GNGTS 2017 - 36° Convegno Nazionale

GNGTS 2017 S essione 2.2 437 Polarization analysis. We investigated the polarization properties of seismic noise by applying the covariance matrix method (Montalbetti and Kanasevich, 1970) to the three- component seismograms, which allows to obtain the rectilinearity, the azimuth and the incident angle of the polarization vector through a diagonalization procedure. 1-hour-long samples of seismic noise recorded in September 2017 were band-pass filtered in the 1-5 Hz frequency range. Moreover, data recorded by T1361 were also analysed in the 3-7 Hz frequency band, over which the H/V ratio has the maximum amplitude. The polarization parameters were estimated using sliding time-windows containing two wave cycles, with an overlap of 50%. Narrow azimuthal distributions of the polarization vector were obtained: the overall pattern is shown in the rose plots of Fig. 2b. Data analysis and results. Sesimic noise. In this paper we show only four representative station: IOCA and IMTC of the permanent network, T1361 and T1364 of the mobile network (Fig. 2). Clear and stationary amplitude peaks are evident in the H/V ratios of IOCA and T1361. The station T1361 shows a peak centred at 5 Hz and has the maximum in the directivity diagram around the nearly E-W direction. Looking at the polarization analysis we observe that the direction of the polarization changes from a nearly NS direction, in the 1-5 Hz frequency range, to the E-W direction in the 3-7 Hz frequency range (yellow rose diagram in Fig. 2b). The higher frequency interval analysis covers the same range in which the H/V ratio reaches its maximum in the directivity analysis and highlights the same polarization direction. The station IOCA has the maximum amplitude peak centred at 1.5 Hz and it also has a maximum in the directivity diagram around the E-SE direction (~ N120). This is in agreement with the polarization direction (Fig. 2b) and with the prevalent geological structures in the northern island. Fig. 3 - Top: MD 4.0 earthquake accelerograms for the three components of motion recorded at IOCA station. The red circle identifies the maximum acceleration value equal to 2.6 m/s 2 . Bottom: H/V ratio obtained for the MD 4.0 earthquake (in blue) and H/V ratio obtained for seismic noise (in red) at the station IOCA. The vertical bars indicate the standard deviations.