APPLICATION OF NUMERICAL WAVE-PROPAGATION TECHNIQUES TO STUDY LOCAL SOIL EFFECTS - THE CASE OF BENEVENTO (ITALY)

Many of the numerical techniques used for seismic zonation studies tre at one-dimensional structural models and/or the incidence of plane pol arized body waves. These techniques are often not adequate for lateral ly heterogeneous structures and for sources that are not located benea th the site of interest. In such cases a more rigorous treatment of th e combined effects of the source, the path and the site response is ne eded. This can be accomplished with a hybrid approach combining modal summation and the finite-difference technique. To demonstrate the diff erences between these techniques, the ground motion in the city of Ben evento (Italy) is modelled. We first compare the results obtained with one- and two-dimensional structural models for vertical incidence of plane polarized body waves. These results are then compared with those obtained with the hybrid approach for two-dimensional structural mode ls. The comparisons have allowed us to find important differences in t he response obtained with the different modelling techniques. For the same site, these differences consist of strong variations in amplitude and in the shape of the spectral amplifications. For a seismic source which is not located beneath the site, vertical incidence of waves si gnificantly overestimates the local hazard in a laterally homogeneous structure. For a laterally heterogeneous area, we can conclude that on e-dimensional modelling fails to estimate the seismic hazard, whereas for a seismic source which is not located beneath the site of interest , two-dimensional modelling with vertical incidence of plane polarized body waves may not allow reliable estimates to be made of the frequen cy bands at which amplifications occur. The results obtained for two-d imensional structural models are used for a zonation of the city of Be nevento.