The Effect of Pre-Existing Structures on the Moosfluh Landslide and its Lateral Propagation (Great Aletsch Glacier, Switzerland)

Truttmann, Sandro; Herwegh, Marco; Schreurs, Guido; Ebert, Andreas; Hardmeier, Sibylla (2021). The Effect of Pre-Existing Structures on the Moosfluh Landslide and its Lateral Propagation (Great Aletsch Glacier, Switzerland). Geomorphology, 377, p. 107530. Elsevier Science 10.1016/j.geomorph.2020.107530

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The retreat of the Great Aletsch Glacier is accompanied by a series of slope failures in solid bedrock, which are heavily influenced by the presence of pre-existing deformation structures. Since the 1880's, the Great Aletsch Glacier has shortened by more than 3 km and decreased about 400 m in thickness. As a reaction to the loss of the stabilizing effect of the ice, one of the largest active deep-seated landslides in the European Alps with an affected surface area of about 1.5 km2, called Moosfluh landslide, is evolving. In this study, a multimethod approach combining field work, remote sensing techniques and microseismic monitoring is used to assess the effect of pre-existing structures on the landslide deformation processes.

The landslide evolution from 2008 to 2018 could be reconstructed with high spatial resolution. Surface deformation analysis reveals the concentration of high deformation in narrow zones, allowing to directly link pre-existing tectonic and exhumation structures with landslide deformation processes. Toppling as the main gravity-driven process is enabled by reactivation of NE-SW striking, steeply SE dipping Alpine Handegg phase shear zones. Differences in the lateral detachment processes are attributed to shear zone bridges in the NE as well as fractures and shear zones similarly oriented to Alpine Oberaarb phase shear zones in the SW. At the landslide toe, a transition from toppling to sliding mechanism due to the formation of a continuous basal detachment surface can be observed, which is favored by the presence of exfoliation joints. The dramatic acceleration of the Moosfluh landslide in autumn 2016 is directly related to an increase in glacier height loss rate, which implies that glacier retreat is the main trigger of the landslide. A temporal stabilization of the landslide is recorded after 2017, most probably caused by the self-stabilizing properties of flexural toppling. However, microseismic data records a lateral propagation of the landslide, following the retreating Great Aletsch Glacier.

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