A methodology to account for local geology at large scale in the SHA approach through numerical modelling of theoretical geological sections

Local geology in the standard procedure of Seismic Hazard Assessment (SHA), at national scale, is not taken into account nowadays. Microzonation studies are very expensive and time consuming for an extensive application to regional territories, while quick and simplified methods could help in this context. Therefore, in the present paper an innovative simplified approach is proposed, aimed at quickly collecting information on the local geology of small towns and villages in order to define a first estimate of the geological contribution to the bedrock-referred SHA. We defined a set of ten 1D and 2D geological and morphological settings, represented by schematic cross-sections that characterise most of the Italian small towns and villages, along with six lithotype classes defined on the basis of their shear wave velocity. These sections were numerically modelled, to include also nonlinear effects. Response spectra and amplification factors were calculated for three different kinds of seismic input at the bedrock. In order to apply the results obtained to large parts of the Italian territory, and possibly integrate them in the seismic hazard maps, Italian municipalities’ technicians were asked to classify their small towns or villages on the basis of geological settings here presented by filling in a form. These data, organised in a database and a GIS, will make it possible to associate the results of the numerical modelling to every Italian small municipality. In this way, the amplification factor values obtained could contribute to the definition of a first estimate of the role played by the local geology to bedrock-referred SHA at a large scale.