A STEP INTO THE DEFINITION OF THE SEISMIC RISK FOR THE CITY OF BENEVENTO (ITALY)

This paper gives a contribution in the definition of the seismic hazard for the city of Benevento in Southern Italy, from a geotechnical engineering viewpoint. To pursue this goal, an extensive geotechnical characterization of the city subsoil was achieved collecting data available at the Department of Geotechnical Engineering (DIG), University of Napoli and Benevento municipal technical office. A quantitative definition of all soil parameters needed for seismic response analyses has been achieved in 19 verticals. These verticals were selected because seismic shear wave velocities were available. Attention was paid in defining strain dependent shear stiffness and damping ratio for the geomaterials present in the urban area. A new method to correct the Masing criteria was adopted, when associated to Ramberg-Osgood model. This methodology consists in adding an initial damping factor to the damping-shear strain curve and in adopting an empirical factor  to correct the area loop of the  curve to match experimental data. The corrective value  was deduced by interpreting the relationship existing between several G() and D() curves obtained from RCTS tests performed at the DIG, on both natural and artificial soils. A value of  equal to 0.8 was suitable to match the experimental damping-shear strain curves. The seismic hazard in the city was evaluated on the basis of two seismic scenarios, respectively characterized by high and low acceleration levels. The first one is representative for Sannio 1688 earthquake (PGA = 0.38g at bedrock). The second seismic scenario is aimed at simulating the effect of the 11/23/1980 Irpinia earthquake in the city of Benevento (PGA= 0.03g at bedrock). Seismic analyses were conducted first by using simplified approaches, such as the site classification suggested by the Eurocode 8, then considering 1-D layered visco-elastic subsoil, behaving linearly and successively adding non-linearity. According to the EuroCode8 guidelines in the whole urban area of Benevento site effects are not relevant, since an almost homogeneous distribution of acceleration response spectra was obtained. With the non linear analyses the zonation map of the city of Benevento was sketched grouping together the response spectra having similar features. From the two seismic scenarios, it clearly emerges the influence of the input motion variability on the results of a seismic zonation. Since Integral parameters such as the Arias intensity or the rms acceleration, are representative of the energy content of a seismic signal, the Arias Intensity at ground level, normalized to the value pertaining to the input motion, was mapped for the two seismic scenarios. It was observed that the areal distribution of this parameter approximately matches that depicted on the base of response spectra.