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Repeating low frequency icequakes in the Mont-Blanc massif triggered by snowfalls
  • Agnes Helmstetter
Agnes Helmstetter
CNRS Université Grenoble Alpes, CNRS Université Grenoble Alpes

Corresponding Author:agnes.helmstetter@univ-grenoble-alpes.fr

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Deformation mechanisms of glaciers are highly sensitive to basal temperature; the motion of temperate glaciers is dominated by basal slip while cold-based glaciers deform mainly by internal creep. While basal slip is usually aseismic, unstable slip sometimes occurs and can be detected by seismometers. I have detected clusters of repeating low-frequency icequakes (LFIs) in the Mont-Blanc massif. Some properties of LFIs are similar to the high-frequency icequakes (HFIs) located at the base of Argenti\‘ere glacier (Helmstetter et al., 2015). Both HFIs and LFIs occur as bursts of tens to several thousand events lasting for days or weeks, with typical inter-event times of several minutes during bursts. Unlike HFIs that have a broad spectra, LFIs have a characteristic frequency of about 5 Hz at all stations, suggesting a rupture length of about 100 m. Seismic amplitudes and seismic waveforms of LFIs progressively evolve with time within each cluster, suggesting changes in either rupture length or rupture velocity. Most LFIs are detected during snowfall episodes while HFIs are not correlated with snowfall episodes. In this study, I used all available seismic stations within or around the Mont-Blanc massif between 2017 and 2022. I found LFIs located all over the massif but mainly above 3000 m. Some clusters are clearly associated with cold basal ice (near Mont-Blanc summit) while others below 2700 m a.s.l. are likely located under temperate glaciers and two clusters could be associated with landslides. This observation of LFIs on cold glaciers is consistent with laboratory friction experiments suggesting that cold ice promotes unstable slip.