Regionally heterogeneous uppermost inner core observed with Hi‐net array

Studies of velocity structure in the Earth's inner core and its volumetric variations illuminate our understanding of inner core dynamics and composition. Here we use an extensive number of seismograms recorded by the Hi‐net array to construct complete empirical curves of PKP BC ‐PKP DF differential traveltimes. The nature of these curves implies that significant variations in traveltimes are accumulated during the passage of PKP DF waves through the uppermost inner core and rules out outer core structure effects. Uniform cylindrical anisotropy of a plausible strength in the uppermost inner core can also be ruled out as a cause of the observed traveltime variations because the range of sampled ray angles is too narrow. The configuration and strength of inhomogeneities from a recent tomographic model of the lowermost mantle cannot account for the observed traveltime variations. Therefore, we infer that either variations of P wave isotropic velocity on the scale of about hundred km and less are present in the uppermost inner core or the material may be organized in distinctive anisotropic domains, and both of these features may be superimposed on long‐wavelength hemispherical structure. If the former holds true, the absolute magnitude of required P wave velocity perturbations from referent values is 0.60 ± 0.10% in the quasi‐eastern and 1.55 ± 0.15% in the quasi‐western hemisphere (0.85 ± 0.05% and 1.10 ± 0.10%, respectively, with the lowermost mantle correction). The existence of these variations is a plausible physical outcome given that vigorous compositional convection in the outer core and variations in heat exchange across the inner core boundary may control the process of solidification.