GNGTS 2024 - Atti del 42° Convegno Nazionale

Session 1.3 GNGTS 2024 apparent velocity almost equal or slightly smaller than the direct P- wave and the arrival tme diference with the direct P- wave is constant. None of the main later phases associated to a subducton system and described in literature seems to be compatble with the seismological features found for the x-phase . The x-phase is a P- wave that propagates downward into the deepest porton of the subducted lithosphere below the Southern Tyrrhenian Sea. The seismological constraints derived from the observatons made in this work, allow us to design a simple 2D modelling by means of ftng the arrival tmes. The arrivals of the direct P- wave are well fted by a subductng lithosphere with an average increment of 1.5% of IASP91 velocity model, whereas the x-phase requires much faster velocites, at least 3% higher than IASP91 (Fig. 3a). Hence, we introduced a high velocity layer, HVL, in the region where we observe the hypocentre of the earthquakes with the later phase with an average increase of the velocity up to 3% with respect to IASP91. Following Pino and Helmberger (1997), the 410-km discontnuity is raised up to a depth of 370 km, as generally observed in subducton zones (Collier et al., 2001). The model can ft the arrival tmes of the x-phase for all the deep earthquakes below the Aeolian Island we have modelled (Fig. 3b-c). According to the fnal model, the x-phase is interpreted as a compressional wave that propagates downward in a “High Velocity Layer” (HVL) located in the subducted lithosphere and refected from a shallower 410 km- discontnuity, located at 370 km of depth. Discussion and conclusion The 2D modelling states that a combinaton of velocity structure and geometric characteristcs can reproduce rather well the x-phase observatons and its travel tmes. A 2D approach and a kinematc determinaton, however, is a frst approximaton to a more complex three-dimensional problem which need to be accompanied with a dynamic calculaton in future research. A HVL, as the one we have introduced in this work, has not been previously described from a seismological point of view. The tomographic images available for the Tyrrhenian subducton zone do not show such a HVL. However, the thickness of the HVL could be too narrow to be detected by the course grid used to model the mantle at those depths. The fact that we see the later P- arrival only in the Southern Tyrrhenian Subducton Zone, is probably due to the peculiar combinaton of the velocity structure, geometric conditons, and the staton distributon in front of the slab. Compressional velocites in the HVL between 250 and 370 km depth are from 8.9 to 9.15 km/s. A simple explanaton for the high velocity values of the HVL at those depths come from recent laboratory experiments on mineral transformatons conducted at upper mantle conditons. The rocks that consttute the subductng lithosphere are locally hydrated with water incorporated into OH-bearing minerals (e.g., Hacker et al., 2003). One of the meta-stable minerals which compose the upper-mantle deep slabs is the dense magnesium hydrated silicate phase A. This mineral is considered the main responsible of the water transport into the deep Earth. Recent ultrasonic measurements of compressional waves on phase A in a cold subducton show an increase of P - velocites to the level introduced in the HVL model and at depths greater than 200 km (Cai et al., 2021). These depths are consistent with the range where we model the HVL in the Tyrrhenian subducton. Therefore, we interpret the HVL as related to the presence of the phase A, as inferred from laboratory experiments in cold subducton zones (van Keken et al., 2011; Cai et al., 2021), as the Tyrrhenian subducton seems to be. This is the frst direct seismological observaton of the phase A in the subducton process.