On the Performances of Site Parameters for Soil Classification

The selection of specific elastic response spectra according to soil categories is the standard to account for site effects in engineering design and general-purpose hazard maps. Most of the international seismic codes are based on the average shear wave velocity of the upper 30 m (Vs(30)) to discriminate between soil categories. The works of Borcherdt and Glassmoyer (1992) and Borcherdt (1994) were the first to propose the adoption of the Vs(30) as a tool to discriminate soils with similar seismic response. Nevertheless, after Borcherdt (1994), some doubts arose about the capability of Vs(30) in site effects estimation (among others, Steidl 2000; Park and Hashash 2004; Stewart et al. 2003; Castellaro et al. 2008; Lee and Trifunac 2010). The objectives of this study are: (i) to find soil classes with a similar response to an earthquake; (ii) to develop empirical amplification factors for 5 % damped response spectral acceleration in the period range T 0.04-4 s; (iii) to identify proxies for site classification (Vs(30), stratigraphy, fundamental frequency, etc.). We propose a soil classification, which is not based on a priori subdivision, but it relies on the natural aggregation of empirical amplification function of the sites, obtained by normalizing the spectra of recorded motions by a reference (rock) spectrum from a ground motion prediction equation (GMPE, developed by Bindi et al. 2011, based on the ITACA 1.0 database). The empirical amplification functions of sites are aggregated by means of cluster analysis.