Shear-wave velocity tomography of the lithosphere–asthenosphere system beneath the Mediterranean area

Abstract

The purpose of this study is deriving a 3D tomographic image of the shear-wave velocity structure of the lithosphere–asthenosphere system of the Mediterranean region from Rayleigh wave fundamental mode dispersion data. The database consists of almost 200 wavetrains corresponding to regional events recorded at the MedNet very-broad-band stations placed in the Mediterranean area. On the basis of the path-averaged group velocities derived for each epicentre-station trajectory, local group velocities over the area covered by the seismic paths have been obtained for the 10–90 s period range by means of Yanovskaya’s formulation for laterally heterogeneous media. According to the resolution of these local velocities, a grid of 1° of latitude per 1° of longitude has been defined over the Mediterranean area and a local group velocity dispersion curve has been then assigned to each gridpoint. The stochastic inversion of almost 450 local dispersion curves permits deriving the respective 1D shear-velocity structures, from which the elaboration of a 3D tomographic image of the lithosphere–asthenosphere system of the Mediterranean region is straightforward. As expected, taking into account the complexity of this area, the shear-velocity model reveals significant lateral changes in the crust and uppermost mantle elastic structure. As a main feature, low shear-velocities are deduced for most part of the central and eastern Mediterranean basin at shallow depths, down to 35–40 km, whereas significantly higher velocities are obtained for the basins of the western Mediterranean at this depth range. This velocity pattern suggests a thicker crust in the centraleastern part, with a greater thickness of sedimentary layers, on account of the lower velocities deduced for the uppermost levels. On the contrary, for depths >80 km, low shear-velocities are obtained towards the west, while the highest shear-velocities are derived for the eastern Mediterranean Sea, the Aegean Sea, Greece, the centre and the south of the Italian Peninsula and the Adriatic Sea, Sicily and Tunisia. The velocity pattern at this depth range suggests a deeper beginning of the asthenosphere (varying between 100 and 150 km) under these areas, while a shallower top (around 70 km depth) is found beneath the western Mediterranean Sea. The areas where a thicker lithosphere has been deduced depict the main contact between the Eurasian and African plates, this feature being related to the collision and subduction processes linked to the convergence of both lithospheric plates.