Ground Shaking Scenario at the Historical Center of Napoli (Southern Italy) for the 1456 and 1688 Earthquakes

Ground motion is computed at the historical center of Napoli for the 5 December 1456 and 5 June 1688 earthquakes responsible for VIII maximum felt intensity (MCS scale). Computations are performed using the hybrid technique, which is based on the mode summation and finite difference methods. These consider the source, propagation and local site effects. The approach is fully justified by the detailed knowledge of the physical parameters of the local subsoil based on the models inferred from noise cross-correlation measurements between two receivers. Moreover, the propagation model is validated through the fitting of synthetics with recording of a moderate earthquake at the historical center of Napoli (29 December 2013, M-W = 5.2) close to the seismogenic fault responsible for the 1688 earthquake. Ground motion is computed along a two-dimensional section crossing the historical center for the seismogenic sources as known from the literature. A consistency exists between the computed peak ground acceleration and intensity data if we attribute higher moment magnitudes of 7 to the 1688 earthquake and 7.3-7.4 for the 1456 earthquake. In light of the uncertainties related to the macroseismic intensities and estimated magnitudes of these historical events and the relevant masonry heritage of the historical center of Napoli, the highest values of the computed ground motion are suggested for seismic retrofitting of the masonry heritage. A scenario earthquake like those of 1688 (M-W = 7) and 5 December 1456 (M-W = 7.4) is predicted by the seismic code for limit states of life safety or collapse, depending on the site and the true material damping.