Preseismic slip and foreshocks on rough faults embedded in a damage zone

Faults exhibit geometrical heterogeneity at all scales, which induces spatial variations in normal stress and hence strength. Additionally, fault zones comprise multiple fractures which can host seismicity and further modify the stress state on the mainshock fault. Here we study how geometrical complexity affects the precursory phase of large earthquakes. We model seismic cycles on fractal faults with uniform velocity-weakening rate-state friction, loaded by a uniform far-field stressing rate. We also include the effect of surrounding damage, represented by a collection of smaller faults with a power-law decay of density with distance from the main fault.
We find that heterogeneity in normal stress σ induced by roughness controls slip behavior: regions with low σ begin to slip aseismically early in the cycle, loading high σ regions (asperities) which eventually fail seismically generating foreshocks. The precursory phase is characterized by a positive feedback between aseismic slip and foreshocks, with stress changes from each process accelerating the other. In simulations including subparallel secondary faults in the damage zone, this process does not take place on the main fault but instead on smaller, off-fault structures. In both cases, mainshocks nucleate on strong asperities at the edge of the preslip area, which is significantly larger and spatially distinct from mainshock nucleation. These features are consistent with a number of observations at different scales, including laboratory experiments, sub-glacial slip events, and foreshock sequences of megathrust earthquakes.