Microseismicity appears to outline highly coupled regions on the Central Chile megathrust

The seismogenic zone of subduction zone megathrusts is commonly thought to be made up of frictionally strong patches (``asperities'') that rupture in large earthquakes surrounded by weaker regions where a part of the deformation occurs aseismically. Knowledge about the size and location of such asperities can be valuable for hazard estimation purposes as well as for better understanding active processes that occur along the plate interface. We analyzed 4.5 years of seismicity (from mid-2014 to 2018) on the megathrust of Central Chile, obtaining a catalog of 8750 events located with state-of-the art double-difference techniques. Earthquake locations outline three half-ellipse shapes that are open towards the trench, with the northernmost one coinciding with the rupture area of the 2015 Mw 8.3 Illapel earthquake. These elliptical shapes may delineate asperities that concentrate strain build-up in this mature part of the plate interface. To check whether these shapes indeed outline highly coupled asperities, we combined the seismicity geometries with GPS-based inversions for interplate locking and 3D mechanical models. Prescribing high locking degree to nodes inside the seismicity features, we ran a series of constrained inversions, which achieved data fits comparable to the unconstrained inversion. When trading off data fit against the number of free parameters with the help of the Bayesian Information Criterion, the constrained inversions are even preferred. Locking inversions that make use of seismicity information improve the stability of achieved results and allow to identify locked zones that are not detected by inversions of GPS data alone due to lack of resolution. By using a mechanical frictional model, we simulate the evolution of the state of stress and estimate mechanical coupling on the plate interface throughout a seismic cycle. These mechanical models predict stress concentrations at the downdip edges of highly coupled asperities after prolonged interseismic loading, whose shapes qualitatively correspond to the observed seismicity geometries. The observed narrow trench-perpendicular bands of seismicity that separate aseismic regions in along-strike direction are found to correspond to regions where the subduction of seafloor features may promote a predominance of creep processes. Our results shed light on the relationship between observed seismicity patterns and the mechanical behavior of asperities. The direct observation of asperities' seismicity signature can independently constrain and thus improve geodetic locking inversions.