GNGTS 2013 - Atti del 32° Convegno Nazionale

Horizontal to vertical spectral ratio of ambient noise according to Nakamura method (NHV, 1989) have been also considered to detect resonance frequencies. Data acquired at different hours of night and day, excluding possible seismic events, both local and teleseismic have been analysed. A common response can be observed at all investigating sites in Ferrara, with an amplification band at 0.6-1 Hz, in accordance with Martelli et al. (2011) investigation in the area of the right bank of the Po, between Modena and the mouth. Also the site OG004, near Ficarolo village, and place on the left bank of the Po river, the resonance frequency of 0.6-1 Hz is clearly recognisable and it has been interpreted as a stratigraphic discontinuities, intercepted by the Casaglia well at 135 m depth below ground level, and interpreted as the Quaternary basement (Cocco et al. , 2001). Site responses are estimated considering spectral ratios for earthquakes of the Emilia sequence and for local and regional events. Each individual event has been analysed and examined manually. For the different methods used here, calculations assume the same input Fourier spectra of the S and coda waves. The window length is not less than 20 s for events of Emilia sequence, for M>3.5; for events of 2013, the time length varies from a minimum of 10 s to a maximum of 30 s for regional events. The events of the Emilia sequence were not recorded at FERB site; some tests were made to investigate the possibility of applying the generalized inversion technique, considering some events recorded in Casaglia in 2013. Closest events, which have epicentral distances similar to the Emilia sequence events, and the mainshock of Lunigiana sequence, have been analysed. These tests have demonstrated the non-applicability of the method of inversion when the average energy of the spectra at the site of interest and the reference is greater than three orders of magnitude, and there is at least one event synchronous for all sites. Fig. 2 shows the comparison of the two site-reference methods RSSR and GIT only for the sites working in 2013, when the records of FERB were available. The two methods show very similar results, while when they are compared with the horizontal-to-vertical ratio of earthquakes (EHV), some discrepancy arise, with the exception of the site OG010. Fig. 3 – Comparison between the theoretical PGA derived by the shake maps and the observed PGA at the OGS stations for the main events of 2012 Emilia sequence. 182 GNGTS 2013 S essione 2.2