Seismic evidence for distinct anisotropy in the innermost inner core

Knowledge of the seismic structure of the innermost inner core is important for understanding the formation of the inner core(1-3). It has been suggested recently that this region may exhibit distinct seismic anisotropy(4,5). Here, we use the difference in travel times between seismic waves reflected at the underside of the inner core boundary and those traversing the inner core to constrain the seismic anisotropy. We calculated travel-time residuals for waves generated by two deep earthquakes that occurred in Indonesia and Argentina respectively, recorded by seismic arrays in Venezuela and China. The travel-time residuals are systematically larger, by about 1.8 s, for waves that travel roughly along the equatorial plane of the inner core (Indonesia-Venezuela) than for those travelling in a direction at an angle of similar to 28 degrees to the equatorial plane (from Argentina to China). The difference in travel times is arguably most sensitive to the structure near the centre of the Earth, and thus provides evidence for deep layering within the inner core. Our results are consistent with models invoking seismic anisotropy of the innermost inner core with the slowest direction tilted at an angle of similar to 45 degrees to the equatorial plane(4).