Stratigraphic amplification factors based on parametric 1D site response analyses and KiK-net downhole recordings: evaluation and comparison with code provisions

The aim of paper is to assess the performance of stratigraphic amplification factors SS as specified by the Italian National Technical code NTC18. The variation of SS with input PGA, taking therefore into account soil nonlinearity, was assessed by means of about 2800 parametric numerical 1D site response analyses carried out on virtual soil profiles as well as from the processing of more than 5500 recordings from Japanese KiK-net stations. The stratigraphic factors were computed in terms of PGA as well as ratio of integral spectral amplitude in the range of period 0.05-2.5s. The results of parametric numerical analyses indicate that NTC18 is too conservative for subsoil class D in the whole range of input PGA; the opposite is observed for class E where NTC18 shows a significant underestimation of seismic response. Moreover, the NTC18 foresees an excessive enlargement of the spectra towards the high periods for all the classes due to an overestimation of the shape parameter CC. The empirical amplification factors are quite higher than numerical ones especially at low values of input PGA: this behavior has been ascribed to the high stiffness of bedrock encountered in the KiK-net profiles, well above the VS=800 m/s assumed for numerical schemes and in the NTC18 code. © 2019 Associazione Geotecnica Italiana, Rome, Italy.