GNGTS 2021 - Atti del 39° Convegno Nazionale

GNGTS 2021 S essione 1.1 22 RESULTS As a selection criterion for directivity, we consider a regression coefficient R 2 > 0.5 for at least 10% (7 out of 69 values) of the frequencies investigated in a defined range for the standard deviation’s azimuth distribution (standard deviation of  W 0 smaller than 20°). According to this selection, we get that about 36% (162 out of 456) of the analyzed events present directivity.   Fig. 3 a) , b) and c) show the histograms of the number of the events compared with the exponent η , which represents a kind of amplitude for the C d model at 1, 5 and 10 Hz respectively. As we can observe, the percentage of events with directivity with respect to the total number increases as the frequency increases with the values of η covering a rather wide range between 0.5 and 2.5 when all events have a median value of n equal to about 0.5. As expected, directivity impacts both the high- and low- frequency intervals. At low frequencies, directivity can be responsible for coherent, potentially very destructive pulses with large amplitudes; at high frequencies, it can cause energy to arrive in short time intervals through its control over the duration of shaking. Fig. 2 - Azimuthal variation of the aleatory residuals fitted with cosine function (black curve) and C d function (blue curve) for the M4 Event of 30 October 2016 at 11:58:17: a) f=1Hz, b) f=5Hz, c) f=10Hz, d) f=20Hz. Results for C d function are obtained considering k =0.85 and Mach number v r /c =0.5.