Intermediate-depth icequakes and harmonic tremor in an Alpine glacier (glacier d 'Argentiere,France): evidence for hydraulic fracturing

Abstract : We detected several thousand deep englacial icequakes on Glacier d’Argentiere (Mont-Blanc Massif) between 30 March and 3 May 2012. These events have been classified in eight clusters. Inside each cluster, the waveforms are similar for P-waves and S-waves, although the time delay between the P-waves and S-waves vary by up to 0.03 s, indicating an extended source area. Although these events were recorded by a single accelerometer, they were roughly located using a polarization analysis. The deepest events were located at a depth of 130 m, 60 m above the ice/bed interface. The clusters are separated in space. The largest cluster extends over about 100 m. For this cluster, the strike of the rupture plane is nearly parallel to the direction of the open crevasses, and the dip angle is 56◦. Deep icequakes occur in bursts of activity that last for a few hours and are separated by quiet periods. Many events occurred on 28-29 April 2012, during the warmest days, when snow melting was likely important. The distribution of events in time and space obeys a power law, as also observed for earthquakes, but with larger exponents.The polarity of the P-waves for all of the events is consistent with tensile faulting. Finally, between 25 April and 3 May, we observed a gliding harmonic tremor with a fun- damental resonance frequency that varied between 30 Hz and 38 Hz, with additional higher frequency harmonics. During this time we also observed shallow hybrid events with high-frequency onsets and a monochromatic coda. These events might be produced by the propagation of fractures and the subsequent flow of water into the fracture. The strongest resonance was observed just after a strong burst of deep icequakes, and during an un- usually warm period when the snow height decreased by 60 cm in one week. The res- onance frequency shows a succession of several sharp decreases and phases of progressive increases. One of the strongest negative steps of the resonance frequency on 28 April coincides with a burst of deep icequakes. These events appear to be associated with the propagation of fractures, which can explain the decrease in the resonance frequency. Finally, we observed an acceleration of glacier flow on 29 April and suggested that melt-water had reached the ice/bed interface. These observations suggest that deep icequakes are due to hydraulic fracturing, and that they can be used to track fluid flow inside glaciers.
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Journal of Geophysical Research : Earth Surface, American Geophysical Union/Wiley, 2015, 120 (3), pp.402-416. 〈10.1002/2014JF003289〉
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Agnes Helmstetter, Luc Moreau, Barbara Nicolas, Pierre Comon, Michel Gay. Intermediate-depth icequakes and harmonic tremor in an Alpine glacier (glacier d 'Argentiere,France): evidence for hydraulic fracturing. Journal of Geophysical Research : Earth Surface, American Geophysical Union/Wiley, 2015, 120 (3), pp.402-416. 〈10.1002/2014JF003289〉. 〈hal-01118889〉

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