S-wave splitting of SK(K)S-phases and laterally varying anisotropy in the Upper Rhine Graben area
Splitting of seismic shear waves (birefringence) can be used to study anisotropy in the Earth. Existing teleseismic S-wave (SK(K)S) splitting measurements vary with backazimuth in the Upper Rhine Graben area and therefore indicate complex including laterally varying anisotropy. Motivated by this observation further S-wave splitting measurements were carried out at broadband seismic stations to improve the model of the anisotropy underneath the Upper Rhine Graben area. To address anisotropy in the mantle PKS-, SKS- and SKKS-phases of teleseismic earthquakes within an epicentral distance range [90,140]° were considered. The splitting parameters, fast polarization direction ϕ given as angle to the north and delay time δt accumulated between the two quasi S-waves, were determined with the MATLAB-program SplitLab with the plugin StackSplit. As corner frequencies for the bandpass-filter [0.020,0.066] Hz (lower corner) and [0.15,0.20] Hz (upper corner) were chosen, and the rotation-correlation-method and the minimum-energy-method were applied both for comparisons. For the development of structural anisotropy models the determined (apparent) splitting parameters were compared with forward-calculated (apparent) splitting parameters of synthetic anisotropy models. Synthetic anisotropy models with one layer and two layers with transverse isotropy with horizontal and tilted symmetry axis were tested. The station-related results will be used to outline basic anisotropy regimes to find a laterally varying anisotropy model for the Upper Rhine Graben area.
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