Delayed dynamic triggering and enhanced high-frequency seismic radiation due to brittle rock damage in 3D multi-fault rupture simulations

Abstract: Using a novel high-performance computing implementation of a nonlinear continuum damage breakage model, we explore interactions between 3D co-seismic off-fault damage, seismic radiation, and rupture dynamics. Our simulations demonstrate that off-fault damage enhances high-frequency wave radiation above 1~Hz, reduces rupture speed and alters the total kinetic energy. We identify distinct damage regimes separated by solid-granular transition, with smooth distributions under low damage conditions transitioning to localized, mesh-independent shear bands upon reaching brittle failure. The shear band orientations depend systematically on the background stress and agree with analytical predictions. The brittle damage inhibits transitions to supershear rupture propagation and the rupture front strain field results in locally reduced damage accumulation during supershear transition. The dynamically generated damage yields uniform and isotropic ratios of fault-normal to fault-parallel high-frequency ground motions. Co-seismic damage zones exhibit depth-dependent width variations, becoming broader near the Earth's surface consistent with field observations, even under uniform stress conditions. We discover a new delayed dynamic triggering mechanism in multi-fault systems, driven by reductions in elastic moduli and the ensuing stress heterogeneity in 3D tensile fault step-overs. This mechanism affects the static and dynamic stress fields and includes the formation of high shear-traction fronts around localized damage zones. The brittle damage facilitates rupture cascading across faults, linking delay times directly to damage rheology and fault zone evolution. Our results help explain enhanced high-frequency seismic radiation and delayed rupture triggering, improving our understanding of earthquake processes, seismic radiation and fault system interactions.