GNGTS 2021 - Atti del 39° Convegno Nazionale

427 GNGTS 2021 S essione 3.2 velocity model for the tower. No obvious influence of precipitation is noticed on the resonance frequency values. However, a clear decrease in f1 spectral amplitude is depicted immediately after the highest precipitation peak (18 mm/h in June 2020, Fig. 2b, magenta arrow). The heavy rainfalls of December 2019 (red arrow) triggered a significant opening of the fracture at the base of the tower, as well as a dramatic increase in the microseismicity. A clear perturbation in the ambient noise spectral content is also found in correspondence. Later increases in the number of microseismic events are associated to less marked increases in released seismic energy and likely associated to displacements at the top of the tower. Perturbation in the ambient noise spectral content are depicted in the same time windows (e.g. orange and black arrows in Fig. 2). More in general, steep increases in microseismicity are found until April 2020, in parallel with resonance frequency and velocity change increases. In the following period, the microseismic activity becomes more stable. Fig. 2 - (a) Air temperature and hourly precipitation. (b) Zoom on H2/V2 spectral ratio around f1 and f2 (amplitude 1 to 10, blue to red). (c) Azimuth of vibration. (d) Seismic velocity changes retrieved in the frequency bands overlapping f1 and f2. (e) Peak frequency of the microseismic events recorded at S2. (f) Cumulative number of natural events recorded at S1 and S2. (g) Cumulated energy. (h) Displacement measurements at C1 (tower base), C2 and C3 (tower top).