The lowermost mantle beneath northern Asia - II. Evidence for lower-mantle anisotropy

We have analysed prediffracted S-waves with turning points beneath northern Siberia in a study of anisotropy in the lowermost mantle. Deep-focus earthquakes beneath the Marianas, Izu Bonin and the Sea of Japan recorded at stations in western Europe are used. A correction for upper-mantle anisotropy is applied to the data. Comparisons of the data with synthetic data for models with and without a high velocity D layer suggest that there is a velocity discontinuity at the top of the D region and that the style of anisotropy is transversely isotropic in this region. Time separations between S-waves on the radial and transverse component show a weak trend where the separation increases with epicentral distance. A normalization of this separation with the travel distance within D (300 km thick in this region) suggests that the anisotropy is uniformly distributed within this layer and has an average value of 0.5 per cent. A combination of different studies which investigate the structure of the lowermost mantle beneath Europe and northern Siberia reveals a complicated picture. Tomographic models from this area and evidence of D anisotropy, lower mantle scatterers, reflections from a D discontinuity and ultra-low-velocity zones suggest two distinct regions. One exhibits high velocities, D anisotropy, a D discontinuity and no evidence of scatterers or ultra-low-velocity zones. These features are likely associated with the palaeosubduction of the Izanagi plate well into the lowermost mantle. The other region has a lower overall velocity and shows evidence of scatterers and ultra-low-velocity zones, perhaps suggesting the presence of partial melt. These results suggest dramatic lateral variations in the nature of the lowermost mantle beneath northern Asia over a length scale of roughly 30 degrees.