GNGTS 2017 - 36° Convegno Nazionale

208 GNGTS 2017 S essione 1.3 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 (Fig. 3a). In order to assess whether ground motion is linearly polarized, the ellipticity is also plotted versus frequency. The shear wave velocity of Mt. Vetore deposits (Fig. 3b) were investigated through non- invasive techniques such as fk analysis (Capon, 1969). A “L” array configuration (red lines in Fig. 2) was used for the ambient vibration measurements, recording 20 minutes of noise. The array was settled using a 26-channel seismograph and 4.5 Hz geophones. Time windows of 20 s were considered to calculate dispersion curves of the fundamental mode and the average of the dispersion curves was computed, excluding those not showing a clear dispersion or in which higher modes were dominant. In present study, the Rayleigh wave dispersion curves, obtained from the experimental setup, were inverted using the DINVER software (www.geopsy.org ) which provide a set of dispersion curve models compatible with the experimental dispersion curve (Fig. 3b). The inversion process needs a rough definition of the free parameters. Since in our case, this information was not available, the input parameters were directly deduced from the fundamental mode of the Rayleigh wave dispersion curves. To invert the dispersion curve, a set of 3 uniform layers with homogeneous properties were considered, taking into account five parameters: shear waves velocity (V S ), thickness (H), compressional waves velocity (V P ), Poisson’s ratio (ν) and density (ρ). Results and discussions. The present study shows preliminary results obtained through ambient vibrations measurements on the top of Mt. Vetore pyroclastic cone. The HVSR measurements (see Fig. 2, for locations) point out a clear seismic site effect, quite noticeable in the frequency range 1.0-4.0 Hz, showing a preferential resonance direction highlighted through rotated spectral ratios (Fig. 2). In particular, such effect occurs at angles of about N80°-90°. The Fig. 3 - a) Polar plots showing the strike, dip and ellipticity obtained at site01. b) Experimental dispersion curve and inversion process results in term of theoretical dispersion curve and shear wave profile.