Xinlei Sun and Xiaodong Song
Department of Geology, University of Illinois at Urbana-Champaign
Recent studies suggest that the inner core is more complex than a simple model of uniform anisotropy. The uppermost inner core was found to be nearly isotropic, western and eastern hemispheres of the inner core were found to be different in velocity and their anisotropy. Yet differential PKP travel times at near antipodal distances show convincingly that significant anisotropy exists over the bulk of the inner core. To investigate how the inner core anisotropy changes laterally and with depth, we combine differential PKP travel-time measurements, obtained by us and others over the years, at various distances sampling different depths of the inner core. We examine large-scale structure of the inner core by dividing the inner core into four regions: a quasi-eastern hemisphere (40-160 degrees east) and a quasi-western hemisphere (the rest of the longitudes); and two layers in the radial direction, which are allowed to have different thickness in different hemispheres. Our results suggest that that the differential times can be fitted with a nearly isotropic upper inner (UIC) core and an anisotropic lower inner core (LIC). The depth of the UIC/LIC boundary is about 200~km in the western hemisphere of the inner core and 700~km in the eastern hemisphere. Our estimate of the degree of anisotropy in the LIC is from 4\% to 5\% in eastern hemisphere and from 6\% up to 8\% in western hemisphere.