GNGTS 2017 - 36° Convegno Nazionale

398 GNGTS 2017 S essione 2.2 during several months, in order to make the analysis reliable. Estimated HVSR curves are reliable and representative of each studied site, as confirmed by the stability of results and by the small values of standard deviation. We confined HVSR analysis in the 0.5 – 20 Hz frequency range, enough to investigate the main contribution from the geological structures present in the area and to make the analysis useful for engineering purposes. Te second step was the HVSR analysis applied to 83 local and regional earthquakes recorded by the 15 seismic stations shown in Fig. 1, for a total amount of 276 HVSR curves. Regional earthquakes were selected by requiring the SNR greater than 3, while highest SNR local earthquakes in the 0.5 – 20 Hz frequency band were chosen among a large number of recorded events. Signal spectra were computed on 20 s windows containing the direct S wave and early coda for regional earthquakes, while 10 s window length was used for local events. Then polarization analysis was computed on a moving window on bandpass filtered signals in different frequency bands, in order to investigate the frequency dependence: 1 – 2 Hz and 2 - 4 Hz, 3 - 5 Hz and 4 - 8 Hz. The stacking of polarization azimuth is plotted in rose diagrams with intervals of ten degrees. Results. The analysis of site effects, taken out by computing the HVSR curves on the background seismic noise and local and regional earthquakes at 15 sites, has been improved by computing the polarization analysis. Fig. 2 shows the average HVSR curves obtained by noise recordings (red lines), compared with that computed from earthquakes recordings (black lines). The general characteristics of the two types of diagrams at the some station appear very similar at the most studied sites, significant differences being evident only at MMN, MMN2 and MMN4. We correlated polarization results with topographic irregularities and known faults (Guerra et al. , 2015) in the area, in order to explain those differences in a wide range of frequencies (1 - 2 Hz, 2 - 4 Hz, 3 - 5 Hz and 4 - 8 Hz). In Fig. 3 we show the rose diagrams computed Fig. 1 - Station location.