GNGTS 2013 - Atti del 32° Convegno Nazionale

from the H/V ratio of acceleration response spectra of the strong motion recordings. The set of accelerometric recordings and the information about recording sites allowed the conduction of a study on seismic site classification. Analysis. The analysis has been preceded by data selection, which implies that each station should have at least 3 recordings with the following characteristics: records should be normally triggered, that is pre-event noise, P and S phase should be present in the record. S-wave triggered waveforms have been rejected; epicentral distances should be lower than 200 km and the event depth < 35 km, to fulfil the criteria of the GMPE by Bindi et al. (2011); moment magnitude of events should be > 3 [Ml has been converted into M w using the relationship proposed by Castello et al. (2007)]. Records of low magnitude events, i.e. 3 < M w < 4, have been excluded if the number of records for each station was larger than 3. For each station the acceleration response spectra of single records were normalised to the expected response spectra predicted for an average rock site (for the same style of faulting, magnitude and distance) and the geometric mean of the empirical amplification functions was evaluated for each station. The distance between a couple of normalised spectra, relative to station i and j, is evaluated as the mean Euclidean distance, over m periods, between spectral ordinates, as: (1) The rank of the matrix containing the distances between couple of stations has been progressively reduced adopting a hierarchical clustering algorithm (Davis, 1986). The analysis converged on 7 soil classes of normalised spectra, identifiable on the base of resonant frequency and amplitude of amplification (Fig. 1): 1. F (flat) 2. B1 (broad band with low amplification) 3. B2 (broad band with moderate amplification) 4. L1 (low frequency band with moderate amplification) 5. L2 (low frequency band with large amplification) 6. H1 (high frequency band with low amplification) 7. H2 (high frequency band with moderate amplification) Fig. 2 shows the mean empirical amplification function (red curves in Fig. 1) relevant to the 7 soil classes identified by the cluster analysis of normalised spectra. The zoom on the right side of the figure clearly evidences the differences between each class. It should be noted that the mean value of the ‘F’ curve is significantly lower than 1. This means that there is a Fig. 2 – Mean empirical amplification function relevant to the 7 soil classes identified by the cluster analysis of normalised spectra. 315 GNGTS 2013 S essione 2.2