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

<p><em>Glaciers or ice-streams have many common points with tectonic faults. Glaciers can move by</em></p> <p><em>stable or unstable slip or by creep within the glacier thickness. Like faults, glacier sliding can</em></p> <p><em>produce &#8220;icequake&#8221; signals over a huge range of frequencies, rupture length and signal</em></p> <p><em>duration, as well as tremor. But because glaciers are shallower, the sliding interface can be</em></p> <p><em>accessed directly much more easily, by boreholes or cavities. And they move much faster than</em></p> <p><em>tectonic faults, so that deformation is easier to estimate and icequake inter-event times are</em></p> <p><em>much shorter than for earthquakes.</em></p> <p><em>Here I present some observations of high- and low-frequencies repeaters of basal icequakes</em></p> <p><em>in the Mont-Blanc areas. Both types of events occur as bursts lasting for a few days or weeks,</em></p> <p><em>with quasi-regularly inter-events times of the order of a few minutes or hours, and progressive</em></p> <p><em>changes in amplitude and inter-event times. High-frequency events (around 50 Hz) occur all</em></p> <p><em>over the year, with no clear triggering mechanism, and are located on the lower-part of</em></p> <p><em>glaciers, where ice is at the melting point temperature and the glacier mainly moves by stable</em></p> <p><em>sliding. Low frequency events (around 5 Hz) are mainly located at higher elevations (mainly</em></p> <p><em>above 3000 m asl), on steeper slopes, and have larger magnitudes (-2<m<0). They are mainly</em></p> <p><em>observed during or shortly after snowfalls. At these elevations, glaciers are possibly coldbased,</em></p> <p><em>or close to the melting-point temperature, so that they are stuck to their bed and</em></p> <p><em>mainly deform by creep within the ice. We observe progressive changes in waveforms that</em></p> <p><em>suggest slow and evolving rupture velocities. These low-frequency icequakes may be the</em></p> <p><em>analog of low-frequency earthquakes, which also occur near the transition between stable and</em></p> <p><em>unstable slip.</em></p>