Equatorial anisotropy in the inner part of Earth's inner core from autocorrelation of earthquake coda

The Earth's solid inner core exhibits strong anisotropy(1-5), with wave velocity dependent on the direction of propagation due to the preferential alignment of iron crystals(6). Variations in the anisotropic structure, laterally and with depth(7-11), provide markers for measuring inner-core rotation(12) and offer clues into the formation and dynamics of the inner core(13,14). Previous anisotropy models of the inner core have assumed a cylindrical anisotropy in which the symmetry axis is parallel to the Earth's spin axis. An inner part of the inner core with a distinct form of anisotropy has been suggested(15), but there is considerable uncertainty regarding its existence and characteristics(16-19). Here we analyse the autocorrelation of earthquake coda measured by global broadband seismic arrays between 1992 and 2012, and find that the differential travel times of two types of core-penetrating waves vary at low latitudes by up to 10 s. Our findings are consistent with seismic anisotropy in the innermost inner core that has a fast axis near the equatorial plane through Central America and Southeast Asia, in contrast to the north-south alignment of anisotropy in the outer inner core. The different orientations and forms of anisotropy may represent a shift in the evolution of the inner core.