Dynamic soil properties and site coefficients for aseismic building design in some typical soil deposits of the Marche region

Although in Central Italy seismicity is generally moderate and the maximum expected local Richter magnitudes are about 6-6.5, in the past many urban settlements and villages have experienced, even during low intensity earthquakes, very severe damage, which can be attributed to the sharp topography or the presence of thick deposits or the particular dynamic properties of the soils. Therefore, in order to provide new updated guidelines for seismic building design, great research effort has been aimed at the quantification of site responses in various Italian historical centres over the latest years. Moreover, since Eurocode 8 (EC8) will be adopted within a short period in countries of the European Union, these studies have also been aimed at controlling reliability of EC8 spectra for Italian sites. This paper describes the research developed for site effects evaluation at Offida, a historical centre in The Marche with important monuments and environmental heritage. A specific program of seismological investigations, consisting in measuring low amplitude earthquakes and microtremors, and geotechnical testing, including boreholes, dynamic field and laboratory tests, was carried out. A variety of methods were utilised and compared, considering different earthquake scenarios and ground motions. The obtained spectra were finally compared with those proposed by Eurocode 8.