Jeena Yun , Alice-Agnes Gabriel , Dave A May, Yuri Fialko 

Numerical and laboratory models of earthquake cycles on faults governed by rate-and-state friction often show cycle-invariant behavior, while natural faults exhibit considerable variability in slip history. Possible explanations include heterogeneities in fault stress and frictional properties. We investigate how various types of heterogeneity in simulations of quasi-dynamic sequences of seismic and aseismic slip affect rupture complexity, hypocenter location, and slow slip events (SSEs). We model a 2D vertical strike-slip fault and study the roles of self-affine fractal heterogeneities in normal stress, rate-and-state parameter (a-b), and characteristic slip-weakening distance, as well as the effects of a low-rigidity fault zone. We find that only a combination of heterogeneous parameters introduces variability in the modeled rupture extent, hypocenter depth, and recurrence interval. In particular, variable hypocenter depths require velocity-strengthening patches within the velocity-weakening seismogenic zone. A low-rigidity fault zone can encourage pulse-like ruptures but adds little to slip complexity. Slip law simulations produce fewer partial ruptures, smaller stress drops, and lower peak slip rates compared to aging law simulations. We show that the ratio of the seismogenic zone thickness to nucleation size does not entirely predict slip complexity. The most complex aging law model, combining multiple heterogeneities, features system-size earthquakes preceded by cascades of partial ruptures and spontaneous SSEs. For such models, a transition from aperiodic to quasi-regular regimes requires more cycles than is typically needed to erase the effect of initial conditions. These results highlight the importance of heterogeneity in reproducing natural fault slip complexity in numerical models of earthquake sequences.