Clusters of low- and high-frequency repeating icequakes in the Mont-Blanc massif

Glaciers or ice-streams have many common points with tectonic faults. Glaciers can move by

stable or unstable slip or by creep within the glacier thickness. Like faults, glacier sliding can

produce “icequake” signals over a huge range of frequencies, rupture length and signal

duration, as well as tremor. But because glaciers are shallower, the sliding interface can be

accessed directly much more easily, by boreholes or cavities. And they move much faster than

tectonic faults, so that deformation is easier to estimate and icequake inter-event times are

much shorter than for earthquakes.

Here I present some observations of high- and low-frequencies repeaters of basal icequakes

in the Mont-Blanc areas. Both types of events occur as bursts lasting for a few days or weeks,

with quasi-regularly inter-events times of the order of a few minutes or hours, and progressive

changes in amplitude and inter-event times. High-frequency events (around 50 Hz) occur all

over the year, with no clear triggering mechanism, and are located on the lower-part of

glaciers, where ice is at the melting point temperature and the glacier mainly moves by stable

sliding. Low frequency events (around 5 Hz) are mainly located at higher elevations (mainly

above 3000 m asl), on steeper slopes, and have larger magnitudes (-2<m<0). They are mainly

observed during or shortly after snowfalls. At these elevations, glaciers are possibly coldbased,

or close to the melting-point temperature, so that they are stuck to their bed and

mainly deform by creep within the ice. We observe progressive changes in waveforms that

suggest slow and evolving rupture velocities. These low-frequency icequakes may be the

analog of low-frequency earthquakes, which also occur near the transition between stable and

unstable slip.