Fracture and joint patterns in the source areas of rock slope instabilities are often analyzed with structural and kinematic analysis, which has traditionally been considered an important component of pre-disposition and displacement studies prior to failure of such mass movements. These analyses are based on the assumption that fractures are fully persistent and that available surface outcrops represent the statistical conditions of fractures at depth, especially at the elevation of basal rupture planes. However, we have observed several cases where kinematic analyses from surface outcrops are ambiguous, or even misleading regarding kinematic feasibility. An impressive example of such a case is the 2012 Preonzo rock slope failure, where pre-failure kinematic analyses could not explain the observed slope movements and where we have the possibility to compare in detail the pre-failure kinematic analyses with the actual properties of the failed rock mass in the head scarp area and the basal rupture plane. As Terzaghi (1962) and others suggested, for an explanation of the temporal behavior of natural rock slope failures, the interaction between existing natural discontinuities and brittle rock fracture propagation through intact rock bridges must be considered. Failure of a rock mass along complex failure surfaces additionally requires internal deformation of the unstable rock mass body; internal damage accumulates and shear planes develop through progressive failure (Eberhardt et al. 2004). At the Preonzo rock slope instability complex, such rupture planes from different failure events can be analyzed in detail which gives an insight into the structural evolution and progressive failure of brittle rock slope instabilities in time and space.
The Preonzo rock slope instability complex is located on the western slope of the Riviera valley in Ticino, southern Switzerland, above the village Preonzo. The head scarp of the active instability is situated at 1520 m a.s.l., at Alpe di Roscera. The instability complex is made up of crystalline units of the Simano nappe (Penninic domain) and is composed of amphibolitic gneisses and augengneisses that dip about 20–25° to the WSW into the slope, as already described by Willenberg et al. (2009).