GNGTS 2021 - Atti del 39° Convegno Nazionale

425 GNGTS 2021 S essione 3.2 Methods Ambient seismic noise spectral analyses and cross-correlation were computed on the acquired data set to characterize the seismic response and the stability of the tower and to monitor possible modifications in the seismic parameters with time. Power spectral densities, spectral ratios in single-station and site-reference configurations were computed for all the stations to extract the resonance frequencies of the tower (following Colombero et al. , 2018a). Spatial directivity and amplitude of the highest spectral peaks were further investigated in comparison with the orientations of the fractures bounding the tower. The azimuth of vibration in the horizontal plane of the 2-10 Hz frequency components was calculated from the composition of the hourly E and N spectra of each station. A 3D numerical model of the tower was built in Comsol Multiphysics to quantitatively validate the experimental resonance frequency values and vibration orientations and to deepen the investigation on the tower constraints through an eigenfrequency simulation. Fig. 1 - a) Geographical location and b) frontal view of the tower (highlighted by a red rectangle). c) Wide- angle view of the top of the quartzite tower (S1 to S4: seismic stations; C1 to C3: extensometers). d) Simplified planar view. Fracture directions are highlighted by colored dashed lines.