Integrated multi-temporal analysis of the displacement behaviour and morphology of a deep-seated compound landslide (Cerentino, Switzerland)

Abstract

Deep-seated gravitational slope deformations (DSGSDs) are complex slope instabilities commonly involving long-term evolution with significant spatial and temporal variations in slope kinematics. The Cerentino compound landslide in southern Switzerland exemplifies the long-term evolution of DSGSDs as well as short-term movements causing substantial damage to infrastructure. Using an integrated analysis of surface geomorphology, displacement monitoring data, borehole core and cosmogenic nuclide dating, we discuss the preconditioning and driving factors, kinematic behaviour, and long-term evolution of the instability. We demonstrate that foliation orientation and lithological boundaries, together with slope geometry and alpine faults, likely played significant roles in preconditioning the slope for failure. Based on morphology mapping and hierarchical clustering analysis of 2007–2018 detrended surface displacement data, we show that the landslide is separated into up to eight compartments at surface, revealing differences in kinematic behaviour from very active at the toe to insignificant movements in the upper region of the landslide above the village of Cerentino. These surficial compartments likely correspond to secondary sliding zones within the landslide mass. The analysis of data from inclinometer and fibre optic strain sensors installed in a new deep borehole (230 m) provided insight into movement within the landslide mass. Results show vertical heterogeneity in total displacement, which appears to be controlled by an active discrete shear zone located at 107 m, by shallower secondary sliding surfaces, and by distributed movement, particularly in the upper 20 m of the instability. Extrapolation of cosmogenic nuclide dating results from five samples taken on the exposed upper sliding scar reveals a possible age of failure initiation of 5–6 ka, with long-term decelerating movement trends. These generally slow averaged movements show multi-annual responses to long-term changes in precipitation and locally rapid responses to strong recharge events. The rare combination of dating and detailed analysis of displacement allows us to demonstrate the behaviour of DSGSDs at long and short timescales, with implications for hazard assessment and mitigation of these complex failures.