Anisotropic Love and Rayleigh wave phase velocity maps for central to northern Europe

Abstract:

We processed all freely available data in Europe, from 1990 to October 2013, obtained through the
European Integrated data archive (EIDA) to calculate surface wave phase velocity maps. With an
automated inter-station method, we have obtained highly consistent, high quality, very broad band
(10 s ->200 s), phase velocity measurements for both Love and Rayleigh waves. We performed
more than 8 million cross correlations to obtain around 1.5 million individual phase velocity
measurements. Path-wise averaging resulted in more than 22,000 average dispersion curves for
different inter-station paths, with an average standard deviation of less than 1.5%. These average
phase velocity dispersion curves are then used to obtain Love and Rayleigh wave phase velocity
maps.

We carried out number of resolutions tests to check the reliability of the isotropic and anisotropic
components of the maps. A quasi checkerboard test shows the areas of the best sampling, while the
reliability of azimuthal anisotropy is tested using a synthetic model with fast directions at 90° to
those in the actual phase velocity maps. The results of the tests show that we obtain a lateral
resolution from 100 to 200 km.

At shortest periods (10 s) the isotropic maps clearly image the Central European basin system
(CEBS) extending from north German basin in the north towards the Polish basin further southeast.
In southern Central Europe low velocities are associated with thick sedimentary cover in the Po
basin and in the Pannonian basin. At these periods, there is a N-S trend of the anisotropy in the
north, and E-W trend in the Alpine region in south, and in the Pannonian basin. Remarkably,
isotropic velocities of the central European mantle lithosphere show no imprint of the Caledonian
and Variscan sutures. In the uppermost mantle, a sharp lateral gradient at the Teisseyre-Tornquist-
Zone (TTZ) is observed as compared to a more gradual lateral transition across the Sorgenfrei-
Tornquist-Zone (STZ). We denote the shallow asthenosphere south west of the TTZ as Tornquist-
Teisseyre-Asthenosphere (TTA), analogous to the TTZ. A pronounced high velocity anomaly in the
western Alps is interpreted as subducting European lithosphere. At 200 s period, a high velocity
anomaly beneath the Pannonian basin, may be due to a slab subducting into the Mantle Transition
Zone (MTZ).