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. Copyright © 1995, Wiley Blackwell. All rights reserved