GEOLOGICAL STRUCTURES OF THE VAJONT LANDSLIDE

The Vajont rockslide is not the largest rockslide in the Italian Alps, but it is a reference at a worldwide scale due to its complex behaviour and catastrophic effects in terms of economic losses and human casualties. An essential aspect to be considered in any kind of approach to the Vajont landslide, whether hydrogeological, geomorphological or related to geomechanical modelling, is the highly three-dimensional character of the geological structures of the Monte Toc northern slope. The knowledge of the geometry and shape of the sliding surface as well as of minor structures, such as folds and steps, represents the starting point for any subsequent analysis and interpretation of the landslide. In order to reach an in-depth evaluation of the structural setting, remote sensing analyses (LIDAR and photogrammetric DTMs) integrated by field data surveys were applied. Our results have clarified the structural relationships between major folds and faults affecting the Monte Toc slope and shown at various scales the primary importance of tectonic folds in controlling the kinematics of the 1963 event. Two pre-existing fold systems (ca. E-W and N-S trending), deforming the sliding surface, and associated minor structures (small scale faults and fractures) have most likely conditioned the sliding process. Particularly the concave shape of the sliding surface, reflecting a major syncline (Massalezza syncline), leads to the separation of the landslide into two distinct blocks with different kinematics.