The average shear wave velocity over the top 30 m of a soil profile (VS,30) represents an usual parameter for soil classification in a modern building code for seismic design. In this work the ground response of about 100 soil profiles in Tuscany and Molise (Italy) is studied through 1-D numerical simulations in order to evaluate the reliability of European and Italian soil classifications based on the VS,30 criterion. The amplification factor, Fa, defined here as the ratio of the pseudo-velocity response spectrum intensity (Housner 1952) at the surface, SIs, to the pseudo-velocity response spectrum intensity at the rock outcrop, SIr, is related to some soil parameters, such as VS,30, the fundamental frequency of vibration of the soil column, F0, and seismic impedance contrast, Iw. Analyzing the standard deviation of the residual obtained from regression analyses of Fa versus VS,30, F0, and Iw shows that F0 is the most helpful parameter for the prediction of Fa. Hence F0 appears to be more appropriate than VS,30 and Iw for the characterization of the seismic response of a site and, therefore, should not be disregarded in building code soil classifications.

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