GNGTS 2015 - Atti del 34° Convegno Nazionale

reliable such amplitude increase. In particular, we believe that these peaks could be interpreted as related to the presence of a discontinuity located at depth, linked to the clay overlaying the limestone. According to Aiuppa et al . (2004) this discontinuity could be the natural location of a hydrocarbon reservoir from which ��� ��� ����� ������� �� ��� ���� ������� �������� the mud rises through an old lava conduit (Carveni et al. , 2001). Another signature of the presence of this reservoir can be extracted by inspecting the HVNRs, that show a strong HVNR de-amplification at about 0.4 Hz ����� ������ �� ���� ���� (grey arrows in Fig. 3a). Summarizing, low-frequency surficial waves (particularly Rayleigh waves), propagate through the subsoil with strength which varies in time. Then, as suggested by Lambert et al. (2007), when Rayleigh waves interact with the reservoir the resulting radiation pattern consists essentially in P-waves, along the vertical direction, and S-waves in the horizontal one. The observed ���low frequency ���� ��� ��� ���������������� �� ��� ����� ����� �� ����������� �� ������ �� ��� peak and the de-amplification in the HVNRs could be interpreted as linked to the body waves generated at depth by the reservoir. At frequency values greater than 1.0 Hz the HVNRs show peaks variable both in frequency and in amplitude which could be related to the presence of the volcanic sequence characterized by blocks, free to oscillate and fractures filled by mud. The frequency peaks at values higher than 6.0-7.0 Hz could be interpreted as related to the Salinelle deposits whereas, at frequencies higher than 10.0 Hz the effects linked to noise generated by the gas emission can be observed (���� ���� Fig. 3a). We also investigated the existence of directional effects in the site response by rotating the horizontal components of the spectral ratios obtained at each measurement site (see examples in Fig. 2). ����� ����������� �������� ���� �� ����� �� ����� ��������� �� �� ��� ��������� ����� C �� lear directional effects, with an angle of about 50 -80 N, in the frequency range 0.1-0.2 Hz, were detected. Conversely, different resonant frequencies and directions, that could be ascribed to the vibration of smaller blocks, can be observed at frequencies greater than 1.0 Hz. Furthermore, the rose diagrams of the noise polarization strikes, in the frequency range 0.1-0.5 Hz, are plotted (example in Fig. 3b). Rose diagrams are circular histograms in which instantaneous polarization azimuthmeasurements are plotted as sectors of circles with a common origin (class width 10 ). In literature exists several studies (Panzera et al., 2014 and references therein) discussing the role played by oriented fractures on seismic wavefield. In particular, in this kind of anisotropic medium ��� ������ ����������� ������ �������� �� �������� ������ the faster shear-waves became parallel to possible dikes, fissures and tensional cracks. On the contrary, the amplification of ground motion takes place orthogonally with the azimuth of the main �������� ����� �������� fracture field (Panzera et al., 2014 and references therein)� �� ����������� ��� ������� ������������ ������������ �������� �� ���� �� ��� ����� . As consequence ������������ �������� �� ���� �� ��� ����� the ENE-WSW polarization orientations observed in most of the sites could explained with the presence of a fracture field NNW-SSE oriented. Fig. 3 – a) HVNR computed at each site along the Tr#1 and Tr#2 profiles. Black and grey arrows point out the low frequency peak and the spectral ratio de-amplification, respectively. b) Equal- area polar diagrams of the polarization azimuth obtained by filtering the noise in the frequency band 0.1-0.5 Hz. c) Dispersion curves obtained by ESAC and MASW tests. GNGTS 2015 S essione 1.3 147