The innermost inner core of the earth: Evidence for a change in anisotropic behavior at the radius of about 300 km

Since the discovery of the inner core in 1936, no additional spherical subshell of the Earth has been observed. Based on an extensive seismic data set, we propose the existence of an innermost inner core, with a radius of ∼300 km, that exhibits a distinct transverse isotropy relative to the bulk inner core. Specifically, within the innermost inner core, the slowest direction of wave propagation is ∼45° from the east-west direction. In contrast, the direction of the slowest wave propagation in the overlying inner core is east-west. The distinct anisotropy at the center of the Earth may represent fossil evidence of a unique early history of inner-core evolution.