The contrast in upper mantle shear-wave velocity between the east european platform and tectonic Europe obtained with genetic algorithm inversion of rayleigh-wave group dispersion

Abstract

The stable, Precambrian East European Platform adjoins the younger, tectonically active regions of central and western Europe along a distinct crustal boundary, the Tornquist-Teisseyre Zone (TTZ). Seismic body- and surface-wave studies indicate that there may be a significant change in S-wave velocity at the top of the mantle in the region of the TTZ, with higher velocities under the East European Platform. To confirm these results we use a genetic algorithm (GA) to invert fundamental Rayleigh group-velocity estimates for wave paths across western and central Europe and across the East European Platform to determine ‘average’ layered S-velocity models separately for each region. The use of the GA method allows identical model parametrizations and broad parameter search ranges to be used for both regions so that a relatively unbiased estimate of the difference in structure can be obtained. The GA is a guided search technique which requires neither a linearized forward method nor a single starting model and which can be applied to very large model-spaces. Consequently, fewer assumptions and physical approximations are required and a greater range of possible solutions is examined than with many other inversion methods. Here we employ the GA to produce a large set of acceptable solutions and associated misfit values, in contrast to inversion for a single, ‘optimum’ solution. The scatter in the set of acceptable solutions gives an estimation of uncertainty, resolution and parameter trade-offs of the non-linear inversion. The scatter of the solutions for the dispersion data shows velocity-depth trade-offs around the Mohorovičić discontinuity, indicates the maximum depth resolution of the inversion, and shows the uncertainty in upper mantle S-velocity estimates. The results indicate a thicker crust and up to 0.3 km s−1 (7 per cent) higher ‘average’ S-wave velocities in the upper 100km of the mantle under the East European Platform than under western and central Europe.