Fault-Zone Damage Promotes Pulse-Like Rupture and Back-Propagating Fronts via Quasi-Static Effects

Damage zones are ubiquitous components of faults that may affect earthquake rupture. Simulations show that pulse-like rupture can be induced by the dynamic effect of waves reflected by sharp fault zone boundaries. Here we show that pulses can appear in a highly damaged fault zone even in the absence of reflected waves. We use quasi-static scaling arguments and quasi-dynamic earthquake cycle simulations to show that a crack turns into a pulse after the rupture has grown larger than the fault zone thickness. Accompanying the pulses, we find complex rupture patterns involving back-propagating fronts that emerge from the primary rupture front. Our model provides a mechanism for back-propagating fronts recently observed during large earthquakes. Moreover, we find that slow-slip simulations in a highly compliant fault zone also produce back-propagating fronts, suggesting a new mechanism for the rapid tremor reversals observed in Cascadia and Japan. Plain Language Summary Damage zones are zones of fractured rock that surround faults and can influence how earthquakes propagate. Previous computer models show that damage zones promote an inchworm-like (rather than zipper-like) pattern of earthquake propagation, known as pulses. This finding has been previously attributed to the effect of seismic waves reflected at the boundaries of the damage zone. Here, we show that pulses are generated in highly fractured damage zones independently of the reflection of seismic waves. We reach this conclusion by scaling arguments confirmed by numerical simulations of sequences of earthquakes in which we ignore the reflection of seismic waves. Moreover, our models produce an unexpected pattern of earthquake propagation: Secondary rupture fronts emerge from the primary rupture front and propagate in the opposite direction. Similar back-propagating fronts have been previously observed during slow earthquakes in subduction zones and more recently during large earthquakes. Our work reveals a possible connection between an observable structural feature of faults and complicated patterns of earthquake propagation. Key Points Highly damaged fault zones promote pulse-like ruptures even without the dynamic effects of reflected waves Slip complexity induced by fault damage involves multiple back-propagating rupture fronts A new mechanism for rapid tremor reversals is observed during episodic tremor and slip