Mantle deformation or processing artefact?

Measurements of shear-wave splitting represent an important tool for determining seismic anisotropy and for quantifying deformation processes in the Earth. Wookey et al. 1 claim to have observed significant seismic anisotropy in the mid-mantle between the Tonga–Kermadec subduction zone and Australia. We argue that their results are likely to be methodological artefacts and that the available data can be explained by moderate anisotropy in the upper mantle close to the seismograph stations. The lack of evidence for anisotropy in the mid-mantle nullifies any related geodynamic inferences.