Influence of uncertain bedrock seismic velocity structure on numerical simulation of earthquake ground motion (EGM): case study of the Le Teil earthquake (November 11, 2019, France)

On November 11, 2019 a very shallow magnitude Mw 4.9 earthquake shook Le Teil (Ardèche, France), some ten kilometres from two nuclear facilities. This earthquake, the strongest in metropolitan France in the last twenty years, occurred on a fault which was not identified as an active fault in the BDFA (Jomard et al., 2017) given its selection criteria, highlighting the importance of studying seismic hazard in low-seismicity areas. This earthquake occurred in a region with many assets, most of which are located in the Rhône valley and therefore subject to significant site effects due to a complex geology. The region has indeed been affected by a major erosion phase during the Messinian salinity crisis ca. 6 My ago, followed by the rapid flooding of the resulting canyon, thereby creating a basin filled with sediments of Pliocene to Quaternary ages embedded in a substratum composed of Secondary to Tertiary formations.

In this study, we present our exploratory work in view to build a seismological model of this region. This will be done through the comparison between observed ground motions and physics-based numerical simulations accounting for uncertainties related to geological layers on the wave path. We want to explore these uncertainties to faithfully replicate the data when modelling the propagation of seismic waves for frequencies between 0.5 and 5 Hz, for scenarios of the Le Teil earthquake and its aftershocks.

We focus particularly on the complexity of the pre-Messinian substratum and its impact on EGM outside the sedimentary basin. We propose a method for building geological interfaces bounding the velocity structure that can easily be parameterized, especially when multiple faults are present, facilitating the investigation of many potential structures.

We were able to produce a first velocity model of the region incorporating faults and a velocity structure with non-planar interfaces, and to confront our numerical simulation of the November 23, 2019 aftershock to field data recorded by several seismic stations. We also identified model parameters subject to uncertainties (shear-wave velocities, layer interfaces, source depth…) and the associated uncertain spaces (approx. 1500-3500 m.s^-1, up to 1.5 km variations, approx. 1-2 km depth). Following this work, a comprehensive sensitivity study including basin structure and source parameters will be conducted to understand which physical and structural parameters primarily control the prediction of EGM.