Community Code Verification Exercises for Simulations of Earthquake Sequences and Aseismic Slip (SEAS): Effects from Fluids and Fault Friction Evolution

Numerical simulations of Sequences of Earthquakes and Aseismic Slip (SEAS) have rapidly progressed to address fundamental problems in fault mechanics and provide self-consistent, physics-based frameworks to interpret and predict geophysical observations across spatial and temporal scales. Challenges in SEAS modeling include resolving the multiscale interactions between slow slip, earthquake nucleation, and dynamic rupture; and understanding the physical factors controlling observables such as seismicity and deformation. To advance SEAS simulations with rigor and reproducibility, we pursue community efforts to verify numerical codes in an expanding suite of benchmarks, including problems considering earthquake sequences on 2D and 3D fault models obeying rate-and-state friction with different treatments of inertial effects and fault dip under slow tectonic loading (Erickson et al., 2020; Jiang et al., 2022; Erickson et al., 2023).

Here we present code comparison results from a new set of benchmark problems that focus on aseismic processes and earthquake nucleation, including the influence of (1) changes in effective normal stress and pore fluid pressure due to fluid injection and diffusion and (2) different formulations of fault friction evolution. Benchmark problem BP6-QD-A/S/C is a 2D problem that considers a single aseismic slip transient induced by changes in pore fluid pressure consistent with fluid injection and diffusion in fault models with different treatments of fault friction, including rate-and-state fault models using the aging (-A) and slip (-S) law formulations for frictional state evolution, respectively, as well as a constant friction coefficient (-C). BP7-QD/FD-A/S is a 3D problem with a 2D rate-and-state fault considering a circular velocity-weakening asperity that can produce sequences of repeating earthquakes or alternating seismic and aseismic ruptures, under different considerations of fault friction evolution and inertial effects. Comparisons of problems using the aging versus slip law illustrate how models of seismic and aseismic slip can differ in the timing and amount of slip achieved with different treatments of fault friction, including for individual aseismic slip events induced by the same perturbations in pore fluid pressure for BP6. 

We utilize simulations from different groups to explore how various numerical factors affect the simulated evolution of pore pressure and interaction between aseismic and seismic processes. We achieve excellent quantitative agreement across participating codes that utilize distinct numerical methods, by ensuring sufficiently fine time-stepping, large enough domain size for volumetric methods and consistent treatment of boundary conditions. Through these comparative studies, we seek to determine best practices for improving the accuracy and efficiency of SEAS simulations and develop quantitative metrics for benchmarking modeling results. These community-led exercises will foster the development of more realistic multi-physics SEAS models and their integration with geophysical observations, contributing to an improved understanding of fault dynamics.