Sackung is a widespread post-glacial morphological feature affecting
Alpine mountains and creating characteristic geomorphological expression
that can be detected from topography. Over long time evolution, internal
deformation can lead to the formation of rapidly moving phenomena
such as a rock-slide or rock avalanche. In this study, a detailed
description of the Sierre rock-avalanche (SW Switzerland) is presented.
This convex-shaped postglacial instability is one of the larger rock-avalanche
in the Alps, involving more than 1.5 billion m3 with a run-out distance
of about 14 km and extremely low Fahrböschung angle. This study presents
comprehensive analyses of the structural and geological characteristics
leading to the development of the Sierre rock-avalanche. In particular,
by combining field observations, digital elevation model analyses
and numerical modelling, the strong influence of both ductile and
brittle tectonic structures on the failure mechanism and on the failure
surface geometry is highlighted. The detection of pre-failure deformation
indicates that the development of the rock avalanche corresponds
to the last evolutionary stage of a pre-existing deep seated gravitational
slope instability. These analyses accompanied by the dating and the
characterization of rock avalanche deposits, allow the proposal of
a destabilization model that clarifies the different phases leading
to the development of the Sierre rock avalanche.