GNGTS 2024 - Atti del 42° Convegno Nazionale

Session 1.3 GNGTS 2024 The x-phase is prominent and easy to recognise. It ofen appears in the vertcal and radial component as an impulsive arrival with a higher amplitude and frequency content than the frst P - arrival. The x-phase shows a typical P- wave partcle moton, and has an incidence angle always greater than the direct P. The travel tme diferences between the direct P wave and the x-phase decrease with increasing distances and with increasing depths of the earthquakes. The later phase has higher apparent velocites than P direct wave (about 11 km/s). These characteristcs indicate that the x-phase is a compressional P- wave which leaves the source, travels downward, and interacts with an interface deeper than the hypocentre, in a less atenuatng medium than the direct P- wave. We observe this phase at statons from 6° to 9° from the epicentre, towards north (Fig. 2). Only seismograms of earthquakes located in a well-defned region of the slab, in the depth range of 215–320 km and below the eastern side of the Aeolian Arc, show the later x-phase (case 1 in Fig. 2). At greater distances, from 11° onwards, we found another arrival afer the direct P- wave that is well reproduced by the 410 km discontnuity (P410P in the Fig. 2b, case 1 and 2). It is detected on earthquakes in the depth range of 200-400 km. Earthquakes deeper than 400 km, do not show neither of the two phases (case 3 in Fig. 2b).