Sedimentary layers enable extra-wide rupture jumps across extensional strike-slip stepovers

Fault stepovers commonly arrest earthquake ruptures, yet shallow sedimentary layers can substantially modify this behavior. We use three-dimensional spontaneous dynamic-rupture simulations to examine how sediment velocity, thickness, background stress, rupture regime, and fault burial affect critical jump distance across extensional and compressional stepovers. In the reference models, sedimentary layers strongly promote transfer across extensional stepovers under high-prestress conditions with supershear rupture. Sediment-guided Mach waves reflect between the free surface and sediment–bedrock interface and sustain favorable dynamic Coulomb stress at large fault-normal distances. The sediment effect is weaker and regime-dependent across compressional stepovers, where transfer is associated primarily with the stopping phase and may be inhibited after supershear transition. At the highest stress level, coexisting and merging Burridge–Andrews and free-surface-induced fronts accompany maximum extensional critical jump distances of 35 km in the low-velocity sediment model and 40 km in the higher-velocity model. These maxima are conditional on the reference stress and friction assumptions. Additional tests show that shallow stress and residual strength substantially change the absolute jump distance. Independent changes in main- and secondary-fault prestress distinguish source-side and receiver-side sensitivity. Fault burial suppresses near-surface supershear development and reduces the enhancement. The results identify a conditional mechanism for sediment-assisted rupture transfer and motivate including shallow structure and fault properties in physics-based assessments of multifault earthquake potential.