Quantifying plate interface coupling in the Mexican subduction zone from InSAR and GNSS using Bayesian inversion methods

6 months, starting January or February 2024

Subduction zones host the world largest earthquakes, and understanding how stresses accumulate and release along these plate boundaries is essential for a better assessment of the seismic hazard.
In subduction zones, thanks to the monitoring the surface deformation of the upper plate, we can infer the processes taking plate along the subduction interface, during the different phases of the seismic cycle. Specifically, during the loading periods (i.e., interseismic), it is possible to characterize the interseismic coupling along the plate interface. Coupling equals 1 when the interface is fully locked—indicating that strain and stress are accumulating—and 0 when the interface is freely slipping.
Surface deformation also helps characterize the various slip modes that contribute to stress release. The slip between tectonic plates along the subduction interface consists of both seismic (i.e., earthquakes) and aseismic slip (i.e., slow slip events, post-seismic), and understanding the partitioning between these slip modes is crucial for assessing a fault’s seismic potential.
In this project, we focus on the Mexican subduction zone, where large damaging earthquakes frequently occur, as illustrated by the recent 19 September 2022 Mw=7.6 earthquake in the Michoacan region. This subduction zone also hosts large and frequent slow slip events (SSEs) that are predominantly aseismic (Radiguet et al., 2012). In this study, we will analyse datasets coming from two surface geodetic techniques to monitor the surface deformation : InSAR observation from the Sentinel-1 satellites and GPS. Previous studies done by our team have shown the interseismic signal can be extracted from InSAR observation (Maubant et al 2022), but have been limited to the 2016-2019 period, and were not completely homogeneous in term of processing. The large amount of data needed to cover the whole Mexican Subduction zone was one challenging issue. We now have a extended InSAR dataset, processed through the national FLATSIM facilities (The ForM@Ter LArge-Scale Multi-Temporal Sentinel-1 InterferoMetry Service) covering the 2016-2022 period. An initial post-processing of the InSAR and GNSS datasets have allowed to estimate the interseismic velocities over the Mexican subduction zone.

During this intership, the objective is to invert the interseismic velocities to infer the interseismic coupling along the Mexican subduction zone. Compared to previous work, the originality of our study is (1) a longer dataset (especially for InSAR, which extend over 6 years), (2) more realistic Green’s function (computation of the Forward problem, considering 3D variations in elastic properties and topography and (3) a Bayesian inversion scheme, that allows to estimate the uncertainties on the retrieved models. A first step in the project will consist in estimating the uncertainties in each datasets, and verify the compatibility of the two datasets. Then the intern will compute the forward model (3F Green’s function). Finally, the intern will perform the Bayesian inversion of the interseismic velocities, to assess the coupling ratio on the palte interface ; Different scenarios will be investigated, considering velocities that have been corrected of not from the transient signals (slow slip events or postseismic).
The resulting coupling and associated uncertainties will be discussed with respect to the past earthquake history of that region.

This internship will be part of the ANR project SSDYN “Identifying slow slip dynamics combining seismic and geodetic data”. This project funded by the French Research Agency (ANR) is lead by Mathilde Radiguet. It involves Mexican and French scientists from 5 different research institutions. The student will be hosted at ISTerre a multidisciplinary laboratory, whose research focuses primarily on the physical and chemical study of the planet Earth. This context provides an ideal setting for the students to engage in scientific discussions and to develop skills in various domains.

What can you gain from this internship ?

• Get into exciting scientific questions related to the earthquake cycle working on one of the best-documented subduction zone showing large slow slip events.
• Learn how to compute surface deformations from dislocations source at depths
• Learn Bayesian inversion techniques
• Learning to process large datasets on the intensive computing infrastructure provided by University (GRICAD)

Required Skills :

• Curiosity and interest for the earthquake cycle.
• Interest for programming and for working on large data sets using computing center.

Bibliography :
Maubant, L., Radiguet, M., Pathier, E., Doin, M.-P., Cotte, N., Kazachkina, E., & Kostoglodov, V. (2022). Interseismic coupling along the Mexican subduction zone seen by InSAR and GNSS. Earth and Planetary Science Letters, 586, 117534. https://doi.org/10.1016/j.epsl.2022.117534
Radiguet, M., Cotton, F., Vergnolle, M., Campillo, M., Walpersdorf, A., Cotte, N., & Kostoglodov, V. (2012). Slow slip events and strain accumulation in the Guerrero gap, Mexico. Journal of Geophysical Research : Solid Earth, 117(B4), https://doi.org/10.1029/2011JB008801
Ragon, T. and Simons M. (2021). Accounting for uncertain 3-D elastic structure in fault slip estimates. Geophys. J. Int. (2021) 224, 1404–1421. https://doi : 10.1093/gji/ggaa526