Dynamic characterization of the hill of Civita di Bagnoregio (Viterbo, Central Italy) for seismic response analysis

Located on the top of a volcanic butte, Civita di Bagnoregio is one of the most beautiful villages in Central Italy. This hamlet, which dates back to the Etruscan era, is a medieval cluster of enchanting beauty and a massive tourist attraction that counted more than six hundred thousand visitors in 2016. The peculiar combination of geological conditions, regional seismicity, and the impressive touristic flow caused the city to be listed among the 100 World's most endangered sites. The definition of a reliable geophysical and geotechnical model of the subsurface is essential for any quantitative evaluation of seismic hazard, which is the key for knowledge-based decisions to design the proper stabilization measures in order to preserve the historical centre. Several geophysical techniques (ambient noise recordings, surface wave methods, electrical resistivity tomography and seismic downhole) were performed to assess the geometry and the low-strain stiffness properties of the different geological units and to provide continuous subsurface imaging throughout the urbanized area. The surface geophysical investigations were complemented by borehole seismics and geotechnical laboratory testing to provide a detailed dynamic/cyclic characterization of the lithotypes, which was used for investigating seismic site effects. The integration of different investigation techniques allowed circumvention of logistic limitations imposed by the historical center, which are a potential source of over-simplified earthquake engineering models with no or limited experimentally-determined seismic properties, thus resulting in unrealistic seismic response estimates. The investigation approach used in this case study can be extended to historical centers with similar geotechnical conditions to enhance the safety and resilience as a common basis for a sustainable future.