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

The purpose of this study is deriving a 3D tomographic image of the shear-w ave velocity structure of the lithosphere-asthenosphere system of the Medit erranean region from Rayleigh wave fundamental mode dispersion data. The da tabase consists of almost 200 wavetrains corresponding to regional events r ecorded at the MedNet very-broad-band stations placed in the Mediterranean area. On the basis of the path-averaged group velocities derived for each e picentre-station trajectory, local group velocities over the area covered b y the seismic paths have been obtained for the 10-90 s period range by mean s of Yanovskaya's formulation for laterally heterogeneous media. According to the resolution of these local velocities, a grid of 1 degrees of latitud e per 1 degrees of longitude has been defined over the Mediterranean area a nd a local group velocity dispersion curve has been then assigned to each g ridpoint. The stochastic inversion of almost 450 local dispersion curves pe rmits deriving the respective 1D shear-velocity structures, from which the elaboration of a 3D tomographic image of the lithosphere-asthenosphere syst em of the Mediterranean region is straightforward. As expected, taking into account the complexity of this area, the shear-velocity model reveals sign ificant 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 th e western Mediterranean at this depth range. This velocity pattern suggests a thicker crust in the centraleastern part, with a greater thickness of se dimentary layers, on account of the lower velocities deduced for the upperm ost levels. On the contrary, for depths >80 km, low shear-velocities are ob tained towards the west, while the highest sheer-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. Th e velocity pattern at this depth range suggests a deeper beginning of the a sthenosphere (varying between 100 and 150 km) under these areas, while a sh allower top (around 70 km depth) is found beneath the western Mediterranean Sea. The areas where a thicker lithosphere has been deduced depict the mai n contact between the Eurasian and African plates, this feature being relat ed to the collision and subduction processes linked to the convergence of b oth lithospheric plates. (C) 2000 Elsevier Science B.V. All rights reserved .