Imaging one of the largest Alpine slope instabilities with 3D seismic first-arrival traveltime tomography

Understanding the internal structure and geometry of large-scale gravitational slope instabilities is crucial for hazard assessment and risk mitigation in mountainous regions. This study presents a high-resolution 2D and 3D seismic first-arrival traveltime tomography analysis of the Cuolm da Vi (CdV) slope instability, one of the largest active mass movements in the Alps. To achieve this, we conducted an extensive seismic survey, deploying over 1000 autonomous nodes across a 0.7 km 2 area and acquiring data from 144 controlled-source shots. Our resulting 2D and 3D tomographic models reveal significant subsurface heterogeneities, including extensive low-velocity zones up to depths of 200 meters, indicative of severe rock mass disintegration. Additionally, strong lateral velocity variations persist throughout the unstable zone, further corroborating its structural complexity. Our findings align with previous studies that suggest toppling as the dominant deformation mechanism. The comparison between 2D and 3D velocity models highlights the critical role of out-of-plane effects, such as observed lateral ray bending, emphasizing the importance of 3D imaging for accurate characterization of complex instability structures. The 2D and 3D velocity models provide important constraints for estimating the total unstable rock volume and serve as a foundation for future geotechnical analyses and hazard assessments. This study also demonstrates the feasibility and effectiveness of large-scale nodal seismic deployments in alpine terrains, paving the way for further applications in monitoring and characterizing deep-seated slope instabilities. • Large-scale 3D seismic tomography with > 1000 receivers at the Cuolm da Vi instability. • Subsurface imaging reveals extensive rock disintegration zones up to 150 - 200 m depth. • Significant lateral velocity variations highlight the internal structural complexity. • Comparison of 2D and 3D models emphasizes the significance of out-of-plane effects.