent characteristics of aftershock sequences: spatially, aftershocks cluster within positive Coulomb stress lobes and are suppressed within stress shadows, with additional localization near fault intersections; statistically, the cumulative frequency–magnitude distributions follow Gutenberg–Richter scaling over a broad magnitude range. Importantly, the synthetic catalogues consistently exhibit a two-branch frequency–magnitude scaling behavior, in which the lower-magnitude branch is dominated by partial ruptures and premature arrest, whereas the higher-magnitude branch corresponds to self-sustained ruptures whose moment magnitudes scale with fault area and are therefore more strongly constrained by fault network geometry. We further show that the transition between these regimes is governed by fault criticality and fracture energy dissipation, providing an alternative mechanics-based explanation for the commonly observed roll-off in frequency–magnitude distribution. Overall, our framework mechanically connects fault network structure and rupture dynamics to explain aftershock statistics, enabling physics-based interpretation of seismic catalogues and supporting improved seismic hazard assessment.