Back‐Propagating Earthquakes on Simple Faults

Abstract Back‐propagating earthquakes, characterized by a secondary front reversing into previously ruptured areas, challenge conventional models of rupture dynamics and have been increasingly documented in recent years. The conditions for this phenomenon remain poorly understood: while rupture complexity is often attributed to fault heterogeneity, back‐propagating fronts have also been reported on simple faults. Here, we employ earthquake simulations with slip‐rate and state‐dependent friction to reveal that back‐propagating fronts arise spontaneously during unilateral rupture propagation on frictionally homogeneous faults, when the rupture exceeds a critical length about 100 times larger than the nucleation dimension. We propose simple theoretical arguments suggesting that back‐propagating fronts are an intrinsic feature of unilateral ruptures under velocity‐weakening friction. These conditions preclude self‐similar crack‐like solutions with constant stress drop; instead, ruptures alternate between pulse and crack‐like modes, with back propagation generated at pulse‐to‐crack transitions. The crack‐to‐pulse transition is driven by restrengthening and happens when the local slip rate decreases below a specified threshold. An analytical criterion for the extent of back propagation shows that they are enhanced by low rupture velocities and stress drops, and observational examples of back propagation on simple faults are consistent with these requirements. Our study presents a simple, fundamental model for back‐propagating earthquakes, suggesting that they may be more prevalent than previously recognized.