Linking Postseismic Deformation and Slab Rheology to Seismic Segmentation in Central Chile

Understanding the rheological structure of the lithosphere and the frictional behavior of the interface is essential to evaluate the mechanisms controlling surface deformation and seismic behavior along subduction margins. Postseismic deformation following large megathrust earthquakes provides a unique opportunity to constrain these properties, as it is strongly influenced by afterslip and viscoelastic relaxation processes.

In this study, we analyze the postseismic deformation associated with the 2015 Mw 8.3 Illapel earthquake in Central Chile by jointly exploiting GNSS and InSAR time series spanning up to eight years after the event. While GNSS data offer high temporal resolution, InSAR provides continuous spatial coverage, allowing us to characterize postseismic deformation at both local and regional scales. To separate the contributions of different deformation processes, we perform an Independent Component Analysis (ICA) on GNSS time series, and a pixel-by-pixel parametric decomposition on InSAR data.

Our results reveal two main postseismic deformation patterns. The first one is spatially correlated with the coseismic rupture area and displays a logarithmic temporal decay, consistent with afterslip-driven deformation. The second pattern is located north of the main rupture zone and is characterized by a nearly linear temporal evolution. This signal spatially coincides with a region of persistently low interseismic coupling, suggesting a distinct physical origin.

Based on these observations, we perform numerical modeling using the finite-element solver PyLith to investigate the potential sources of deformation. The models incorporate realistic fault geometry and rheological layering, and are driven by the imposed coseismic slip distribution. Our results indicate that the observed deformation patterns are best explained by the presence of a low-viscosity channel at a ~40 km depth, located at the base of the slab interface. This rheological anomaly spatially correlates with the subduction of the Challenger Fracture Zone (~30°S).

We propose that the subduction of this bathymetric anomaly enhances fluid release, which, through serpentinization processes, reduces the effective viscosity of the medium. These findings have important implications for seismic segmentation and earthquake behavior, as this region commonly acts as a boundary for the rupture of large megathrust earthquakes.