Rheological insights from Illapel postseismic deformation through GNSS and InSAR time series analysis

Understanding the inner structure of the crust and upper mantle is essential to evaluate those mechanisms driving Earth’s dynamics. Usually, surface deformation provides valuable constraints on viscoelastic parameters.  Postseismic deformation following large megathrust earthquakes, offers a unique opportunity to explore the viscoelastic properties of the shallower earth structure since it is strongly influenced by viscoelastic relaxation processes. This postseismic deformation is often recorded by GNSS stations, which offer high temporal resolution and therefore are useful to constrain the relaxation time along convergent margins. However, the spatial coverage of GNSS networks is often sparse,  inhibiting our ability to study the large scale variations in viscoelastic properties of the medium. 

To solve these issues, we rely on InSAR time series which provide continuous spatial resolution of surface deformation. In this work, we exploit the FLATSIM project (Thollard et al., 2021) initiative considering Sentinel-1 data  over Central Chile that has been processed using the NSBAS processing chain (Doin et al., 2013). Particularly, we focus on Central Chile, with special emphasis on the 2015 8.3 Mw Illapel earthquake. The InSAR data spans 8 years and has been corrected using the global atmospheric models ERA-5. Complementary, we use GNSS time series from 25 stations deployed over the Illapel rupture area, combining stations from Centro Sismologico Nacional and the DeepTrigger project.

Since both data sets contain the contribution from multiple tectonic and non-tectonic processes, we employ different techniques to isolate the postseismic deformation of the 2015 Illapel earthquake. Actually,  for GNSS, we apply Independent Component Analysis while for InSAR time series, we perform  a parametric decomposition pixel by pixel. Our findings reveal a very strong postseismic signal with a typical logarithmic decay, lasting at least 8 years.

In this work, in order to investigate the underlying rheological properties of the medium, we exploit the PyLith software,  a finite-element model that can take into account the complex rheological structure of the system. To do so, we impose the co-seismic slip model coming from averaged slip solutions, thereby initiating the model to distinguish between viscoelastic and afterslip contributions. By reproducing the surface deformation patterns given jointly by GNSS and InSAR data, we aim to determine the geometrical and rheological variations beneath the Illapel rupture area, particularly those viscoelastic parameters characterizing the crust and upper mantle regions. Our analysis provide insights to better understand how these properties affect both the seismic cycle and long-term deformation patterns at local and regional scales.