Predictive mechanism for anisotropy development in the Earth's inner core

Abstract

We present a model for the grain growth and deformation of the Earth's inner core that explains the dominant features of seismic velocity and attenuation anisotropy. At the surface of the inner core, the dominant deformation mechanism is diffusion creep. Grain growth continues via grain-boundary migration recrystallization such that, depending upon the magnitude of stress and grain size, dislocation-controlled creep dominates in as little as 1–2 m or as much as ~ 100 km below the crystallization horizon. Subsequent dislocation link climb leads to the development and maintenance of lattice-preferred orientation (LPO) increasing seismic velocity anisotropy with depth. The model predicts an inner core viscosity of 10 20–22 Pa-s deforming in a regime with a deviatoric stress of 10 3–4 Pa, consistent with an inner core anisotropic growth model. We provide explanations for the development of shape-preferred orientation of grains elongated in the equatorial direction in the context of the fast direction of seismic waves correlating to the direction with the greatest attenuation. We predict high attenuation at the top of the inner core, decreasing with depth, with an attenuation that is 30% greater along polar paths than for equatorial paths through the center of the Earth.