Assessing ground-motion amplifications in a slow-moving landslide (Gourette, French Pyrenees) using seismological data and numerical simulations

Predicting the behaviour of existing landslides in mountain regions prone to earthquakes requires a good understanding of the dynamic response of the slopes. Indeed, previous studies, in particular using numerical simulations, have highlighted the role of ground-motion amplifications in the triggering or reactivation of landslides. Unlike seismically active mountain areas where landslides are known to be triggered by earthquakes of magnitude greater than 5, little is known about the potential of repetitive small-magnitude earthquakes to trigger instabilities. This project focuses on an existing slow-moving landslide overhanging the ski resort of Gourette in the French Pyrenees. This landslide, which is approximately 700 m long by 500 m wide at its toe, is active at least since 1990. It is composed of one deep global landslide and several more localized and superficial landslides. The driving force behind this landslide is primarily precipitation. However, if we consider that this region is regularly affected by repetitive small-magnitude earthquakes, the question then arises as to the response of this landslide to seismic shaking. To answer this question, we have started to deploy seismic stations on the slope to measure ground-motion amplifications in the landslide The idea is also to investigate whether the slope ground response varies in time as a consequence of local earthquakes or landslide internal deformations. In parallel, we will also characterize site effects in the landslide area using two methods: the Frequency-Scaled Curvature method and more complex 3D numerical simulations (FLAC3D software). The objective is to characterize site effects within the landslide mass and investigate if combined geological/topographic site effects may develop. In the next months (Interreg POCTEFA SPIRAL project co-financed by EU), additional sensors (GNSS, weather station) will be deployed on the landslide to enable a joint analysis of exogenous factors (precipitation, seismicity) and the landslide's response in terms of surface displacements.