Large-scale variations in inner core anisotropy

I analyze nearly 2000 handpicked differential times of core-penetrating com pressional waves to image lateral variations in the anisotropic structure o f the solid inner core. The inner core is strongly anisotropic (2-4% on ave rage) throughout most of the western hemisphere from near the surface to it s center and into the lowermost several hundred kilometers of the eastern h emisphere. In contrast, the outer half of the eastern hemisphere from 40 de grees to 160 degrees E (the quasi-eastern hemisphere) exhibits very weak an isotropy with an average amplitude of only 0.5%. The symmetry direction is the fast direction and lies on or near the spin axis. Voigt's isotropic ave rage of compressional wavespeeds is the same in the eastern and western hem ispheres, suggesting that there are no large-scale lateral variations in th e chemistry or temperature in the inner core. Instead, I suggest that the v ariations seen in anisotropy represent lateral variations in the degree of crystal alignment. The inner core appears to be organized in a very simple way with 60-90% of its volume containingwell-aligned crystals and the remai ning part (uppermost 400-700 km of the quasi-eastern hemisphere) containing less well aligned crystals. A mechanism consistent with axi-symmetry and l arge-scale variability of anisotropy is discussed. It incorporates the infe rence that the inner core is rotating faster than the mantle and that gravi tational coupling to the mantle forces large-scale inner core deformation, inducing flow with strain rates as high as 10(-14) s(-1). A positive feedba ck mechanism, related to anisotropic viscosity, may reinforce lateral varia tions in the strength of anisotropy.