Shear-wave velocity variations in the upper mantle beneath central Europe

Abstract

A new model, EUR-S91, of shear-wave velocity variations in the upper mantle beneath central Europe and surrounding regions, down to a depth of 670km, is presented. The model is derived from the inversion of the waveforms of 217 seismograms, using the partitioned waveform-inversion method for the seismogram in the time window from the S-wave arrival to the fundamental mode of the Rayleigh wave. The seismograms were mostly assembled from digitially recording long-period and broad-band stations in Europe. The resulting 3-D model accurately predicts most of the observed waveforms, in a wide band of frequencies. Body waves were fit for frequencies up to 60mHz. The fundamental Rayleigh mode, which at high frequencies is more prone to scattering and multipathing, was generally low passed at 25 mHz. The resolving power of the data set depends strongly on the density of available wave paths and varies as a function of geographical position. Small-scale heterogeneities like the subducted lithosphere in the Hellenic collision zone were imaged in the region with the highest density of wave paths, which indicates an optimum resolution of better than 200 km. The main new results of this study pertain to the transition between east and central Europe. We present new information about the structure below the Tornquist-Teisseyre Zone (TTZ). The TTZ is generally regarded as the boundary between the Precambrian crust of the Baltic Shield/Russian Platform and the younger crust of central Europe. Our 3-D S-velocity model reveals that below this line a sharp lateral boundary extends down to at least 140 km depth, with high velocities beneath the Baltic Shield and Russian Platform contrasting to the low velocities beneath the younger regions of Europe. The observed velocity contrast across the TTZ is largest between the Pannonian Basin and the Russian Platform, where it is equal to 12 per cent at 80 km depth. At depths of 300–400 km, the TTZ is underlain by a zone of low S velocity, which indicates a local thinning of the deeply rooted high-velocity structure or tectosphere that underlies most of eastern Europe. In addition to these new results on the transition region between east and central Europe, a number of features that are present in other tomographic results have been confirmed: the Precambrian provinces of Europe are characterized by high S velocities; low velocities are found beneath the Pannonian Basin, the western Mediterranean, northern Aegean Sea/Turkey and a small region south-west of the Massif Central. The lithosphere beneath the Paris Basin has a positive S-velocity anomaly. The resolution of all these anomalies has been established with sensitivity tests.