Quantifying local seismic amplification from regional charts and site specific numerical analyses: a case study

Three levels of seismic microzonation are considered in the Italian guidelines: 1st level allows to qualitatively recognise those areas characterised by the same seismic performance, while 2nd and 3rd levels quantify the modification of the reference signal due to the specific site conditions. The paper summarises and compares the results of 2nd and 3rd level seismic microzonation studies for the Dovadola urban area (Emilia-Romagna, Italy). The site was selected due to the availability of in situ and laboratory investigations for the definition of a detailed geotechnical seismic model of the subsoil. The 2nd level study was conducted adopting the regional charts for the quantification of the stratigraphic amplification phenomena and the regional procedures for assessing the influence of topography. The 3rd level analysis was developed through mono- and two-dimensional seismic site response analyses adopting a numerical non-linear approach. Amplification factors for the peak ground acceleration and for the velocity spectrum intensity (i.e. the integral of the pseudo-spectral velocity of a ground motion from 0.1-0.5 to 0.5-0.1 s) were determined along two geotechnical sections of the site, both characterised by a central large and surficial valley bordered by irregular topography. The study indicates a general satisfactory match between the results of the simplified regional approach and the advanced numerical tool, especially when one-dimensional seismic site conditions prevail. Further comparative studies are encouraged to validate the 2nd level microzonation as a reliable tool for seismic risk mitigation.