Seismic monitoring of debris flows using Distributed Acoustic Sensing

Debris flows are among the most destructive geohazards in alpine regions. Within minutes, hundreds of thousands of cubic meters of water, sediments, and rocks may discharge in an uncontrolled way at velocities exceeding 5 m/s. Seismic monitoring offers perspectives for detection and warning, and thus for protecting human lives and infrastructure. Distributed Acoustic Sensing (DAS) is a new alternative to conventional seismic sensors and can be applied to pre-existing telecommunication fibers repurposed as seismic sensors. With the high sensitivity to ground displacement and the distributed nature of DAS measurements, this approach allows detection and location of debris flows kilometers upstream of affected regions, thus maximizing warning times.

Between September and August 2022, we interrogated a 450-meter-long telecommunication fiber in the municipality of Susten, located on Illgraben’s debris cone in Switzerland’s Rhône valley. Illgraben is among Europe’s most active debris flow catchments, producing 2-10 debris flows per year (Badoux et al., 2009). One event was recorded on 8 September 2022, with first signals registered by the DAS system 20 minutes before the debris flow reached the village of Susten. At that time, the debris flow was still located 4 km upstream in the Illgraben catchment, demonstrating the early warning capabilities of DAS.

In a second DAS investigation between 2024 and 2026, a 2-kilometer-long fiberoptic cable was trenched along the Illgraben channel, only tens of meters away from the torrent bed. Such near-torrent observations illuminate the interaction of the debris flow material with the torrent bed and enable us to better understand the seismogenesis of debris flows. The strongest signals are observed at the boulder-rich debris flow front. Using DAS, such moving sources can be tracked along the torrent, and their velocity can be estimated. During later flow stages, the bulk composition changes, and only fine-grained sediments are transported. During these flow stages, large boulders generate the strongest seismic signals. Their ground impacts can be located with the DAS system, elucidating boulder transport within debris flows and their contribution to the hazard potential.

The 2-kilometer-long fiber also resolved surge fronts and roll waves within several debris flows. Such unsteady flow features increase peak discharge and dynamic complexity, which contributes much to the hazard potential (Aaron et al., 2025). Our along-torrent DAS measurements capture the evolution of debris flow surges and roll waves. This provides unprecedented insights into their formation and propagation, which is essential to more accurate predictions of the destructive potential of surging debris flows.

Badoux, A., et al. A debris-flow alarm system for the Alpine Illgraben catchment: design and performance. Nat Hazards 49, (2009). https://doi.org/10.1007/s11069-008-9303-x

Aaron, J., et al. Detailed observations reveal the genesis and dynamics of destructive debris-flow surges. Commun Earth Environ 6, (2025). https://doi.org/10.1038/s43247-025-02488-7