Mapping the rheology of the Central Chile subduction zone with aftershocks

The postseismic deformation following a large (M-w>7) earthquake is expressed both seismically and aseismically. Recent studies have appealed to a model that suggests that the aseismic slip on the plate interface following the mainshock can be the driving factor in aftershock sequences, reproducing both the geodetic (afterslip) and seismic (aftershocks) observables of postseismic deformation. Exploiting this model, we demonstrate how a dense catalog of aftershocks following the 2015 M(w)8.3 Illapel earthquake in Central Chile can constrain the frictional and rheological properties of the creeping regions of the subduction interface. We first expand the aftershock catalog via a 19month continuous matched-filter search and highlight the log-time expansion of seismicity following the mainshock, suggestive of afterslip as the main driver of aftershock activity. We then show how the time history of aftershocks can constrain the temporal evolution of afterslip. Finally, we use our dense aftershock catalog to estimate the rate and state rheological parameter (a - b)sigma as a function of depth and demonstrate that this low value is compatible either with a nearly velocity-neutral friction (a approximate to b) in the regions of the megathrust that host afterslip, or an elevated pore fluid pressure (low effective normal stress sigma) along the plate interface. Our results present the first snapshot of rheology in depth together with the evolution of the tectonic stressing rate along a plate boundary. The framework described here can be generalized to any tectonic context and provides a novel way to constrain the frictional properties and loading conditions of active faults. Plain Language Summary The slow postseismic deformation, or afterslip, that lasts several years following a major earthquake can be as strong as the earthquake itself and is therefore a key component in understanding the seismic hazard along tectonic plate boundaries. Afterslip is typically studied with GPS that measures the deformation at the surface, but with a low spatial precision. It is therefore difficult, if not impossible, to tease out the finer details of how the plate interface responds to a large earthquake and to identify which parts of the plate interface could rupture next. Here we develop a new framework to study what happens after a major earthquake using the precise evolution in time and space of aftershocks. We leverage the high-resolution aftershock distribution to determine how the friction of the plate boundary varies in depth, which allows us to better understand which parts of the plate interface are susceptible to afterslip. Our methods described here can be generalized to any tectonic plate boundary and provide a novel way to constrain how active faults are influenced by major earthquakes.