GNGTS 2014 - Atti del 33° Convegno Nazionale

240 GNGTS 2014 S essione 2.2 mainly relative to the Ferrara province, ambient vibrations were measured in continuous mode for about two months (more details are described in Priolo et al. , 2012). Examining the overall trend of the HVSR curves, 4 main patterns were identified which are representative of 4 geographic macrozones (Fig. 2): - zone 1, which includes the southern part of the investigated area, is characterized by the most widespread HVSR pattern. In particular, HVSR curves show a main broad peak in the frequency band 0.8-0.9 Hz and a secondary peak at about 0.25-0.3 Hz; - zone 2, which includes the municipalities of Concordia sulla Secchia and San Possidonio, where HVSR curves are generally characterized by a main peak 0.9-1 Hz (sharper than that in zone 1) and, again, a secondary peak at about 0.25-0.3 Hz; - zone 3, which includes mainly the municipality of Mirandola, showing HVSR curves with a single main sharp peak at 0.8-1.3 Hz; - zone 4, localized in the NE of the study area where HVSR curves point out � ���� ����� a main broad peak in the frequency range 0.8-0.9 Hz, a slight “hump” in the range 1.5-2 Hz and another maximum at about 0.25 Hz. Within zones 1 and 4, the shape of the main peak of individual HVSR curves does not show any clear spatial pattern variation of the corresponding frequencies in the range 0.8-0.9 Hz: random spatial variations are observed also at the scale of a single settlement. This suggests the presence of a relatively broad transition in the seismic impedance values at depth. As concerns zones 2 and 3, the sharpness of main peak indicates the presence of a single strong impedance contrast at depth. In all the above four zones, we can notice that the two peaks are both characterized by significant and spatially erratic variations of amplitude values (between 1.5-2 and 5-6, Fig.2). This feature suggests that amplitude of the HVSR peaks could be driven more by the structure of the ambient vibration wavefield than of the subsoil structure. To explore this possibility, these amplitude values were correlated with the sea-wave activity in the seas surrounding the area. Data provided by ISPRA - Servizio Mareografico “Rete Ondametrica Nazionale” (Bencivenga et al. , 2012; http://www.telemisura.it/ ) were considered on purpose; in particular, sea-wave average height obtained each half hour at the measuring stations of Ancona, Cagliari, Fig. 1 – Localizations of the passive seismic measurements: green crosses related to the seismic arrays, red dots to the single stations measurements. Dashed black lines indicate 6 MCS isoseismal (modified from Galli et al. , 2012), the stars are the epicenters of two mainshocks (May 20 and 29).