The new Italian seismic code, NTC 2008, as well as the Eurocodes and other international codes, admit the existence of the special seismic class of ground S2 in all the cases in which the use of the simplified response spectra are not adequate to represent the seismic action. The inadequacy may be due for example to the shear wave velocities profile Vs, that shows strong stiffness contrasts or velocity inversions (i.e. soft layers among stiff layers or stiff layers among soft layers). When dealing with S2 soils or more generally in all the studies of local seismic response the seismic code NTC2008 suggests to define a design earthquake in terms of accelerograms at bedrock and to study the 1D or better the 2D propagation of the seismic input from the bedrock to the ground surface. The most delicate step in this process is the definition of the accelerogram at the bedrock, because it has to be seismogenetically compatible to the zone as well as compatible to the response spectra provided by the National Board for Public Works (Consiglio Superiore dei Lavori Pubblici). In this article a simplified but still rigorous method is proposed to calculate the response spectrum on soils of type S2, provided the basic seismic hazard in terms of PGA and the seismogenetic characteristics of the site.
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