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The Cryosphere An interactive open-access journal of the European Geosciences Union
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Preprints
https://doi.org/10.5194/tc-2020-195
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/tc-2020-195
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  31 Jul 2020

31 Jul 2020

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This preprint is currently under review for the journal TC.

Modal sensitivity of rock glaciers to elastic changes from spectral seismic noise monitoring and modeling

Antoine Guillemot1, Laurent Baillet1, Stéphane Garambois1, Xavier Bodin2, Agnès Helmstetter1, Raphaël Mayoraz3, and Eric Larose1 Antoine Guillemot et al.
  • 1Univ. Grenoble Alpes, CNRS, Univ. Savoie Mont-Blanc, IRD, IFSTTAR, ISTerre, 38000 Grenoble, France
  • 2Univ. Grenoble Alpes, CNRS, Univ. Savoie Mont-Blanc, Laboratoire Environnements, Dynamiques et Territoire de Montagne (EDYTEM, UMR 5204), 73000 Chambéry, France
  • 3Canton of Wallis, 1951 Sion, Switzerland

Abstract. Among mountainous permafrost landforms, rock glaciers are mostly abundant in periglacial areas, as tongue-shaped heterogeneous bodies. Passive seismic monitoring systems have the potential to provide continuous recordings sensitive to hydro-mechanical parameters of the subsurface. Two active rock glaciers located in the Alps (Gugla, Switzerland and Laurichard, France) have then been instrumented with seismic networks. Here, we analyse the spectral content of ambient noise, in order to study the modal sensitivity of rock glaciers, which is directly linked to elastic properties of the system. For both sites, we succeed in tracking and monitoring resonance frequencies of specific vibrating modes of the rock glaciers during several years. These frequencies show a seasonal pattern characterized by higher frequencies at the end of winters, and lower frequencies in hot periods. We interpret these variations as the effect of the seasonal freeze-thawing cycle on elastic properties of the medium. To assess this assumption, we model both rock glaciers in summer, using seismic velocities constrained by active seismic acquisitions, while bedrock depth is constrained by Ground Penetrating Radar surveys. The variations of elastic properties occurring in winter due to freezing were taken into account thanks to a three-phases Biot-Gassmann poroelastic model, where the rock glacier is considered as a mixture of a solid porous matrix and pores filled by water or ice. Assuming rock glaciers as vibrating structures, we numerically compute the modal response of such mechanical models by a finite-element method. The resulting modeled resonance frequencies fit well the measured ones along seasons, reinforcing the validity of our poroelastic approach. This seismic monitoring allows then a better understanding of location, intensity and timing of freeze-thawing cycles affecting rock glaciers.

Antoine Guillemot et al.

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Antoine Guillemot et al.

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Latest update: 21 Oct 2020
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Short summary
Among mountainous permafrost landforms, rock glaciers are composed of boulders, fine frozen materials, water and ice in various proportions. Displacement rate of active rock glaciers can reach several m/yr, addressing emerging risks linked to gravitational hazards. Thanks to passive seismic monitoring, resonance effects related to seasonal freeze-thawing processes of the shallower layers have been monitored and modeled. This method is then an accurate tool for studying rock glacier at depth.
Among mountainous permafrost landforms, rock glaciers are composed of boulders, fine frozen...
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