It is well-known that the nature of ground shaking at a construction site is strongly influenced by the lithostratigraphic characteristics of a specific soil deposit. According to the Italian Building Code (NTC08) the seismic input in stable (e.g. non-liquefiable) soils can be defined using a simplified procedure that is similar to the method proposed by Eurocode 8 (Part 1) to account for local site effects. For the horizontal component of ground motion this is based on introducing five different soil categories to which are associated specific soil factors that are used to modify amplitude and shape of elastic acceleration response spectra calculated on stiff ground (reference spectra).

Questions have been raised about the accuracy of this approach to predict site-specific ground motion and specifically on the ability of soil factors to properly quantify local site effects considering the large variability of soil profiles that may be associated to a given soil category, the uncertainty of the reference seismic input and the frequency range of interest of most structures.

With the intent to clarify and at the same time quantify the magnitude of the above problem, a series of 1D linear-equivalent, fully stochastic ground response analyses have been carried out using Monte Carlo simulations coupled with the Latin hypercube sampling technique. The variability of the reference object motion has been taken into account by adopting spectrum- and seismo- compatible natural accelerograms recorded on outcropping stiff ground. The variability of soil model parameters have also been systematically considered for each soil category by randomly generating a large population of different soil models with the only restriction to have a V_S,30 compatible with the soil category specified by NTC08.

Median and median plus-one-standard-deviation elastic acceleration response spectra have been calculated at a finite number of sites in Italy corresponding to different seismic hazard and compared with the spectra predicted by the NTC08 at the same sites and for the same soil category.