Imaging of shear-wave velocity structure beneath Iberia

A detailed dispersion analysis of Rayleigh waves propagating across the Iberian Peninsula is carried out. The starting data are high-quality long-period data recorded at the broad-band NAPS stations installed in the Iberian Peninsula during the ILIHA. project. We apply methods to obtain a correct selection of data and subsequent two-station surface-wave velocity measurements. A total of 64 teleseismic events recorded by the NAPS array and 143 seismic paths have been studied. Several techniques which provide a significant improvement in the signal-to-noise ratio are employed to remove higher-mode interference efficiently and improve the isolation of the fundamental-mode Rayleigh wave from the seismograms. Thereafter, the interstation Rayleigh wave phase and group velocities are determined. We perform simultaneous inversion of phase- and group-velocity dispersion data by means of the stochastic inverse operator, and test the reliability of the results by computing resolving kernels and also by forward modelling. A regionalization procedure based on the Backus-Gilbert approach for linear inversion of traveltime data is applied. Both the inversion results and the contoured shear-wave velocity panoramas display the main features of the deep structure of Iberia. We find a subcrustal low-velocity channel which extends over practically the whole peninsular area and spans a depth interval of approximately 40-50 km; it exhibits velocities of between 4.30 and 4.50 km s(-1). At depths of 66-81 km, we find the highest velocities in the lithosphere, which reach values of 4.85 km s(-1) in many cases. The low-velocity channel of the asthenosphere spans a large depth interval of approximately 80-180 km; it shows the lowest velocity values computed by us. We find velocities decreasing with depth, which are of the order of 4.25-4.36 km s(-1) for the first 40 km and of the order of 4.00-4.25 km s(-1) for the rest. The upper mantle under the asthenosphere exhibits high velocities, which range between 4.62 and 4.82 km s(-1) in most cases. The shear-wave velocity structure of the Iberian subcrustal lithosphere and asthenosphere is mapped at 11 depth intervals from 24 to 201 km. At the top of the mantle, relatively low velocities span the Ebro Valley and also the southern third of the peninsula. Low velocities appear in the south-southwest quadrant, and high velocities occur over the Hercynian basement. At greater lithospheric depths, very low velocities extending over the whole peninsula suggest a low-velocity channel of non-uniform lateral structure, where a reduced zone to the west of the Iberian plateau shows relatively high velocities. At the greatest lithospheric depths, the whole Iberian block is fairly homogeneous laterally. The asthenosphere shows a notable lateral heterogeneity as well. We distinguish two parts: the upper asthenosphere, a 40 km thick layer with predominant velocity values of 4.25 km s(-1) or more; and the lower asthenosphere, a 60 km thick layer with velocity values generally below 4.25 km s(-1). The upper asthenosphere seems to be less laterally heterogeneous than the lower asthenosphere. The lower asthenosphere exhibits a more pronounced negative velocity gradient than the upper asthenosphere.