GNGTS 2013 - Atti del 32° Convegno Nazionale

frequency in the range 2 – 18 Hz. On the contrary, for this earthquake the coherence of the S wave and early coda is very high only at frequency lower than 3 Hz. The very high coherence of any signals at low frequency is a consequence of the array extension, which is smaller than the signal wavelength for frequency lower than about 6 Hz (S wave). In other words, UND array is not appropriate to measure the spatial coherence of seismic waves at frequency smaller than about 3 Hz. This consideration is also independent from the window length. In fact, whatever the window length is, the low frequency signals will be always very similar to each Fig. 1 – Regional earthquake 200909200350. a) Seismograms of one station, Spectrogram of the vertical component and Coherence among the array stations. b) Results of the array analysis with the Beam Forming method focused at 6 Hz: Coherence at 6 Hz, spectral amplitude, backazimuth and slowness. Different symbol and color refer to the three components of ground motion. 65 GNGTS 2013 S essione 1.1