Shear-wave attenuation tomography of the lithosphere–asthenosphere system beneath the Mediterranean region

3-D tomographic images of the shear-wave attenuation structure of the lithosphere–asthenosphere system beneath the Mediterranean region are derived from the inversion of anelastic attenuation coefficients obtained for the fundamental mode of nearly 3100 Rayleigh wavetrains recorded at very-broad-stations placed in the Mediterranean region. The two-station method is used to compute 124 interstation path-averaged attenuation coefficients within the 15–120 s period range. An adaptation to attenuation coefficients of the Yanovskaya–Ditmar formulation, specifically devoted to path-averaged surface-wave dispersion data and body waves, permits to obtain the local attenuation coefficients and their uncertainties for all the period range, at every node of a grid covering the studied region. Maps of stretching, azimuths and mean radii of the resolution ellipses are also obtained for every period. All of these parameters are strongly related to the spatial path-coverage, data quality and data coherency. A local attenuation dispersion curve is defined then in each grid node and the stochastic inversion of these curves yields 1-D models of inverse shear-wave intrinsic quality factor (Qβ− 1). The shear-wave velocity models obtained for every grid node in an elastic tomography developed in a previous work for the same domain are taken as reference models for the inversion process. The kriging contouring algorithm is used to represent the resulting 3-D Qβ− 1 structure by means of horizontal and vertical cross-sections down to 160 km depth. The main patterns of this anelastic structure are mostly in agreement with the structural and tectonic features obtained in the reference shear-velocity tomography. The local attenuation maps manifest notable lateral changes for periods of 20 and 40 s, whereas more moderate spatial variation is observed for longer periods. The 3-D anelastic models show that, at shallow levels, the attenuation in the western Mediterranean is usually slightly higher than in the central-eastern part. The area from Corsica–Sardinia Islands to Italy presents high Qβ− 1 values of 40.0–70.0 × 10− 3 down to 75 km depth, whereas Qβ− 1 does not exceed 20.0 × 10− 3 in the rest of the Mediterranean basin, except for the south-eastern part, where slightly higher values are obtained (20.0–30.0 × 10− 3). Those high Qβ− 1 values suggest the presence of asthenospheric material at shallow levels, which would be related to distensive tectonic processes responsible for the recent opening of extensional basins, as the Tyrrhenian Sea and the Ligurian–Provençal basin. In the eastern Mediterranean, the vertical cross-sections at different latitudes and longitudes reveal at deep levels some structural patterns consistent with signs of subduction, which are linked to the convergence between the African and Eurasian plates.