Modelling the post-seismic deformations measured by GNSS and InSAR, following the 2014 Iquique earthquake, Chile

Large earthquakes are followed by a post-seismic period during which the stresses induced by the co-seismic phase are relaxed through different processes. This post-seismic phase participates to the redistributions of stresses in the earth, and understanding its mechanism is a key to understand interactions between earthquakes at different spatial and temporal scales. In subduction zones the most important terms are the afterslip and the visco-elastic relaxation. It is generally considered that the two mechanisms affect different spatial and temporal scales: the afterslip is prevalent the first months in the surrounding of the fault, while the visco-elastic relaxation process affects a larger area and lasts a longer time. The time-space pattern of the measured deformation can help to characterize the rheology of the underlying structure.

In this work we look at the processes involved after the Iquique earthquake.

 

To explore the processes driving the post-seismic deformation, we use a finite element model (FEM) (2D model, using the FEM software Pylith) that is constrained with InSAR and GNSS data. The GPS time series (processed with GipsyX) include 83 stations located in North Chile, Peru and Bolivia. The post-seismic signal is isolated using a trajectory model. The InSAR data consist in two Sentinel-1 time series (ascending and descending tracks) processed with the NSBAS chain, they include 514 interferograms, starting 7 months after the earthquake up to the end of 2019. In the model we impose a co-seismic displacement on the plate interface and explore the influence of the structure and the rheology on the predicted surface displacement.

 

Our tests reveal that the viscosity in the continental and the oceanic mantle both have an impact on the displacement produced at the surface. The difference between these viscosities controls the movements allowed at depth. The crust thickness and the presence of a cold nose have a clear impact on the wavelength and the location of the maximum of amplitude, respectively.

The afterslip is the major contribution at short time. At longer time, it affects weakly the near trench displacements. To fit the long-term data, we show that visco-elastic relaxation is needed. After 7 months, the InSAR data show a clear spatial wavelength with a strong signal 150 to 300 km from the trench which can be explained by the visco-elastic process.

We pointing out that these is a trade-off between the contribution of afterslip and visco-elastic relaxation. However, both processes affect different space and time, and the comparison with GNSS data and two InSAR tracks allows to strongly constrain the model and reduce the range of plausible models.