On the possibility of anisotropy in the D″ layer as inferred from the polarization of diffracted S waves

Abstract

S waves diffracted along the core-mantle boundary (Sd) show in a few cases anomalously large SV components. To test if azimuthal anisotropy in D″ can be a cause for these anomalies, a method based on the Langer approximation and on perturbation theory is developed to model Sd waves in the presence of anisotropy in D″. By comparing the synthetics in azimuthally anisotropic models with those in isotropic or transversely isotropic models, we search for the characteristic features in the data which would discriminate between the different models. The SVd/SHd amplitude ratio turns out not to be a discriminating feature. A ratio of 30% at an epicentral distance of 115° can, for example, be explained by 1% of azimuthal anisotropy at the core-mantle boundary (CMB), decreasing linearly to zero 150 km above, by a mean of 0.5% azimuthal anisotropy through D″, by a realistic negative velocity gradient in an isotropic model of D″, or by transverse isotropy in D″. The phase difference between the SH and SV components and their frequency content are not discriminating either. Thus, with a single recording, azimuthal anisotropy cannot be discriminated from isotropy or transverse isotropy in D″. The only effect characteristic of azimuthal anisotropy is that the horizontal particle motion of Sd hardly depends on the focal mechanism, as opposed to the linear dependence which exists in isotropic or transversely isotropic models. Also, it becomes invariant with epicentral distance beyond 115°. To discriminate between different kinds of models, the particle motion of Sd waves from several events with known focal mechanisms therefore needs to be analysed. To test the hypothesis of Vinnik et al. (Geophys. Res. Lett., 16: 519–522, 1989) concerning the origin of the anomalous SVd they observed, we analyse three of their data. These are Fiji events recorded at the North American Geoscope station WFM and at the North Atlantic World Wide Standardized Seismograph Network station BEC, all showing large SVd waves. The Sd waveforms of the two events recorded at the smallest epicentral distance cannot be explained simultaneously by a laterally homogeneous isotropic or a transversely isotropic model for D″. An azimuthally anisotropic model with 1% azimuthal anisotropy at the CMB explains them better. However, neither an isotropic nor an anisotropic model could be found which explains the waveforms observed at the furthest station. The hypothesis of azimuthal anisotropy in D″ under the Pacific Ocean to explain these clearly anomalous waveforms is not confirmed. A much larger data set should, however, be analysed before drawing a conclusion for that region. The example of the data of Vinnik et al. shows that the clear predictions which are made here as to which waveforms are expected if D″ is anisotropic allow discriminating studies of D″.