GNGTS 2021 - Atti del 39° Convegno Nazionale

249 GNGTS 2021 S essione 2.2 opportunity to study signals less affected by human activities. Figure 1 shows the HVSR versus time and frequency for three permanent seismic stations where the day-night variation of the peak amplitude is particularly evident. A considerable increase of the peak amplitude occur during day hours, while the same peak almost disappear by night. This feature of the HVSR has been observed for several sites, and it is not affected by the reduced contribution of antropic noise that characterized the two months of March and April 2020. We computed the RMS (root mean square) versus time for continuous signals to follow the day-night variations of signal amplitude in time domain. The RMS was computed on a 300 s sliding window for the three components of ground motion on bandpass filtered signals to include only the frequency range shown by the peak of HVSR result. Then we computed the mean RMS among the three components, and the ratio between the RMS or horizonthal and vertical components. This last analysis, the H/V ratio in time domain (hereafter said H/V to distinguish from HVSR in frequency domain), allows to easily Fig. 1 - HVSR versus time and frequency at the three sites CHC2, MNGA, SCAI, for 12 days of data. A strong strong day-night variation of the peak amplitude characterizes these sites.