GNGTS 2021 - Atti del 39° Convegno Nazionale

23 GNGTS 2021 S essione 1.1 We also use histograms to compare the azimuthal distribution of events for the C d model, as shown in Fig. 3 d) , e) and f) . The distribution is bimodal with two peaks around 150° and 330°, corresponding to the average direction (approximately towards the SE and NW, respectively) and is aligned with the fault strikes of the Central Apennines events (Tinti et al ., 2016; Improta et al ., 2019). From a global perspective, in addition to a certain frequency range, the percentage of events with directivity effects with respect to the total number, varies as a function of magnitude. For small events (M < 3.6), directivity occurs at about 25-30% of the events, in the frequency range between 5 and 10 Hz. As the magnitude increases, the percentage of events affected by directivity tends to slightly decreases, involving lower frequency; in particular, in case of events with M > 5 it reaches the maximum at frequency lower than 1 Hz. CONCLUSIONS AND FUTURE PERSPECTIVES In this work, we identified the events with directivity effects by fitting the residuals with the C d function. Generally, events with smaller magnitude (M < 4.0) show directivity amplification at higher frequencies (>10 Hz), while the largest earthquakes exhibit more significant effects only at intermediate frequencies. Some relations between the directivity effects and the high-frequency source attenuation parameter (Sommerville, 1997) suggesting that the rupture mechanism may be complex also in case of small or moderate events. From the preliminary results, we observed directivity not only during seismic sequences, but also in background seismicity. This finding could play a key-role in understanding which structures can be responsible for triggering the main events in the Central Apennines. As a Fig. 3 - Histograms of the exponent for all the events (red) and the events with directivity (yellow) for a) 1 Hz, b) 5 Hz and c) 10 Hz. Histograms of the azimuth angle for all the events (red) and the events with directivity (yellow) for (d) 1 Hz, (e) 5 Hz and (f) 10 Hz.