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. To advance SEAS simulations with rigor and reproducibility, we pursue community efforts to verify numerical codes in an expanding suite of benchmarks. Here we present code comparison results from a new set of benchmark problems BP6-QD-A/S/C that consider 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. Ten modeling groups participated in problems BP6-QD-A and BP6-QD-S considering rate-and-state fault models using the aging and slip law formulations for frictional state evolution, respectively, allowing us to explore these ingredients across multiple codes and better understand how various computational factors affect the simulated evolution of pore pressure and aseismic slip. Comparisons of problems using the aging versus slip law illustrate how models of aseismic slip can differ in the timing and amount of slip achieved with different treatments of fault friction given the same perturbations in pore fluid pressure. We achieve excellent quantitative agreement across participating codes, with further agreement being found by ensuring sufficiently fine time-stepping and consistent treatment of remote boundary conditions. Our benchmark efforts offer a community-based example to reveal sensitivities of numerical modeling results, which is essential for advancing multi-physics SEAS models to better understand and construct reliable predictive models of fault dynamics.

The dynamics of the fluid flow within faults plays a critical role in the evolution of fault strength through the seismic cycle. The key processes that control how fluids affect fault slip behavior are the evolution of fault porosity and fluid recharge during slip that, in turn, determine dilational strengthening or compaction weakening. Despite the significance of these processes, high-fidelity lab measurements that include the evolution of porosity, fluid pressure and frictional properties are sparse. Here, we report such data for drained and undrained velocity-stepping experiments from 3 to 300 µm/s on natural fault gouges from the seismogenic zone of injection well 16A (2050 - 2070m) of the Utah FORGE EGS site. We conducted a suite of experiments under fixed normal stresses (44 MPa) and pore fluid pressures (13, 20, 27 MPa) corresponding to pore fluid factors between 0.3 and 0.65. We carefully monitor the volumetric strain and show that the dilatancy coefficient of the material ranged from 5 to 12 x 10-4, and showed minor sensitivity to fluid boundary conditions. In some cases, we see that larger slip velocities cause a transition from dilatancy strengthening to compaction weakening via fluid pressurization. Fluid pressure diffusion across the fault evolves during shear suggesting that permeability asymmetry, up to 4 orders-of-magnitude, is required to explain the interaction between fault stress, dilation and fluid diffusion. We posit that the spatial-temporal pattern of pore connectivity creates a spectrum of fault drainage conditions, ultimately controlling the mode of fault slip.