Global azimuthal anisotropy in the transition zone.

Jeannot Trampert

Surface wave dispersion measurements for more than 100,000 minor and major 
arc Love wave overtones show strong evidence of azimuthal anisotropy in the 
transition zone. This inference is made in a two step procedure where we first 
construct azimuthally anisotropic phase velocity maps. The depth inversion, is 
made using Backus-Gilbert kernels exclusively sensitive to the transition zone. 

The strength of the anisotropy is a few percent and globally distributed 
throughout the transition zone. The $2\psi$ term is somewhat stronger than the 
$4\psi$ term and there is no simple correlation with surface tectonic features. 
This new observation puts strong constraints on the mineralogy and dynamics in 
the transition zone since this anisotropy is the likely result of aligned 
minerals, tilted layering or even organized pockets of partial melt.