GNGTS 2016 - Atti del 35° Convegno Nazionale

GNGTS 2016 S essione 2.2 385 Experimental spectral ratios were also calculated after rotating the NS and EW components of motion by steps of 10 degrees starting from 0° (north) to 180° (south). This approach, first applied to earthquake recordings in studying the directional effects due to topographic irregularities at Tarzana, California (Spudich et al ., 1996), has been used for ambient noise signals by several authors to identify site response directivity in the presence of faults (e.g. Panzera et al ., 2016 and reference therein). In this study we also apply the time-frequency (TF) polarization analysis proposed by Vidale (1986) and exploited by Burjánek et al. (2012 and reference therein). This technique can provide quite robust results, overcoming the bias that could be introduced by the denominator spectrum in the HVSR calculation. Following Burjánek et al. (2012 and reference therein), the continuous wavelet transform (CWT) is applied to signals in order to select time windows whose length matches the dominant period: signals are thus decomposed in the time-frequency domain and the polarization analysis is applied. For each time-frequency pair, polarization is characterized by an ellipsoid and is defined by two angles: the strike (azimuth of the major axis projected to the horizontal plane from North) and the dip (angle of the major axis from the vertical axis). Another important parameter is ellipticity that is defined, according to Vidale (1986), as the ratio between the length of the minor and major axes: this parameter approaches 0 when ground motion is linearly polarized. Polarization strike and dip obtained all over the time series analyzed are cumulated and represented using polar plots where the contour scale represents the relative frequency of occurrence of each value, and the distance to the center represents the signal frequency in Hz. In order to assess whether ground motion is linearly polarized, the ellipticity is also plotted versus frequency. Results and Discussion. We focused the present study on the part in which there is major evidence of fracturing. The HVSR measurements were performed near the cliff edge, where the fractures are more evident and moving away from it (see Fig. 3a, for measurement point’s location). TheHVSR results point out a clear seismic site effect verymarked in the neighbourhood of the fractures, in the eastern part of the studied area, and a decrement moving towards West. In particular, ��� ����� ���� � �������� �� �������� ��� ���������� �� ��� ��������� ����� the HVSRs show a tendency to increase the amplitude, in the frequency range 1.5-4.0 Hz� ���� � ����� ���� ������ �� ��� ��� ����� ����� �� ��� ���� ��� �������� �� � ����� , with a clear “eye shape” in the FFT (e.g. Figs. 3b and 3c). The presence of a clear “eye shape” allowed us to exclude anthropic disturbance on HVSR results. ��� ������� �������� The rotated spectral ratios (e.g. Fig. 3d) show a broadband frequency effect with several adjacent peaks pointing out a preferential direction, which is the typical behavior of directional resonances. In particular, such effects are with angles of about 80°-90°. The results coming out from the use of TF Fig. 1 – Belvedere Place in the S. Caterina village (see Fig. 2 for location), affected by NNW–SSE fractures along: a) the floor and the house; b) the floor and the chapel; c) a wall to the north of the chapel (photographs October 4, 2012, after Barbano et al., 2014); d) the same chapel and e) via Pianeto (photographs 15 September 2016).