Abstract
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
(DEM) 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.