Seismic response analysis of the Civita di Bagnoregio mesa

The design of preservation measures of the historic centre of Civita di Bagnoregio must take into account the seismic hazard associated with the weak rock slab on which it stands and the underlying clay slope. To this end, numerical analyses were carried out to investigate the combined effects of the complex topography and geotechnical/geophysical stratigraphy on local seismic response. A detailed geophysical and geotechnical model was first developed, incorporating the slab of weak pyroclastic rocks and the underlying clay formation. This model was built by integrating various geophysical techniques, including ambient noise measurements, surface wave methods, electrical resistivity tomography and down-hole tests, with cyclic and dynamic laboratory tests on the main lithotypes. Based on this model, two-dimensional (2D) numerical simulations were conducted along two cross-sections, longitudinal and transverse, aligned with the major and minor axes of the mesa, respectively. The results were compared with those from one-dimensional (1D) analyses. The 2D simulations showed significant variations in seismic amplification, both in amplitude and frequency content, between the two sections and along each section. Amplification factors at longer periods were notably higher than those at shorter periods. At short periods, both 2D models showed a pronounced spatial variability in surface shaking with different locations of maximum and minimum values of amplification factors at the mesa top. Furthermore, the 1D analyses tended to underestimate the vertical acceleration component, especially along the major axis of the mesa. These discrepancies between 1D and 2D results, especially those observed between the two sections, highlight the complex and the direction-dependent nature of the seismic response, driven by the interaction between morphological and litho-stratigraphic conditions. These effects will be further explored through future three-dimensional (3D) modelling.