Afterslip of the Mw8.3 2015 Illapel earthquake imaged through a
time-dependent inversion of continuous and survey GNSS data
Abstract
We use continuous and survey GNSS data to image the spatial and temporal
evolution of afterslip during the two-months following the Mw8.3 Illapel
earthquake. Our approach solves for the incremental daily slip on the
subduction interface using non-negative least-squares and
spatial/temporal minimum Laplacian constraints. We find that afterslip
developed at three specific areas at the megathrust, surrounding the
coseismic rupture. The largest patch takes place at shallow depth north
of the coseismic rupture. Smaller patches occur at greater depth north
and south of the rupture, but no afterslip is found downdip. In
addition, well resolved afterslip also occurs within the coseismic area
that experienced 3-5 meters of seismic slip. Our afterslip model shows
striking correlations with the spatial distribution of aftershocks and
repeating earthquakes. A Mw6.8 aftershock occur on November 7 at the
deep patch of enhanced afterslip and our inversion captures the
triggered afterslip. Two Mw6.9 events occurred 100 km north of the
rupture 55 days after the mainshock. The enhanced shallow afterslip
developing northward possibly triggered these remote and delayed events.
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.
The similarity with the behavior observed for the Mw7.8 2016 Ecuador and
Mw7.6 2012 Costa Rica, suggests a common behaviour for heterogeneous
subduction interfaces.