Teleseismic P wave tomography across the Sorgenfrei-Tornquist Zone. Evaluation of TOR project results – Niels Bohr Institute - University of Copenhagen

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Niels Bohr Institute > Calendar > 2007 > Teleseismic P wave...

Teleseismic P wave tomography across the Sorgenfrei-Tornquist Zone. Evaluation of TOR project results

Speciale kollokvium: Lene Vandur Nielsen

The colloquium will be in Danish!

From August 1996 to July 1997 a rectangular array, 900km long and 100km wide, of seismographs were placed across the Sorgenfrei-Tornquist Zone that lies between Denmark and Sweden. The aim of this TOR (Teleseismic TOmography TORnquist) project was to study the lithosphere-asthenosphere system of the significant geological boundary, between the Baltic Shield and the younger European lithosphere in this region. Inversions of the data set have been made with a variety of methods. The general position is that the sharp transition, in the upper mantle below the crust, is dipping towards north-east or nearly vertical, but a dipping towards south-west is indicated in two of the inversion results.
I narrow down and describe these differences. The inversion is performed by using the most common tomographic inversion method ACH, named after the three authors Aki, Christoffersson, and Husebye (Aki et al., 1977), with a non-linear iterative scheme.

The results indicate a vertical slope of the transition zone. The horizontal velocity gradients support this result together with other velocity contrasts seen along the TOR profile. The influences of reference models, on the resulting structures, are examined.

The models do not displace the structures as feared. The resolution is low in the lithosphere beneath Denmark, and in a deeper area beneath Sweden. The dip of the transition zone is nearly resolved. The influence of data weighting gives pronounced differences in the dip of the transition zone. Also phase picking, data fit, robustness of the result, ray coverage, and uncertainties are examined and discussed.