Brittany Angela Erickson, Junle Jiang , Michael Barall, Nadia Lapusta, Eric M Dunham , Ruth A Harris, Lauren S Abrahams , Kali Allison, Jean Paul Ampuero , Sylvain Barbot

Numerical simulations of Sequences of Earthquakes and Aseismic Slip (SEAS) have made great progress over the past decades to address important questions in earthquake physics and fault mechanics. However, significant challenges in SEAS modeling remain in resolving multiscale interactions between aseismic fault slip, earthquake nucleation, and dynamic rupture; and understanding physical factors controlling observables such as seismicity and ground deformation. The increasing capability and 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 (SCEC). Here we report the findings from our first two benchmark problems (BP1 and BP2), designed to test the capabilities of different computational methods in correctly solving a mathematically well-defined, basic problem in crustal faulting. These benchmarks are for 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 >70 simulation results from 11 groups using different numerical methods, uploaded to our online platform, show excellent agreements in long-term and coseismic evolution of fault properties. In BP1, we found that the truncated domain boundaries influence interseismic fault stressing, earthquake recurrence, and coseismic rupture process, and that agreement between models is only achieved with sufficiently large domain sizes. In BP2, we found that complexity of long-term fault behavior depends on how well important physical length scales related to spontaneous nucleation and rupture propagation are resolved. Poor numerical resolution can result in the generation of artificial complexity, impacting simulation results that are of potential interest for characterizing seismic hazard, such as earthquake size distributions, moment release, and earthquake recurrence times. These results inform the development of more advanced SEAS models, contributing to our further understanding of earthquake system dynamics.