Large-scale variations in inner core anisotropy

Abstract

I analyze nearly 2000 handpicked differential times of core-penetrating compressional waves to image lateral variations in the anisotropic structure of the solid inner core. The inner core is strongly anisotropic (2-4% on average) throughout most of the western hemisphere from near the surface to its center and into the lowermost several hundred kilometers of the eastern hemisphere. In contrast, the outer half of the eastern hemisphere from 40° to 160° E (the quasieastern hemisphere) exhibits very weak anisotropy 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 average of compressional wave speeds is the same in the eastern and western hemispheres, suggesting that there are no large-scale lateral variations in the chemistry or temperature in the inner core. Instead, I suggest that the variations 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 containing well-aligned crystals and the remaining part (uppermost 400-700 km of the quasi-eastern hemisphere) containing less well aligned crystals. A mechanism consistent with axisymmetry and large-scale variability of anisotropy is discussed. It incorporates the inference that the inner core is rotating faster than the mantle and that gravitational coupling to the mantle forces large-scale inner core deformation, inducing flow with strain rates as high as 10-14 s-1. A positive feedback mechanism, related to anisotropic viscosity, may reinforce lateral variations in the strength of anisotropy. Copyrighted 1999 by the American Geophysical Union.