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The long-lasting earthquake sequence of Central Italy in 2016 triggered around 1370 landslides, most of which were observed close to ridge areas. The area of San Pellegrino di Norcia (PG) was badly affected by the 6.5 Mw Norcia earthquake that triggered 95 rock falls and slides with volumes ranging from 0.5 m3 to 380 m3 over an area of 0.6 km2. The presence of such a large number of coseismic landslides in a small area provided a suitable case study for investigating triggering mechanisms related to slope morphology and the subsequent propagation dynamics of rockfalls. To this aim, detailed field data collection and up-to-date terrain modelling based on Unmanned Aerial Vehicle (UAV) imagery were carried out. They permitted to perform morphometric analyses oriented at defining: (a) the frequency of occurrence of rockfall with reference to the energy relief, (b) the relationships between extensions of source and propagation areas and (c) the change in the shape of the propagation area as a function of the different terrains crossed by rockfalls. The results of the morphometric study were compared with dynamic analyses of rock-slides onset and simulations of rockfall propagation. They highlighted that, disregarding the presence of steep rock faces, the failures were located close to ridge and slope edges, thus pointing out the role of topography on triggering of landslides. Moreover, it was possible to observe that even small-sized rockfalls had a widening effect in the foothill characterized by longer runouts compared to rockfall simulations. This issue was due to the mobilization of the talus debris covering the limestone bedrock. This study investigates an area where a significant increase in rockfall hazard can be expected due to the coupled effect of topographic amplification and debris remobilization. • A slope highly affected by coseismic rockfalls is studied with integrated approaches. • Rockfalls were triggered close to ridges regardless of rock mass features. • Topographic amplification of the PGA is quantitatively evidenced. • Small rockfalls remobilized slope debris, causing large propagation areas. • Slopes in seismic areas covered by debris can suffer increased rockfall hazard. [ABSTRACT FROM AUTHOR]

Abstract: This study presents the detailed reconstruction of the entire structure of the prehistoric Vajont rockslide (about 270–300millionm3 of rocks and debris) for the first time, describing the complex geometry and the characteristic superimposition of distinct rigid blocks on a very thick shear zone. The prehistoric Vajont rockslide was characterized by an enormous ‘en masse’ motion of a rigid overlying rock mass (100–130m thick) that moved downslope, sliding onto a very thick shear zone (40–50m thick, on average) made up of a chaotic assemblage of blocks, limestone angular gravel, and high plasticity clays (montmorillonitic clays). Coarse loose sediments, still exposed on the 9 October 1963 detachment surface, are always associated with large blocks made of strongly fractured rock masses (Fonzaso Formation: middle‐upper Jurassic) preserving the stratification. The blocks of stratified and folded limestone sequences appear to be ‘sheared off’ from the underlying bedrock and can be considered as displaced rock masses planed off by the motion of the overlying rigid rock mass (‘rock mass shavings’). The prehistoric Vajont rockslide was characterized by a multistage failure with a marked retrogressive evolution. The first rupture (Pian del Toc block) rapidly destabilized the upper slope, mobilizing a second rock mass block (Pian della Pozza block) that, in turn, determined the multiple rupture of the revealed shear zone material (Massalezza lobe). Even if the exact timing of the different phases is not known, the entire multistaged failure process was very rapid. At the end of the multistage retrogressive failure, the slope morphology of the northern toe of Mt. Toc was drastically changed and the large failed rock mass settled into the preexisting Vajont Valley assuming the unusual chair-like geometry. The Vajont rockslide represents a very significant example on how a complex geological situation, if not adequately analyzed and reconstructed, can lead to dangerous misinterpretations or even to erroneous engineering–geological and geotechnical models. Accurate fieldwork and modern technologies can be fundamental in solving such a very intriguing ‘geological puzzle.’ [Copyright &y& Elsevier]