Bokelmann, G.H.R., 1995, P-wave array polarization analysis and effective
anisotropy of the brittle crust, Geophysical Journal International,
120, 145-162
P-wave polarization constrains local anisotropy in the vicinity of the receivers. Using three-component
and array data from the
regional GERESS array in southeastern Germany, we measure polarization a and propagation vectors s
for P-phases of 120 events.
Angular deviations a-s between these normalized vectors often approach 10 degrees, rendering them easily
measurable. The effect of
anisotropy can be distinguished from remote effects, since all remote effects such as source mislocation,
distant lateral
heterogeneity or distant anisotropy affect polarization and propagation vectors simultaneously. Averaging
removes sensitivity to
near-receiver heterogeneity, and local anisotropy is left as sole cause of the effects in a-s.
This method hence gives local effective
anisotropy in the near-receiver crust averaging over a depth interval of a wavelength (approximately 6 km). We resolve strike and dip of the
symmetry plane and also two dimensionless numbers eta and tau
which give constraints on four of the elastic parameters. The optimum
model (variance reduction 44%) has symmetry plane orientation of strike 113 deg and dip 49 deg to the North, which corresponds closely
to the consistently observed gneiss foliation direction in the area (120 deg, 50-60 deg). Comparing eta and tau with predictions from different
physical models we find that the data are fit by a gneiss model assuming that the anisotropy is dominated by the mica, if 3% to 8% of
the mica are well-aligned. This suggests that anisotropy in the region studied is dominated by the effect of local foliation rather than
the regional stress field.
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