The June 2024 Mattertal slope destabilizations: zoom into the Gugla rock glacier

The last weeks of June 2024 were a very active period in the Alps with various floods, landslides, rockfalls and debris flows. In particular, the Mattertal valley (Switzerland) was hit by intense rainfall on June 20-22, following a very snowy winter and rainy spring. This led to various floods and debris flows, including the cutting off of the road and railway to the famous town of Zermatt. Also, some exceptional slope destabilization were also observed before the late June storm activity. Forecasting such natural hazards and anticipating the effects of rapid erosion processes is key for public managers, especially for energy and communications infrastructures and tourist resorts in mountainous valleys.

Using passive seismic sensors placed on the Gugla rock glacier (2700 m a.s.l) above Herbriggen village, Mattertal, we have detected landslides and quakes around the rock glacier almost continuously from 2016 to 2024 [1]. Using the same seismic instrument, we were also able to measure relative seismic velocity changes on a daily basis, which are indicative for the variations in stiffness at depth undergone by the rock glacier [1]. We observe seasonal variations of relative velocity changes and rockfall activity, mainly controlled by the freeze-thawing cycles. Melting seasons and wet summer episodes (storms) generally lead to seismic velocity drops of 2-3% in May-June. In June 2024, however, we observed a significant decrease in seismic velocity (-6.5%), which corresponds to a significant decrease in stiffness (ice melting) and a high liquid water content (snow melting infiltration), both lowering ground stability. This reduction in ground stability is likely to be responsible for the observed faster kinematics of the frontal part of the rock glacier, as well as rockfall and debris flow activity increase downstream.

Since this reduction in ground stability is likely to have occurred further in the Mattertal catchment at the same elevation and orientation, our work emphasizes that this reduction in seismic velocity at the catchment scale may be a good proxy for the higher sensitivity of the catchment to environmental triggers such as rainfall, eventually leading to a higher probability of slope destabilization.

[1] A. Guillemot, et al: Seismic monitoring in the Gugla rock glacier (Switzerland) : ambient noise correlation, microseismicity and modelling, Geophys. J. Int. 221, 1719-1735 (2020).

This work was partially funded by the Wallis canton, and by the European Research Council (ERC) under grant No. 101142154 - Crack The Rock project.