GNGTS 2013 - Atti del 32° Convegno Nazionale

other at the array stations, independent of their origin (earthquake, body waves, coda waves, seismic noise) and their regime (diffusive or not). At frequency higher than 3 Hz the coherence varies greatly between seismic noise, earthquake body waves, and along the coda. In this range it is noteworthy the strong similarity between spectrogram and coherence patterns. The high coherence line between 23 and 25 Hz reveals a coherent noise produced inside the laboratory. Looking at the pictures of Fig. 1 we can choose the best windows along the coda for successive analysis. Our purpose is to verify the hypothesis that the late coda is composed by a diffuse wavefield. Our data set allows the investigation of this feature in the frequency range 5 – 23 Hz. Results of BF analysis for the same regional earthquake are also shown in Fig. 1b by differ- ent symbol and color for the three components of ground motion. The second plot contains the coherence, shown by symbol and referred to the left axis, and the spectral amplitude, shown by lines and referred to the right axis (with log scale). The coherence shown here corresponds to the same values readable at frequency of 6 Hz in the coherence of the third plot of Fig. 1a. The spectral amplitude suggests that the earthquake coda at 6 Hz lasts until at least 240 s, perhaps more. Coherence returns to values comparable to the seismic noise preceding the earthquake at about the same time. Important indications about the end of coda come also from the values of backazimuth, that return comparable to the seismic noise after 220 s, and from the slowness, that suggests 250 s. From the earthquake onset (30 s) to 260 s the values of slowness are always smaller than 0.35 s/km, corresponding to 3 km/s of apparent velocity. This demonstrate that the earthquake seismic wavefield recorded at the underground array is composed only of body waves at the frequency of 6 Hz, as expected. The slowness distributions obtained from the anal- ysis performed at different frequencies confirm that surface waves are negligible in the signals recorded at UND array for frequency greater than 3 Hz as also shown by La Rocca et al. (2013). In a wavefield composed of only body waves in diffusive regime (far from the surface) the horizontal to vertical ratio ( H/V ) must be very near to 1. Similarly, the PE hr should be very close to 0.67 (Hennino et al. , 2001). Therefore we have computed the H/V ratio and PE hr along the coda of local and regional earthquakes. To perform the spectral analysis, the time window along the coda has been chosen looking at the values of coherence and spectral amplitude. We require low coherence and spectral amplitude at least 10 times that of the seismic noise preceding the earthquake. The low coherence is a condition required by the diffusive regime, Fig. 2 – Energy partitioning ratio of two horizontal components PE hr and Horizontal over Vertical amplitude ratio (H/V) versus frequency computed as the average among the array stations for the earthquake 200909200350. The different colors (red, black and blue) are referred to time and frequency limits (respectively [0-5], [5-10] and [10- 22] Hz) shown by boxes in Fig. 1a. The green and magenta lines show the expected value for H/V ratio equal to unity (top) and equipartition in an homogeneous halfspace far from free surface (bottom) respectively. 66 GNGTS 2013 S essione 1.1