Fibre-Optic Monitoring of An Alpine Slope Instability Using Seismic Events: A Spatio-Temporal Analysis

Slope instabilities pose an increasing threat to populations and infrastructure across various regions worldwide. Therefore, a fundamental understanding of processes governing slope failure is critical for improving hazard mitigation. While remote-sensing and synthetic aperture radar methods effectively capture surface displacement, they provide limited information on subsurface dynamics. Seismic monitoring and imaging techniques can provide valuable complementary information on the internal structure, material properties, and time-dependent processes associated with unstable slopes.

We present a large-scale application of long-term Distributed Acoustic Sensing (DAS) measurements to investigate the spatial and temporal evolution of microseismicity at Cuolm da Vi (Central Switzerland), one of the largest active slope instabilities in the Alps. We deployed a 6.5 km long fibre-optic array to record continuous DAS data over a five-month period in spring 2023. Using a coherence-based detection method that exploits the dense spatial sampling of DAS, we identified 1,277 local seismic events. Event locations were obtained by adapting a matched field processing (MFP) approach to DAS observations, resulting in a comprehensive microseismic catalogue. The localisation workflow was validated through a controlled-source experiment and by comparison with a traveltime inversion of manually picked arrivals for selected events.

The resulting event distribution shows a pronounced spatial correspondence with known tectonic structures at Cuolm da Vi, particularly steeply dipping fracture systems, suggesting that much of the observed seismicity is linked to internal deformation processes related to a toppling movement. Clusters of elevated event density coincide with regions of reduced seismic velocities or strong velocity contrasts inferred from an independently derived three-dimensional velocity model. During the five-month observation time, the seismicity exhibits three distinct phases of elevated activity, with the first two closely following periods of intense precipitation and snowmelt. In addition, distinct spatial migration patterns of seismic activity emerge across different timescales.

The findings of our study demonstrate that DAS enables long-term monitoring of microseismic activity over spatially extensive and challenging Alpine terrain. The results provide new constraints on the internal structure and evolving dynamics of the Cuolm da Vi instability and additionally, highlight the potential of DAS-based seismic monitoring to improve hazard assessment and advance our understanding of deep-seated slope failure processes.