GNGTS 2017 - 36° Convegno Nazionale

436 GNGTS 2017 S essione 2.2 band, accelerometer and short period sensors, transmit the seismic signals in real-time for seismic surveillance purposes. The eight stations currently installed on the island have different technical characteristics and their positions are shown in Fig. 1. Moreover, we have analysed seismic noise data acquired in 2002 by the stations ISF1 and ISF3 installed temporarily in the southwestern area of the island, close to IMTC station. Spectral analysis. Horizontal to vertical spectral ratios (H/V) are computed from seismic noise data using the GEOPSY package (Wathelet et al. , 2004). We selected at least one hour of noise signals, preferably at night hours, and we calculated, on 40s moving time windows with 30% overlap, the spectral ratios, their stationarity and directivity showed in Fig. 2 a. Stationarity refers to the stability of the H/V ratios throughout the analysis: which means that the H/V ratios performed on each time-window should be as similar as possible among them, especially with respect to the amplitude peak frequency. The directivity of the H/V ratio is a function of the direction of the signal source on the horizontal plane, which can help to detect any signal polarization along a specific direction. The continuous black curves represent the average H/V ratio, while the dashed black curves are representative of the standard deviation. The grey bands identify the frequency peak, or f0, with the relatively standard deviation bandwidth. The dashed red area in the left part of the graphs indicates an uncertain confidence zone; this is caused by the fact that the dimensions of the time windows used for the data processing are not big enough to goodly resolve the ratios at low frequencies. Fig. 2 - a) Noise H/V ratios (first and third column) as a function of frequency and directivity of the H/V ratio obtained rotating the horizontal components with a step of 10 degrees at four seismic stations (second and fourth column). The coloured curves on the H/V vs frequency graphs represent the H/V ratios calculated on each time-window. b) In red the rose diagrams in the 1-5 Hz ranges. In yellow the rose diagram in 3-7 Hz range for the station T1361.