Poster for EUG XXI General Assembly, May, 1996, The Hague.

The contrast in upper mantle S velocity across central Europe from Monte Carlo inversion of Rayleigh-wave group dispersion

Anthony Lomax and Roel Snieder

Department of Geophysics
Utrecht University, PO Box 80.021, 3508 TA Utrecht, The Netherlands
anthony@alomax.net

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Abstract

The stable East European Platform (EEP) adjoins tectonic Central and Western Europe (TE) along the Tornquist-Teisseyre Zone (TTZ). Seismic studies indicate a significant change in S wave velocity at the top of the mantle across the TTZ, with higher velocities under the EEP. Recently, we used a genetic algorithm (GA) (Lomax and Snieder, 1995b) to invert Rayleigh group-velocity data for paths across TE and the EEP to determine "average", layered S velocity models separately for each region. The GA allows an identical, non-linear search to be used for both regions giving a relatively unbiased estimate of the difference in structure. However, the GA uses a poor sampling of the model space and consequently the uncertainty in the results are not well defined ( Lomax and Snieder, 1995a; Lomax and Snieder, 1996).

Here, we repeat this inversion using an adaptive, Monte-Carlo importance sampling algorithm (AMI) which has improved sampling of the model space. This method produces approximate likelyhood functions at each depth giving a better understanding of the uncertainty and resolutions of the inversion. The new results confirm the earlier GA results but give clearer and more quantitative images of features including velocity-depth trade-offs around the Moho, the maximum depth resolution, and the contrast in upper mantle S velocity.

The adaptive, Monte-Carlo importance sampling method (AMI)

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Group velocity data and model construction

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Results of AMI surface wave inversion

In the figures below we show the AMI inversion results for the EEP and TE dispersion data. In each figure the left frame shows the sampling density at each depth in the model space, the center frame shows relative likelyhood for S velocity at each depth. The right frame shows the acceptable models, those models that fit the data within a cutoff based on the scatter in the data.

Results for the East European Platform (EEP)

Results for tectonic Europe (TE)

Comparison of results for tectonic Europe (TE) and for the East European Platform (EEP)

The figure below shows the likelyhood estimates for S velocity at each depth for TE and EEP.
  • Velocity-depth trade-offs near the Moho are indicated by the widening of the likelyhood estimates along the velocity axis at around 25 km depth for TE and around 50 km depth for the EEP.
  • The maximum depth resolution of the data is indicated by the broadening and flattening of the likelyhood functions with depth. There is little resolution below about 150 km for TE and below about 400 km for the EEP.
  • The contrast of S velocity at the top of the mantle between the two regions is indicated by the difference in location of the peaks in the likelyhood at around 80 km depth. The peak for TE is at about 4.1 km/s and that for the EEP is at about 4.4 km/s, giving a contrast of about 7%.

    References

  • Lomax, A., and R. Snieder, (1995a), Identifying sets of acceptable solutions to non-linear, geophysical inverse problems which have complicated misfit functions, Nonlinear Processes in Geophys., 2, 222-227. (view Abstract)

  • Lomax, A., and R. Snieder, (1995b), 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, Gephys. J. Int., 123, 169-182. (view Abstract)

    Anthony Lomax - anthony@alomax.net

    Last update: 15 Dec 1995