Realistic ground motion scenarios for Napoli.

The realistic simulation of the ground motion at Napoli, recorded at Torre del Greco station for the 1980 earthquake, has stimulated similar simulations to be compared with the effects of the most damaging historical and recent earthquakes - the events happened in 1456, 1688, 1694, 1732, 1805, 1930, 1962 and 1980. A parametric study is carried out by placing the 1980 fault in the epicentral areas of the strongest central-southern Apeninnes events. The Sannio seismogenic area turns out to be responsible of the highest peak ground accelerations (PGA) and seismic response spectra (SRS) at Napoli. In particular, the 1688 earthquake is considered representative of the area (scenario earthquake) and realistic synthetic seismograms have been computed in the historical centre of Napoli with a hybrid technique based on the mode summation and the finite difference methods. The use of such a sophisticated approach is fully justified by the detailed available knowledge about the geological and geophysical properties of the neapolitan subsoil. The modeling makes it possible to recognize that amplifications of about 2 of PGA and higher than 3 for SRS, at frequencies close to the eigenfrequencies of the existing buildings, are to be expected because of the pyroclastic soil cover. Based on the information contained in the available catalogues, different magnitudes have been considered. Taking into account the correlation, valid for the italian territory, between synthetic PGA and observed intensities, it turns out that the most probable magnitude (M) of the 1688 earthquake is lower than 6.7, while M=7.3 should be assigned to a conservative scenario earthquake.