The Origin of the Earth Inner Core's Seismic Anisotropy

Abstract

The Earth's inner core is of axisymmetric anisotropy for seismic waves, and there is an angle of about 11° between the symmetric axis of the inner core's seismic anisotropy and the Earth's axis. In this article, we intend to explore origin of inner core's seismic anisotropy basing on the observation that there is a differential rotation between the solid inner core and the molten outer core of the Earth. According to the theory on crystal growth, the crystal growth rate is proportional to the relative linear rate between the solid and the molten crystal. As a result, the inner core grows faster at the equator than at pole area. Under the gravitational force, the inner core remains quasi-spherical, with a material flow from the equator where the growth is faster to the pole area where the growth is slow, and results in an axial symmetric viscous flow. This flow is accompanied by an axial symmetric stress field, which makes c-axes of hcp iron align with inner core's rotation axis, resulting in observed seismic anisotropy. As a deduction, there may be a nutation as well as a precession of the Earth's inner core relative to the mantle and crust.