Anisotropy in the deep Earth

Anisotropy in the inner core was proposed 30 years ago to explain faster P wave propagation along the direction of the Earth's axis of rotation as well as anomalous splitting of core sensitive free oscillations. There is still uncertainty about the origin of this anisotropy. In particular, it is difficult to explain its strength, based on known elastic properties of iron, as it would require almost perfect alignment of iron crystals. Indeed, the strongly anomalous P travel times observed on paths from the South Sandwich Islands to Alaska may or may not be due to inner core anisotropy, and will need to be explained before consensus can be reached on the strength of anisotropy in the inner core and its origin.