GNGTS 2021 - Atti del 39° Convegno Nazionale

GNGTS 2021 S essione 2.2 250 follow the variations of the HVSR peak amplitude versus time. For a better characterization of the wavefield properties the polarization analysis was performed on continuous signals filtered in a narrow frequency band centered at the HVSR peak frequency. This analysis was performed on 300 s sliding windows applying the covariance matrix method in time domain, obtaining the rectilinearity of particle motion, the polarization azimuth and the polarization incidence angle. Figure 2 shows, from top to bottom, RMS, rectilinearity, incidence (beta), polarization azimuth and H/V for 10 days of seismic signals recorded at the seismic station SCAI. Among these five parameters, only the polarization azimuth is quite stable, taking values in a narrow range around 200 degrees for most of the time, without any appreciable variations between day and night hours. The other four parameters are characterized by variations in time, with the highest values during day hours, and similar results have been found for other investigated sites. These features of the seismic noise are very interesting and require more in depth observations. Therefore we selected the results of HVSR analysis (versus frequency) for a specific hour of the day (11 am) and of the night (01 am) for tens of days to obtain two average HVSR curves representative of day and night hours respectively. The results of this selection are shown in Figure 3, top plots, for the same three sites shown in Figure 1. The HVSR analysis was applied also to local earthquakes, selected for each station based on epicentral distance (<100 km), hypocentral depth (<40km), magnitude (>3), signal-to- noise ratio and backazimuth in order to have a distribution of sources as much uniform as possible. This analysis was performed on windows of few tens of seconds length, including the earthquake signal from the direct P wave to the middle coda waves. Thus we are sure that body wave contribution to the result is much greater than background seismic noise and than surface waves. Results of this analysis are shown in Figure 3, bottom plots, for three of the studied sites. Fig. 2 - Results of amplitude and polarization analysis for 10 days of seismic signal recorded at SCAI site.