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 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.