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 M-w 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 similar to 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 M-w 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.

Automated summary

We used continuous and survey GNSS data to study the spatial and temporal evolution of afterslip following the 2015 Illapel earthquake. Our approach involved solving for incremental daily slip at the subduction interface using nonnegative least squares with spatial and temporal Laplacian regularization constraints. We found that afterslip occurred in specific areas surrounding the coseismic rupture, as well as within the rupture area itself. Our afterslip model showed strong correlations with aftershocks and repeating earthquakes, and we also observed localized afterslip triggered by specific aftershocks. We furthermore discovered enhanced aseismic slip correlated with a seismicity burst, and areas of enhanced afterslip correlated with previous seismic swarms.