Seismic and geodetic observations show that slow slip events (SSEs) in subduction zones can happen at all temporal and spatial scales and propagate at various velocities. Observation of rapid tremor reversals (RTRs) indicates back-propagating fronts traveling much faster than the main rupture front. Heterogeneity of fault properties, such as fault roughness, is a ubiquitous feature often invoked to explain this complex behavior, but how roughness affects SSEs is poorly understood. Here we use quasi-dynamic seismic cycle simulations to model SSEs on a rough fault, using normal stress perturbations as a proxy for roughness and assuming rate-and-state friction, with strengthening behavior at high slip rate. SSEs exhibit temporal clustering, large variations in rupture length and propagation speed, and back-propagating fronts at different scales. We identify a mechanism for back propagation: as ruptures propagate through low-normal stress regions, a rapid increase in slip velocity combined with rate-strengthening friction induces stress oscillations at the rupture tip, and the subsequent ”delayed stress drop’ induces secondary back-propagating fronts. Moreover, on rough faults with fractal elevation profiles, the transition from pulse to crack can also lead to the re-rupture of SSEs due to local variations in the level of heterogeneity. Our study provides a possible mechanism for the complex evolution of SSEs inferred from geophysical observations and its link to fault roughness.

Repeating earthquakes repeatedly rupture the same fault asperities, which are likely loaded to failure by surrounding aseismic slip. However, repeaters occur less often than would be expected if these earthquakes accommodate all of the long-term slip on the asperities. Here we assess a possible explanation for this slip discrepancy: partial ruptures. On asperities that are much larger than the nucleation radius, a fraction of the slip could be accommodated by smaller ruptures on the same asperities. We search for partial ruptures of repeating earthquakes in Parkfield using the Northern California earthquakes catalogue. We find 3991 individual repeaters which have 4468 partial ruptures. The presence of partial ruptures suggests that asperities of repeating earthquakes are much larger than the nucleation radius. However, we find that partial ruptures could accommodate only around 25% of the slip on repeating earthquake patches. A 25% increase in the slip budget can explain only a small portion of the long recurrence intervals of repeating earthquakes.

Dynamic modeling of sequences of earthquakes and aseismic slip (SEAS) provides a self-consistent, physics-based framework to connect, interpret, and predict diverse geophysical observations across spatial and temporal scales. Amid growing applications of SEAS models, numerical code verification is essential to ensure reliable simulation results but is often infeasible due to the lack of analytical solutions. Here, we develop two benchmarks for three-dimensional (3D) SEAS problems to compare and verify numerical codes based on boundary-element, finite-element, and finite-difference methods, in a community initiative. Our benchmarks consider a planar vertical strike-slip fault obeying a rate- and state-dependent friction law, in a 3D homogeneous, linear elastic whole-space or half-space, where spontaneous earthquakes and slow slip arise due to tectonic-like loading. We use a suite of quasi-dynamic simulations from 10 modeling groups to assess the agreement during all phases of multiple seismic cycles. We find excellent quantitative agreement among simulated outputs for sufficiently large model domains and small grid spacings. However, discrepancies in rupture fronts of the initial event are influenced by the free surface and various computational factors. The recurrence intervals and nucleation phase of later earthquakes are particularly sensitive to numerical resolution and domain-size-dependent loading. Despite such variability, key properties of individual earthquakes, including rupture style, duration, total slip, peak slip rate, and stress drop, are comparable among even marginally resolved simulations. Our benchmark efforts offer a community-based example to improve numerical simulations and reveal sensitivities of model observables, which are important for advancing SEAS models to better understand earthquake system dynamics.