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Numerical simulations of sequences of earthquakes and aseismic slip (SEAS) have made great progress over past decades to address important questions in earthquake physics. However, significant challenges in SEAS modeling remain in resolving multiscale interactions between earthquake nucleation, dynamic rupture, and aseismic slip, and understanding physical factors controlling observables such as seismicity and ground deformation. The increasing complexity of SEAS modeling calls for extensive efforts to verify codes and advance these simulations with rigor, reproducibility, and broadened impact. In 2018, we initiated a community code‐verification exercise for SEAS simulations, supported by the Southern California Earthquake Center. Here, we report the findings from our first two benchmark problems (BP1 and BP2), designed to verify different computational methods in solving a mathematically well‐defined, basic faulting problem. We consider a 2D antiplane problem, with a 1D planar vertical strike‐slip fault obeying rate‐and‐state friction, embedded in a 2D homogeneous, linear elastic half‐space. Sequences of quasi‐dynamic earthquakes with periodic occurrences (BP1) or bimodal sizes (BP2) and their interactions with aseismic slip are simulated. The comparison of results from 11 groups using different numerical methods show excellent agreements in long‐term and coseismic fault behavior. In BP1, we found that truncated domain boundaries influence interseismic stressing, earthquake recurrence, and coseismic rupture, and that model agreement is only achieved with sufficiently large domain sizes. In BP2, we found that complexity of fault behavior depends on how well physical length scales related to spontaneous nucleation and rupture propagation are resolved. Poor numerical resolution can result in artificial complexity, impacting simulation results that are of potential interest for characterizing seismic hazard such as earthquake size distributions, moment release, and recurrence times. These results inform the development of more advanced SEAS models, contributing to our further understanding of earthquake system dynamics.

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