Non‐Precursory Accelerating Aseismic Slip During Rupture Nucleation

Accelerating aseismic slip events have been commonly observed during the rupture nucleation processes of the earthquake. While that accelerating aseismic slip is usually considered strong evidence for precursory activity, it remains unclear whether all accelerating aseismic slip events are precursory to an incoming earthquake. Two contrasting nucleation models have been introduced to explain the observations associated with the nucleation of unstable slip: the pre‐slip and cascade nucleation models. Each of these two‐end members, however, has its own limitations. In this study, we employ Discrete Element Method simulations of a 2‐D strike‐slip fault to simulate various rupture nucleation and triggering processes. Our simulation results manifest that the final seismic event is a product contributed by multiple pre‐slip nucleation sites, which may interact, causing clock advance or cascade nucleation rupture processes. We also introduce a strengthening perturbation zone to investigate the role of a single nucleation site in an imminent seismic event. The simulation results reveal a new type of non‐precursory aseismic slip, representing the region favoring the generation of the precursory slip process but not correlating to the incoming main event, which differs from the previous interpretation of precursory slip. Furthermore, we include weakening perturbation zones in some simulations to demonstrate how small earthquakes may or may not trigger a nucleation site depending on spatial and temporal conditions. Our simulation results imply that such non‐precursory but accelerating aseismic slip events may suggest a fault segment that appears weakly coupled but possesses the potential to be triggered seismically. Plain Language Summary It is essential to understand the nucleation of unstable slip to predict an impending earthquake. Slow slip fault activities and their accelerations have been observed before large earthquakes. The accelerating aseismic slip events are usually considered an early warning sign of an incoming earthquake. However, it remains unclear whether all such accelerating aseismic slip events are precursory to the incoming earthquake, and if not, how to distinguish between precursory and non‐precursory aseismic slip. This study employs the DEM to simulate earthquake nucleation along a 2‐D geometrically irregular fault plane. Our simplified DEM models can produce complex slip behavior. Our results manifest that the final seismic event is a produc