Hemispherical variations in seismic velocity at the top of the Earth's inner core

Knowledge of the seismic velocity structure at the top of the Earth's inner core is important for deciphering the physical processes responsible for i nner-core growth(1-3). Previous global seismic studies(4-9) have focused on structures found 100 km or deeper within the inner core, with results for the uppermost 100 km available for only isolated regions(10-12). Here we pr esent constraints on seismic velocity variations just beneath the inner-cor e boundary, determined from the difference in travel time between waves ref lected at the inner-core boundary and those transmitted through the inner c ore. We found that these traveltime residuals-observed on both global seism ograph stations and several regional seismic networks-are systematically la rger, by about 0.8 s, for waves that sample the 'eastern hemisphere' of the inner core (40 degrees E to 180 degrees E) compared to those that sample t he 'western hemisphere' (180 degrees W to 40 degrees E). These residuals sh ow no correlation with the angle at which the waves traverse the inner core ; this indicates that seismic anisotropy is not strong in this region and t hat the isotropic seismic velocity of the eastern hemisphere is about 0.8% higher than that of the western hemisphere.