Seismic anisotropy in the core-mantle transition zone

Abstract

Split S waves observed at Hockley, Texas, from events in the Tonga-Fiji region of the southwest Pacific show predominantly vertically polarized shear-wave (SV) energy arriving earlier than horizontally polarized (SH) energy for rays propagating horizontally through D'. After corrections are made for the effects of upper-mantle anisotropy beneath Hockley, a time lag of 1.5 to 2.0 s remains for the furthest events (93.9°-100.6°), while the time lags of the nearer observations (90.5°-92.9°) nearly disappear. At closer distances, the S waves from these same events do not penetrate as deeply into the lower mantle, and are not split. These observations suggest that a patch of D' beneath the central Pacific is anisotropic, while the mantle immediately above the patch is isotropic. The thickness of the anisotropic zone appears to be of the order of 100-200km. Observations of shear-wave splitting have previously been made for paths that traverse D' under the Caribbean and under Alaska. SH leads SV, the reverse of the Hockley observations, but in these areas the fact that SV leads SH in the HKT data shown here suggests a different sort of anisotropy under the central Pacific from that under Alaska and the Caribbean. The case of SH travelling faster than SV is consistent with transverse isotropy with a vertical axis of symmetry (VTI) and does not require variations with azimuth. The case of SV leading SH is consistent with transverse isotropy with a horizontal axis of symmetry (HTI), an azimuthally anisotropic medium, and with a VTI medium formed by a hexagonal crystal. Given that (Mg,Fe)SiO3 perovskite appears unlikely to form anisotropic fabrics on a large scale, the presence of anisotropy may point to chemical heterogeneity in the lowermost mantle, possibly due to mantle-core interactions.