Topographic amplification and debris remobilization as a cause for increasing rockfall hazard in seismic areas: A case study in Central Italy

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.