GNGTS 2017 - 36° Convegno Nazionale

GNGTS 2017 S essione 1.3 207 Ambient noise recordings were performed randomly in the Mt. Vetore cone area, sampling time series of 20 minutes length through a short period velocimeter, using a sampling rate of 128 Hz and processing data through the HVNR technique. According to common assumptions (Bard, 1998; Parolai et al. , 2001), to compute HVSR, we subdivided the recorded signal taking care that at least 10 cycles of the lowest frequency analysed were included. Then, time windows of 20 s were considered and the most stationary part of the signal was selected excluding transients associated to very close sources. In this way the Fourier spectra were calculated in the frequency range 0.5-20.0 Hz and smoothed using a proportional 20% triangular window. Finally the resulting HVSR were computed estimating the logarithmic average of the spectral ratio obtained for each time window. The experimental spectral ratios were also calculated after rotating the horizontal components of motion by steps of 10 degrees starting from 0° (north) to 180° (south) in order to investigate about the possible presence of directional effects (left and right panels in Fig. 2). However, in presence of lateral and vertical heterogeneities or velocity inversion, the HVSRs can be “not-informative” due to the occurrence of amplification on the vertical component of motion (Panzera et al. , 2015). In this study we also applied the time-frequency (TF) polarization analysis proposed by Vidale (1986) and exploited by Burjánek et al. (2012 and reference therein). Following Burjánek et al. (2012), the continuous wavelet transform (CWT) was applied to signals in order to select time windows whose length matches the dominant period: signals were thus decomposed in the time-frequency domain and the polarization analysis was 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 on the horizontal plane fromNorth) and the dip (angle of the major axis, starting from the vertical axis). Another important parameter is the ellipticity that is defined, according to Vidale (1986), as the ratio between the length of the minor and major axes. This Fig. 2 - Mt. Vetore map showing the location of the single station ambient vibrations recording sites (white rectangles) and the L shape geometry array (red lines). The dashed white line indicates the rough extension of the pyroclastic cone basis. The contours plots show the results of the spectral ratios geometric means, plotted as a function of frequency ( x-axis ), direction of motion ( y-axis ) and HVSR contour plot amplitudes.