Distinct layering in the hemispherical seismic velocity structure of Earth's upper inner core

The existence of hemispherical variation in the Earth's inner core is well-documented, but consensus has not yet been reached on its detailed structure. The uppermost layers are a region of particular importance, as they are directly linked to the growth processes and post-solidification mechanisms of the inner core. Here, we use a large PKIKP-PKiKP differential travel time residual data set to derive a model for the upper inner core, providing new constraints on its isotropic and anisotropic velocity, and the amount of scattering. We find that the eastern and western hemisphere are separated by sharp boundaries. This is incompatible with the recently proposed inner core translation model, but might be explained by differences in outer core convection and inner core solidification rates. The eastern hemisphere displays weak anisotropy of 0.5%-1.0%. The western hemisphere, on the other hand, is characterized by the presence of an isotropic upper layer with a thickness of 57.5 km, with anisotropy of 2.8% appearing at deeper depths. The boundary between the isotropic layer and the deeper anisotropy appears sharp. We also detect, for the first time, a high velocity layer at the top of the eastern hemisphere with a thickness of 30 km, which we interpret as being due to an increased amount of light elements. There appears to be no relationship between the layered structure in the two hemispheres, with abrupt changes in velocity with depth in one hemisphere without any significant change at the same depth in the other hemisphere. Our results indicate that there is a difference in composition and mineral structure between the hemispheres, resulting in differing responses to external processes.