GNGTS 2015 - Atti del 34° Convegno Nazionale

GNGTS 2015 S essione 1.2 115 to the central sector of the Ferrara Arc (Fig. 1a), which is one of the three arcs consisting of blind, north-verging thrusts and folds that represent the external northern Apennines front ������ (Pieri and Groppi, 1981; Bigi et al. , 1982; Boccaletti et al. , 2004)� ��� ������������ ������� ���� �� . The investigated profile runs in the middle of the elongated area investigated by the detailed gravimetric survey, described in the companion paper (see Palmieri et al. , 2015), between Traghetto (near Molinella) and Formignana (Fig. 1b). Because the density variations of the LateQuaternary deposits are negligible, even performing a much denser grid of gravity measurements, the resolution of this approach, as described in the above mentioned companion paper, would not be sufficient to detect and reconstruct the surfaces and geometries of bodies in the shallow subsurface. In addition, the geometry of the sedimentary bodies, accumulated in the alluvial plain, is generally characterized by sharp lateral variations and hence the interfaces commonly lack planar geometry showing curvatures with wavelength varying between one to ten hundred meters and several meters wide. Accordingly, as far as the expected deformation structures ( i.e. fault-propagation folds) in the youngest and hence shallowest deposits could have comparable dimensions, the gravimetric survey was integrated by geophysical measurements sensitive to more variable properties in the shallowest subsurface than density. Therefore, our investigations based on seismic techniques, exploited the ambient seismic noise. In particular, we applied the ESAC (Aki, 1957, 1964; Asten and Henstridge, 1984; Ohori et al. , 2002) strategy to obtain several 1D shear wave velocity profiles, providing quantitative shear velocity models down to 120-150 m depth, and HVSR technique (Nakamura, 1989) to infer the fundamental resonance frequency and an estimate of the depth of major impedance contrast(s) at each site. Afterwards, the local 1D Vs profiles and HVSR curves were assembled in order to reconstruct two independent pseudo-2D sections. In what follow we shall show that it is possible to obtain reliable pseudo-2D sections from surface seismic noise data so emphasizing the occurrence of lateral shear wave velocity and spectral ratio amplitude variations. Further, a characterization beyond the nominal depth of 30m results in very useful information for site characterization especially for the quantitative evaluation of the local site specific seismic response (Abu Zeid et al ., 2012). Data acquisition. Along the investigated profile, we carried out 26 ESAC arrays associated with single station recordings at the centre of the array, roughly one kilometre spaced. The coordinates of the investigated sites are listed in Tab. 1. Fig. 1 – a) Simplified tectonic map of the blind northernApennines showing the studied area [black boxes ESE of Ferrara: modified from CNR-PFG (1991)]. b) Location of the measured sites (blue dots) along investigated profile (black line).