Aspherical structural heterogeneity within the uppermost inner core: Insights into the hemispherical boundaries and core formation

Abstract Lateral heterogeneities at the top of the inner core are investigated using earthquakes that occurred in Indonesia and southeast Asia and were recorded in the southeastern Caribbean. Using seismic observations of attenuation and seismic velocity, we were able to constrain the characteristics of the boundary between the inner and outer core to further investigate the dynamics and evolution of the Earth’s core. Our seismic observations from core phases confirm that the outermost inner core is asymmetrically heterogeneous and we are able to further constrain the morphology and physical properties of this layer. Comparison of data from earthquakes with ray paths traversing from east to the west versus those with ray paths from west to east allow us to map the aspherical heterogeneity of the boundary layer and specifically image the boundary between the proposed quasi-eastern and western hemispheres of the inner core. The variation of differential travel times between PKP df and PKP bc , attenuation in terms of Q factor, and latitudinal changes for both of these observations, can be attributed to localized heterogeneity at the quasi-hemispherical boundaries of the inner core. We constrain the change in the thickness of outermost core boundary layer from 100 to 250 km within a distance of a few 10s of kilometers at 45°E ± 2°, for the western boundary, with an overall P-wave velocity decrease in the western hemisphere of 0.5% and increase of 0.5% in the eastern hemisphere. We constrain the eastern boundary at latitudes greater than 45°N to 173°E ± 4° with an overall P-wave velocity decrease in the western hemisphere of 1.0% in the uppermost 200 km of the inner core. The eastern boundary at equatorial latitudes is constrained to a region