3D GEOPHYSICAL IMAGING OF THE VAJONT LANDSLIDE AND OF ITS SURROUNDINGS

The 1963 collapse of the northern slope of the Monte Toc in the Vajont reservoir is probably one of the most studied landslides worldwide. During the various studies several numerical models have been proposed to explain the collapse dynamics but a comprehensive and reliable insight into the failure kinematics is still missing. A major step forward in the understanding of the landslide mechanisms is represented by the reconstruction of the geometry of the different geological units within the landslide body and of the associated physical properties. A large scale geophysical experiment based on 2D and 3D seismic and resistivity was undertaken to address the issue. Prior to the main survey electrical resistivity and longitudinal and transversal wave velocity of the lithological units involved in the landslide were measured along the exposure of the rock wall below the village of Casso. The geophysical images of the two landslide lobes showed a very good correlation with the reference section. Particularly a conductive unit located in the bottom part of the stratigraphic sequence resulted an excellent geophysical marker. Some important structural records, recently defined on the sliding surface, were imaged in the geophysical profiles of the western lobe indicating a limited internal deformation along the east-west axis during the slide. In the deeper part of the western lobe the geophysical image appears rather complicated because of the partial overlap of the various lithological units occurred during the itslide. In the eastern lobe the geophysical images clearly outlined several detachment planes that disrupt the continuity of the pre-landslide geology. The seismic and the resistivity tomography obtained inverting the data resulted comparable indicating a proper choice of the measuring techniques and of the field parameters