Séminaire ISTerre

Passive seismology on slope instabilities : applications to rock glaciers and insights

mercredi 5 avril 2023 - 10h00
Antoine GUILLEMOT - ISTerre-Séminaire Post-doc (Café croissant à 9h45)
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Among mountain permafrost landforms, rock glaciers are composed of a heterogeneous mixture of rock debris, ice and liquid water. They can reach surface velocities of several m/yr for the most active ones, potentially causing emerging hazards linked to permafrost thawing and debris flows. As a complement to geophysical methods (georadar, active seismics, geoelectrics) providing interesting tools for investigating the subsurface, and to in-situ and remote sensing methods that track kinematics of these instabilities (1), passive seismic instrumentation offers a continuous monitoring at depth. Such instrumentation has been deployed for several years at Gugla, Tsarmine (Valais, Switzerland) and Laurichard (Hautes-Alpes, France) rock glaciers. From seismic ambient noise, Coda Wave Interferometry has been applied to compute daily dV/V (or relative change velocity of the surface waves) (2)(3) which are directly linked to the elastic properties of the medium at depth, and therefore its rigidity and density (4)(5). For the three sites studied, seasonal variations of shear stiffness have been measured, and located by using a 1D coda wave inversion. These changes in mechanical properties of the medium are related to seasonal hydro-thermal forcing. We developed a simple viscoelastic model to explain the seasonal variability of the deformation rate of rock glaciers. By using observed shear stiffness as a parameter varying over time, we reconstructed well the creep rates observed, strengthening the key role of meltwater and rainfall on rock glacier dynamics at a seasonal scale. In the long term, a pluriannual seismic monitoring allows to detect changes in ice content, by tracking long-term changes in rigidity within the rock glacier body. Such permanent instrumentation paves thus the way to quantify the permafrost degradation.

Equipe organisatrice : Organisation labo

Amphithéâtre Killian, Maison des Géosciences, 38400 Saint Martin d'Hères