Single Fault Earthquake Cycle Complexity

Summary

Large earthquakes occur irregularly and often show limited resemblance to an ideal, periodic stick-slip cycle. Numerical models and laboratory experiments have reproduced transitions from periodic to quasi-periodic and irregular behavior, yet the mechanisms controlling these transitions remain debated. Here, we analyze 3D earthquake-cycle simulations governed by rate-and-state friction on a single dipping fault embedded in an elastic half-space. This provides a simple framework for isolating the roles of frictional dynamics, fault geometry, and free-surface effects before analyzing more complex setups. Our tests show that subtle numerical meshing choices beyond insufficient spatial resolution can influence cycle behavior and, in some cases, lead to spuriously periodic solutions. In terms of the physical effects, we expect that the ratio between the seismogenic width, W, and the nucleation length, L∞, W′ = W/L∞ to be a primary control on earthquake-cycle behavior. We further systematically vary fault dip, the down-dip transition between velocity-weakening and velocity-strengthening friction, and lateral velocity-strengthening bounds. Our results confirm that increasing W′ promotes transitions from periodic to quasi-periodic and irregular earthquake cycles. However, we also find that fault dip and frictional heterogeneities can independently induce similar transitions and compete with the influence of W′. The effects of these perturbations are not fixed: the same change in fault dip or frictional properties may either stabilize or destabilize earthquake cycles depending on the cycle regime occupied by the reference model. Together, these findings highlight the sensitivity of physics-based earthquake-cycle simulations to both physical and numerical model assumptions. This has important implications for the interpretation of fault system dynamics and seismic hazard.