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 parametrization and broad parameter search range 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 give 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 Mohorovi i 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 (7%) higher "average" S wave velocities in the upper 100 km of the mantle under the East European Platform than under Western and Central Europe.