The 3-D anatomy of the Cuolm da Vi slope instability

Cuolm da Vi (CdV) is a deep-seated gravitational slope deformation in central Switzerland with an estimated unstable volume of around 150 million m³. In the central part, surface displacement rates are on the order of 10 to 20 cm/yr. The ongoing south-westward deformation, which is dominated by toppling, is expressed by scarps, graben-like structures, tension cracks, and local instabilities. These landforms suggest gravitational movement guided by inherited tectonic structures. Despite detailed geomorphological mapping, geological-geotechnical investigations, and more than two decades of surface-displacement monitoring, fundamental uncertainties remain regarding, for example, the maximum depth of the unstable mass and the internal deformation processes.

Here, we integrate multiple geophysical and geological constraints into a 3-D structural model of the instability. To establish the model, we combined a 3-D P-wave velocity volume from first-arrival travel-time tomography, microseismicity detected during five months of continuous distributed acoustic sensing (DAS) monitoring, and distributed strain sensing (DSS) observations from around two years of periodic measurements, together with detailed mapping of tectonic features and available geotechnical information. We feed the geophysical and geological data into a 3-D structural and probabilistic geological modelling framework to establish a complex model of the structural features of CdV. The model covers about 1 km² at the surface and extends to a few hundred meters depth.

Low P-wave velocities (Vp < 2000 m/s) spatially coincide with mapped unstable terrain, indicating that velocity variations can help delineating comparatively intact versus more fractured/damaged rock volumes. Based on the geometry of the low-velocity domain, the maximum depth of the unstable mass in the central part is estimated at about 180-200 m. Microseismicity is concentrated within low-velocity regions and clusters near mapped tectonic features, consistent with deformation localized on key planar discontinuities. Key tectonic features are also associated with distinct DSS strain events. The resulting 3-D “static” model provides a quantitative framework for future analyses of temporal changes in microseismicity, with direct relevance for process understanding and the continued development of early-warning strategies at CdV.