Propagation of Slow Slip Events on Rough Faults: Clustering, Back
Propagation, and Re-rupturing
Abstract
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.