Depth extent of inner-core seismic anisotropy and implications for geomagnetism

To constrain the elastic structure of the Earth's inner core, we have picked the differential times of 879 core-penetrating body waves from vertical-component, short-period seismograms recorded by global and regional seismic networks. Using a cross-correlation technique, we measure the difference in arrival times of P'(BC)-P'(DF) and P'(AB)-P'(DF) where the P'(DF) (PKIKP) phase penetrates the inner core, while both the P'(BC) (PKP-BC) and P'(AB) (PKP-AB) phases bottom in the outer core, P'(BC)-P'(DF) times for paths that are nearly parallel to the Earth's spin axis are consistently 2-4 s larger than predicted using the Preliminary Reference Earth Model (PREM), while rays in other directions have a mean and standard deviation of 0.2 +/- 0.4 s. P'(AB)-P'(DF) times, which correspond to P'(DF) rays turning deeper in the inner core, are 3-6 s for rays nearly parallel to the spin axis and 0.3 +/- 0.9 s for rays not near the spin axis. These observations lead to the robust conclusion that the inner core is strongly anisotropic. The level of anisotropy in our model is about 3% at a radius of 1000 km (depth of 200 km) and increases to about 4% at a radius of about 700 km. Below this radius, our resolution is poor, but the anisotropy appears to weaken. Resolution is also weak in the outer 200 km of the inner core, but the anisotropy appears to diminish in this region as well, A simple model of hexagonally symmetric anisotropy aligned with the spin axis explains 74% of the variance. The symmetry direction which fits the data the best and explains 79% of the variance is at 80 degrees N, 120 degrees E. The locus of directions which explain 70-79% of the variance includes only 4% of the possible range of directions, and includes the spin axis direction. The observed anisotropy is most likely due to preferred alignment of elastically anisotropic crystals, We propose several new alignment mechanisms and all viable mechanisms seem to be associated with a strong toroidal magnetic field. An outstanding problem that requires further investigation is that first-principles calculations of seismic anisotropy of hexagonal close packed (hcp) iron suggest that anisotropy of order 3% is predicted. Thus, 100% alignment could go a long way towards explaining our observations, but seems highly unlikely.