SEISMIC ANISOTROPY IN THE CORE-MANTLE TRANSITION ZONE

Split S waves observed at Hockley, Texas from events in the Tonga-Fiji region of the southwest Pacific show predominantly vertically polariz ed shear-wave (SV) energy arriving earlier than horizontally polarized (SH) energy for rays propagating horizontally through D ''. After cor rections are made for the effects of upper-mantle anisotropy beneath H ockley, a time lag of 1.5 to 2.0 s remains for the furthest events (93 .9 degrees-100.6 degrees), while the time lags of the nearer observati ons (90.5 degrees-92.9 degrees) nearly disappear. At closer distances, the S waves from these same events do not penetrate as deeply into th e lower mantle, and are not split. These observations suggest that a p atch of D '' beneath the central Pacific is anisotropic, while the man tle immediately above the patch is isotropic. The thickness of the ani sotropic zone appears to be of the order of 100-200 km. Observations o f shear-wave splitting have previously been made for paths that traver se D '' under the Caribbean and under Alaska. SH leads SV, the reverse of the Hockley observations, but in these areas the fact that SV lead s SH in the HKT data shown here suggests a different sort of anisotrop y 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 Sri is consistent with transverse isotropy with a horizontal axis of symmetry (HTI), an azim uthally anisotropic medium, and with a VTI medium formed by a hexagona l crystal. Given that (Mg,Fe)SiO3 perovskite appears unlikely to form anisotropic fabrics on a large scale, the presence of anisotropy may p oint to chemical heterogeneity in the lowermost mantle, possibly due t o mantle-core interactions.