Origin of seismic anisotropy in the D″ layer inferred from shear deformation experiments on post-perovskite phase

Seismic anisotropy is one of the significant features in the D″ layer of the Earth and is thought to be derived from the lattice preferred orientation (LPO) of constituent materials or shape preferred orientation (SPO) of heterogeneous materials such as melt and inclusions. Recent experimental and theoretical studies strongly suggest that the D″ layer consists mainly of a MgSiO₃ post-perovskite phase together with ferro-periclase. To understand the anisotropy in the D″ layer, we have conducted a series of simple shear deformation experiments at high temperature and pressure on polycrystalline CaIrO₃ as an analogue of MgSiO₃ and measured the LPO of the post-perovskite phase. Crystallographic orientation analysis of the deformed post-perovskite phase showed strong LPOs with the dominant slip system being [100](010). Calculation of the elastic wave velocities considering the effect of LPOs of post-perovskite and ferro-periclase showed as azimuthal and polarization anisotropies in the horizontal shear plane where the velocity of horizontally polarized S-wave is considerably faster than that of vertically polarized S-wave. Thus, the seismic anisotropy observed in the D″ layer can be reasonably explained by the LPO of the mixture of post-perovskite and ferro-periclase, where the LPO may result from the horizontal shear flow generated by the mantle convection.