GNGTS 2017 - 36° Convegno Nazionale

GNGTS 2017 S essione 2.2 399 in two of the above-mentioned frequency bands, 2 - 4 Hz and 3 -5 Hz. The results show that at nine sites (MMN, MMN2, MMN4, MMN9, MMNB, T0711, T0716, T0713, MMN7) the polarization direction is nearly transversal to the ridge direction, at four sites (MMN1, MMN3, MMN5, MMN6) it is roughly parallel to it, while at only two sites (MMNA, T0714) it does not show a well defined polarization direction. Hereafter, we took into account the known faults near the analyzed sites, as shown in Fig. 1, considering only those known faults that are closer than 250 m to the analyzed sites. Because of this restriction, only 5 sites are taken into account (MMN4, MMN9, MMNB, T0711, T0713). At three of these 5 sites (MMN4, MMN9, T0711) the polarization direction is roughly normal to the fault strike, as observed by many authors in other regions (Rigano et al. , 2008; Pischiutta et al. ., 2017), while the last two sites show polarization oblique to the fault strike. At the three sites where the HVSR from noise and earthquakes are significantly different (MMN, MMN2, MMN4) the earthquake HVSR curves do not show the peaks that characterize the noise HVSR. This feature could indicate that the peaks in the noise HVSR is an effect related mainly with the topographic irregularities, represented by mountainous reliefes. MMN and MMN2 deserve careful attention, due to their location inside the built-up area of Mormanno, therefore being of particular interest in the view of seismic hazard assessment. In summary, for 11 sites out of the 15 studied the earthquake HVSR curves are very similar to the average noise HVSR. The most evident differences between noiseandearthquakes are seen at MMN, MMN2, MMN4. The good agreement of noise and earthquake results for many sites is an important indication of the reliability of the HVSR method to estimate site effects through the analysis of seismic noise. Fig. 2 - Average HVSR curves. Fig. 3 - Rose diagrams.