Abstract
Assessment of contributions from shallow lithosphere to teleseismic
wave front distortion is a prerequisite for high-resolution regional
teleseismic tomography. Several methods have been proposed in the
past for the correction of these effects, e.g. application of station
correction terms. We propose an approach that is independent of the
subsequent inversion and uses the available a priori knowledge of
the crustal structure to calculate crustal traveltime effects of
teleseismic wave fronts. Our approach involves the construction of
a 3-D crustal model based on controlled source seismology data and
calculation of the associated traveltime anomalies for incoming teleseismic
wave fronts. The model for central Fennoscandia shows a maximum crustal
thickness of 64 km and includes a high-velocity lower crust as derived
for parts of the study area by previous authors. Traveltimes calculated
using finite differences for teleseismic waves travelling through
this crustal model are compared with those from the standard reference
model IASP91 and the residuals are used to correct observed teleseismic
arrival times at the SVEKALAPKO array. To test the performance of
this approach, in a second part of the study a synthetic traveltime
data set is obtained by tracing wave fronts through a mantle structure
with known velocity anomalies and the 3-D crustal model. This data
set is inverted with and without correction for crustal effects.
The 3-D crustal effects alone with a homogeneous mantle are also
inverted and the results show that the crustal effects propagate
down to 450 km. The comparison of the inversion results demonstrates
the need to apply appropriate 3-D crustal corrections in high-resolution
regional tomography for upper-mantle structure beneath the Baltic
Shield.
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