Seismic Velocity Variations as Indicators of Hydromechanical Processes in the Séchilienne Landslide (France)

The Séchilienne landslide in the French Alps is a large, deep-seated landslide whose dynamics are strongly influenced by local hydro-geological conditions. The highly fractured moving zone has a higher hydraulic conductivity than the underlying stable bedrock, creating a perched aquifer. Below, the bedrock hosts a deep saturated zone. Between 2011 and 2016, the landslide underwent an active phase marked by an initial increase and a subsequent decrease of displacement rates. Since then, the landslide has remained largely stable. The site has been instrumented with a seismic network since 2012 [1], providing a unique opportunity to link relative seismic velocity changes to hydro-geological observations and landslide deformation. We use ambient seismic noise interferometry to compute depth-dependent relative seismic velocity changes (dV/V), reflecting variations in the elastic properties of the landslide.

We observe clear seasonal cycles in seismic velocities within the shallow, fractured part of the landslide. Velocities decrease during periods of elevated groundwater levels and increase during dry conditions, indicating a reversible response to water-table fluctuations in the perched aquifer. Superimposed on this seasonal behavior, dV/V shows a long-term trend during the active phase of the landslide. At shallow depths, dV/V decreases during periods of increasing displacement rates and increases as displacement rates decrease. At greater depths in the deep aquifer, dV/V decreases during the deceleration phase. During the stable period, dV/V shows almost no long-term trend. This behavior indicates sensitivity of dV/V to landslide kinematics and a possible coupling between the shallow and deep aquifers.

The results show that seismic interferometry captures both short-term hydrologically driven variations and longer-term changes connected to landslide kinematics, showing distinct responses at different depths. The approach provides valuable insight into hydromechanical processes governing landslide evolution and highlights the potential of continuous seismic monitoring for slope stability assessment.

[1] Seismic data have been acquired by the French National Landslide Observatory (OMIV), and are available at doi.org/10.15778/RESIF.FR and doi.org/10.15778/RESIF.MT

We acknowledge help from the ISTerre-SIG team for operating the seismic network. We acknowledge the support of the European Research Council (ERC) under the grant agreement no. 101142154 (Crack The Rock).