Seismic structure and dynamic process of the Earth's inner core and its boundary

Abstract

Modern seismology has revealed complex structures of the Earth’s inner core and its boundary. The interior of the Earth’s inner core exhibits both velocity and attenuation anisotropy, and hemispherical variations of velocity, attenuation and anisotropy, while the surface of the inner core possesses topography and mushy zone in some localized regions. The inner core velocity and attenuation anisotropy is cylindrical with the axis of high velocity and high attenuation parallel with the Earth’s rotation axis, and depth dependent with the topmost 100–400 km close to be isotropic and the innermost 300–600 km possibly possessing a distinct anisotropy. The hemispherical variations in the inner core are manifested in many aspects: the isotropic velocity of the topmost ~100 km of the eastern hemisphere is 0.8% faster than that of the western hemisphere, the attenuation in the top ~100 km of the eastern hemisphere (Q=250) is stronger than that of the western hemisphere (Q=600), the top isotropic layer is thinner in the western hemisphere (~100 km) than in the eastern hemisphere (~400 km), and the inner core anisotropy in the western hemisphere (~4%) is stronger than in the eastern hemisphere (~0.7%). In the boundary, the inner core exhibits topographic variations with height changes of 1–14 km beneath the Philippine Sea, the Yellow Sea, the western Pacific and Central America, and a mushy zone with a thickness of 4–8 km beneath the southwest Okhotsk Sea. These new findings have motivated a series of new physical mechanisms in the scientific community and demand for reevaluation of outer core composition, thermal-compositional convection in the outer core, inner core solidification process and driving force of Earth’s geodynamo. These results suggest that the inner core solidification process and the driving force of Earth’s geodynamo are not laterally homogeneous, as it has long been held in the traditional views. Solidification may occur in the western hemisphere releasing latent heat and light elements, while melting may occur in the eastern hemisphere absorbing latent heat and light elements. Therefore, the driving forces for the outer core convection in these two hemispheres may be different and may even be in opposite signs. Such alternative solidification and melting should also occur in the localized topographic regions. Moreover, the solidification in the region of a mushy zone would release both thermal and chemical energy, while that in other regions of a sharp inner core boundary only releases thermal energy.

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