Phys. Earth Planet. Int. http://dx.doi.org/10.1016/j.pepi.2011.08.008 (2011)

Seismic waves that pass through Earth's inner core slow down when travelling in some directions, but are unaffected in others. Numerical modelling indicates that this variation may be related to the stratification of the inner core.

Philippe Cardin at the Université Joseph-Fourier, Grenoble, and colleagues combined a numerical model of texture development in iron crystals with simulations of the dynamic evolution of Earth's inner core. The simulations show that as the inner core grows — through the addition of new material around its equator — it becomes radially stratified and forms concentric rings. Flow is concentrated along these rings so that strain is transferred from the equator of the inner core to its upper regions. Deformation of the iron crystals in the mid- to polar regions destroys the ancient texture. In contrast, deeper parts of the inner core are protected, preserving the texture of the crystals.

Together, the ancient iron-crystal texture preserved in deeper parts and deformation of upper parts of the inner core can slow passing seismic waves in the unusual manner measured at the surface.