Abstract
Seismic waveforms recorded at high-density receiver arrays facilitate
the application of new inversion techniques, which take advantage
of the coherent nature of the observations. We use measurements of
shear wave splitting parameters from observed SKS waveforms along
a dense receiver profile and compare them with splitting parameters
obtained from numerical waveform modeling through anisotropic Earth
models. We use two different iterative approaches for the inversion
of the observed splitting parameters (1) a local optimization technique
(the downhill simplex method) and (2) a global genetic algorithm
search. In our forward modeling, we calculate SKS waveforms by a
finite difference (FD) method solving the anisotropic wave equation,
instead of deriving individual anisotropic models for each station
and combining them into one model. By the comparison of FD modeling
and observations we avoid a direct interpretation of the splitting
parameters in terms of medium properties. We apply these techniques
to the analysis of SKS phases recorded along a 100-km profile located
at the Dead Sea transform fault. The measured splitting parameters
show gradual short-scale variations along the profile which depend
on frequency. Lateral and vertical variations of anisotropy are accounted
for by two-dimensional block structures. For the simplex method the
complexity of the models (the number of anisotropic blocks) is increased
gradually, whereas the number of anisotropic blocks is kept fixed
for the genetic algorithm search. The anisotropic structures of the
best fitting models derived from the two inversion approaches agree
well. The results support earlier interpretations of the observations
in terms of a narrow, approximately 20 km wide, vertical decoupling
zone in the mantle that accommodates the transform motion between
the African and Arabian plates.
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