Re-evaluation of code-specified stratigraphic amplification factors based on Italian experimental records and numerical seismic response analyses

For several engineering and seismological applications, site effects can be synthetically evaluated as a soil amplification factor of the reference ground motion on a rock outcrop leading to the peak ground acceleration and/or the response spectrum expected at a soil site, classified according to its equivalent shear wave velocity, VS,30. In this work, nonlinear stratigraphic amplification factors as specified by Eurocode 8 (EC-8) and National Technical Code (NTC) were assessed with reference to selected accelerometric records of the Italian seismic network and well-characterized recording stations classified according to VS,30. A first set of 'empirical' amplification factors has been identified, by selecting those stations on deformable soil for which records of the same events exist also at a nearby reference station located on outcropping rock (4 sites, 102 records). A second set of 'semi-empirical' data has been obtained by analysing the accelerograms recorded at stations where the geotechnical characterization was comprehensive and the reference motion could be back-figured by de-convolution to the bedrock (7 sites, 50 records). A third set of 'analytical' data was obtained from 1D numerical simulations of seismic site response performed on virtual stratigraphic profiles, consistent with the ground classification adopted by EC-8 and NTC, and subjected to 80 acceleration time histories of 22 Italian earthquakes, recorded at 19 stations of the Italian Network classified as rock sites. Empirical, semi-empirical and analytical data have been therefore integrated to express the stratigraphic amplification factor of peak ground acceleration and spectral intensity as a function of the corresponding reference ground motion value for each class of subsoil.