Site response estimation in the Vittorio Veneto area (NE Italy) Part 2: mapping the local seismic effects in the urban settlement

Linear site-specific response spectra for seismic design are defined in the urban area of Vittorio Veneto (NE Italy) on the basis of the results provided by geological studies, some geophysical surveys and numerical modelling. The geological survey and the analysis of the collected stratigraphies located in the Vittorio Veneto basin allowed us to characterize four soil-types. Geophysical surveys consist of seismic noise horizontal-to-vertical spectral ratio analysis (HVSR), seismic site response estimated from earthquake records, S-wave measurements from surface wave dispersion analysis and inversion, and S-wave velocity from borehole and down-hole measurements. In addition, a 2-D spectral element method (SPEM 2D) numerical modelling of the M=5.8, 1936 Cansiglio earthquake was performed along a transect crossing the centre of Vittorio Veneto in order to assess the influence of the geological structure on the seismic response of the basin. The procedure followed to define the distribution of the soil types across the area, the local response spectra and the soil amplification factors is: 1) soil class identification and definition of the maximum value of the soil factor (Smax), according to EC8; 2) computation of the amplification coefficients (Sa) from earthquake and normalized HVSR mean amplitude; 3) zonation of the area into EC8 ground types, with associated elastic response spectra. Since the earthquake dataset includes only weak events, this study provides only site response estimates for the linear behaviour. The proposed site response map identifies three narrow zones of quite high amplification, characterized by C type soils, in Serravalle Vecchia, Vittorio Veneto centre, and Ceneda.