GNGTS 2024 - Atti del 42° Convegno Nazionale

Session 1.1 GNGTS 2024 al. (2016) is compatble with the results obtained with this study, despite the observaton of a greater number of events due to a signifcantly longer observaton period. However, approximately 20 events detected by the HPD with epicentral distances greater than 2000 km are located below the blue line (top plot, Fig. 3). On the other hand, 5 events with a Mw of 6.5 are superfcial. Therefore, with the same magnitude and distance, deep events are not detected by the HPD. Figure 2 – Mw 6.8 earthquake occurred in Morocco on 9 September 2023 at 22:11:00 UTC. Top panel: The blue (Z), green (N-S) and brown (E-W) seismic traces refer to the vertcal, N-S and E-W components, respectvely, recorded by the INGV seismic staton GIGS. Botom panel: black trace is the hydraulic pressure signal recorded in the borehole S13 (i. e., hydroseismogram). Events with an epicentral distance of less than 200 km, the hypocentral depth is low (about 5-20 km, shallow earthquakes). Beyond 200 km of epicentral distance, the frst deep earthquakes that are identfed with distances just over 200 km are those in the Tyrrhenian area, which however are not detected by the HPD system (botom panel, Fig. 3). In accordance with the top plot in Fig. 3, HPD is more sensitve to surface events. Instead, the events detected with hypocentral depth greater than 80 km are signifcantly reduced. Also note in the cluster at the top right (top plot of Fig. 3: A) as among the events at greater depth, i.e. greater than approximately 500 km, the HPD detected only two events (Mw 7.6 event located in the Sea of Japan at a depth of 675 km and epicentral distance of 10527 km, and the Mw 8.1 event located in the sea of Fiji Islands at a depth of 574 km and an epicentral distance of 17089 km). In the most populous cluster (top plot of Fig. 3: B), consistng of events with a hypocentral depth between approximately 5 km and 20 km, HPD detectability is instead correlated to epicentral distance and magnitude. Generally, events with a magnitude less than 3 and with an epicentral distance and hypocentral depth less than 20 km are not detected by the HPD. Instead, most of the events detected by the HPD, regardless of the epicentral distance, have hypocentral depths generally less than 20 km as perhaps with the same magnitude the deep events generate smaller amplitudes of the surface waves which are normally partcularly evident in the hydroseismograms.