Resolving topography of the inner core boundary (ICB) and the structure and composition of the nearby region is key to improving our understanding of solidification of the Earths inner core. Observations of travel times and amplitudes of short-period seismic phases of PKiKP and PcP reflected, respectively, off the inner and outer boundary of the liquid core, provide essential constraints on the properties of this region. We revisit heterogeneities of ICB using a total of more than 1,300 new differential travel times and amplitude ratios of PKiKP and PcP measured at 3.2-35.2 degrees and reflected off the cores boundaries under Northeastern Asia and South America. We observe a statistically significant systematic bias between the measurements collected in the two spots. We carefully examine its origin in terms of contributions by various Earths shells and find that most of variance in PKiKP-PcP differential travel times measured above the epicentral distance of 16.5 degrees in Northeastern Asia can be accounted for by mantle corrections. We find slight disparity of about 1-3 km between the outer core thickness under Asia and America; the ICB density jump under Northeastern Asia is about 0.3 g/cm(3), which is three times as small as under South America. The findings are interpretable either as evidence for inner core hemispherical asymmetry, whereby crystallization dominates in the West and melting in the East (not vice versa), or in terms of two disconnected mosaic patches with contrasting properties.
Funding Agencies|Russian Foundation for Basic Research [18-05-00619]; Seismological Facilities for the Advancement of Geoscience and EarthScope (SAGE) Proposal of the National Science Foundation [EAR-1261681]