Repeating Low Frequency Icequakes in the Mont-Blanc Massif Triggered by Snowfalls

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 Argentiere glacier (Helmstetter, Nicolas, et al., 2015, https://doi.org/10.1002/2014jf003288). 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 3,000 m. Some clusters are clearly associated with cold basal ice (near Mont-Blanc summit) while others below 2,700 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 (McCarthy et al., 2017, https://doi.org/10.1098/rsta.2015.0348; Saltiel et al., 2021, https://doi.org/10.1785/0220200480; Zoet et al., 2013, 2020, https://doi.org/10.1002/jgrf.20052, https://doi.org/10.1029/2020gl088964). Plain Language Summary Glaciers flow due to slip at the base of the glacier and due to internal deformation. When the ice is at the melting point temperature, the presence of water at the base of the glacier promotes basal slip, while for cold ice (below melting temperature) the glacier is stuck to its bed and most deformation occurs within the glacier. The displacement of the glacier is usually slow and continuous, but in some cases slip can occur as intermittent fast slip events. These "icequakes" generate ground vibrations that can be recorded by seismic sensors. In this study, I analyze clusters of icequakes that repeat more or less regularly with time every few minutes, with progressive changes in amplitudes and inter-event times. The signals have a narrow spectrum with a main frequency of about 5 Hz that suggests a rupture length of about 100 m. These events mainly occur during snowfall episodes and are mostly located on glaciers above 3,000 m. At these locations, the ice is often colder than the melting point temperature. These repeating icequakes are probably associated with unstable slip at the base of glaciers. This result is surprising since basal motion for cold ice (below melting point) is believed to be negligible, but it is however consistent with laboratory experiments suggesting that unstable slip is promoted by cold temperatures. A few events occur on or close to glaciers at lower elevations, where ice is at the melting point temperature. They may be associated with basal glacier motion or with landslides induced by glacier retreat.