What Can GNSS Tell About Physical Processes in Slowly Deforming France? Insights From a Community Benchmark Exercise

Abstract

Analysis of crustal deformation is key to understand current tectonics, deformation processes and seismicity. However, there is no consensus to date on how to integrate geodetic strain rates in seismic hazard models for continental intraplate regions. Most of Western Europe and mainland France are located within the Eurasia Plate, with very low deformation and seismicity rates, and GNSS velocities that comprise different sources, from tectonic contributions (e.g., fault motions) to non-tectonic long-term or transient processes (e.g., Glacial Isostatic Adjustment - GIA). Some of these processes also reflect stress changes at depth, loading of active faults, and seismicity. Understanding this deformation is therefore key to better assess seismic hazard in slow straining areas. In order (i) to assess the variability due to the diversity of strain-rate calculation methods and (ii) to test their capacity to resolve low-amplitude deformation, we conduct a benchmark exercise based on synthetic velocity fields comprising background noise (with the same characteristics as that observed in mainland France) plus velocity signals from various geodynamic processes. Comparing to the expected values the strain rates derived independently by eight different methods, four main conclusions can be drawn. (1) The capacity to deal with velocity noise and recover large-scale or local signals vary strongly between the methods, with some standing out from the others. (2) No method is able to retrieve the strain rate signals associated with active faults slipping at rates of 0.3-1 mm/yr (i.e. larger than expected for mainland France). (3) No method is able to retrieve the small deformation associated with a potential hot spot under the Massif Central (similar to, but about twice lower than observed for the Eifel hotspot). (4) Some methods are able to retrieve parts of the large-scale strain rate patterns (but not the amplitudes in general) associated with Alpine GIA or rotating rigid blocks. These results must be interpreted with caution, keeping in mind the strict benchmark design. They point out the potential for improvements in future analyses of geodetic deformation in mainland France and continental intraplate context in general.