Afterslip of the Mw 8.3 2015 Illapel Earthquake Imaged Through a Time‐Dependent Inversion of Continuous and Survey GNSS Data.

We use continuous and survey GNSS data to image the spatial and temporal evolution of afterslip during the 2 months following the Mw 8.3 2015 Illapel earthquake. Our approach solves for the incremental daily slip at the subduction interface using nonnegative least squares with spatial and temporal Laplacian regularization constraints. We find that afterslip developed at three specific areas at the megathrust, surrounding the coseismic rupture. In addition, well resolved afterslip also occurs within the coseismic rupture area that experienced ∼4 m of seismic slip. Our afterslip model shows striking correlations with the spatial distribution of aftershocks and repeating earthquakes. We capture the local afterslip triggered by a Mw 6.8 and two 6.9 aftershocks that ruptured downdip and north of the coseismic rupture, respectively. The latter ones were possibly triggered by the afterslip that developed north of the rupture. We also find a pulse of enhanced aseismic slip lasting a few days south of the rupture that spatially and temporally correlates with a seismicity burst. We finally find that areas of enhanced afterslip spatially correlates with areas having experienced regular seismic swarms observed during the years prior to the Illapel earthquake. This correlation supports the view of localized fluid high pore pressure areas behaving aseismically and surrounding a highly locked asperity, preventing the seismic rupture to propagate into them. Plain Language Summary: After a large earthquake, aseismic slip occurs in the vicinity of the seismic rupture. In this study, we focus on the aseismic slip that occurred after a large subduction earthquake of magnitude 8.3 in 2015 in central Chile. Using GPS time series, we obtain a spatiotemporal view of the aseismic slip during the 2 months following the earthquake. We find that aseismic slip occurred at three preferential areas located at the periphery of the rupture, as observed for many earthquakes. However, we also identify a patch of aseismic slip within the rupture area. We could image the daily afterslip rate and compare it to the daily seismicity and found striking correlations with the spatial distribution of aftershocks. In particular, we find that the largest aftershocks with magnitude 6.8–6.9 are tightly related to the evolution of afterslip. This result suggests that precisely monitoring the evolution of afterslip after a large earthquake might help to define preferential areas of large aftershocks. We also find a potential slow slip event south of the mainshock rupture which correlates with a local increase of seismicity. Finally, we find that the areas of enhanced afterslip had experienced seismic swarms decades before the earthquake, possibly reflecting areas of the fault with high fluid pore pressure. These results highlight new aspects about the dynamics of afterslip. Key Points: A full time‐dependent inversion provides a detailed view of afterslip during the 2.5 months following the Illapel earthquake in ChileAfterslip developed at three preferential areas, one of which overlaps a region of significant coseismic slip (2–4 m)Afterslip shows local and short‐term acceleration after large aftershocks and a slow slip event [ABSTRACT FROM AUTHOR]