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.

Korea Polar Research Institute (KOPRI) has successfully installed two autonomous very broadband threecomponent seismic stations at the King George Island (KGI), Antarctica, during the 24th KOPRI Antarctic Summer Expedition (2010~2011). The seismic observation system is originally designed by the Incorporated Research Institutions for Seismology Program for Array Seismic Studies of the Continental Lithosphere Instrument Center, which is fully compatible with the Polar Earth Observing Network seismic system. The installation is to achieve the following major goals: 1. Monitoring local earthquakes and icequakes in and around the KGI, 2. Validating the robustness of seismic system operation under harsh environment. For further intensive studies, we plan to move and install them adding a couple more stations at ice shelf system, e.g., Larsen Ice Shelf System, Antarctica, in 2013 to figure out ice dynamics and physical interaction between lithosphere and cryosphere. In this article, we evaluate seismic station performance and characteristics by examining ambient noise, and provide operational system information such as frequency response and State-Of-Health information.