Repeating low frequency icequakes in the Mont-Blanc massif triggered by
snowfalls
Abstract
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 Argentière glacier. Both HFIs and LFIs occur as bursts of tens
to several thousand events, with typical inter-event times of several
minutes and last for days or weeks. 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 snowfalls, while HFIs are not
correlated with snowfalls.
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 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.