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

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 been instrumented with seismic networks. Here, we analyze
the spectral content of ambient noise to study the modal sensitivity of rock
glaciers, which is directly linked to the system's elastic properties. For
both sites, we succeed in tracking and monitoring resonance frequencies of
specific vibrating modes of the rock glaciers over several years. These
frequencies show a seasonal pattern characterized by higher frequencies at
the end of winters and lower frequencies in warm 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 in elastic properties occurring in
winter due to freezing were taken into account thanks to a three-phase
Biot–Gassmann poroelastic model, where the rock glacier is considered a
mixture of a solid porous matrix and pores filled by water or ice. Assuming
rock glaciers to be 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 over seasons,
reinforcing the validity of our poroelastic approach. This seismic
monitoring allows then a better understanding of the location, intensity and
timing of freeze–thawing cycles affecting rock glaciers.