Seismological evidence for mosaic structure of the surface of the Earth's inner core

The transition from the Earth's solid inner core to liquid outer core is the location where the inner core grows1 and from which compositional convection in the outer core originates2,3. Most seismological models of the Earth describe the inner-core boundary as sharp4,5 and simple6,7,8, although experimental data requiring the presence of a thin transition layer at the bottom of the outer core have been reported9. The density jump at the inner-core boundary—an important parameter determining gravitational energy release10 and constraining the compositional difference between the inner and outer core—is also not well known. Estimates of this density jump obtained using free-oscillation eigenfrequencies give low values11,12,13 of 0.25–1.0 g cm-3, whereas a method using the amplitude ratio of core-reflected phases yielded values of 0.6–1.8 g cm-3 (refs 14, 15, 16–17). Here we analyse properties of waves precritically reflected from the Earth's inner core (PKiKP phases) that show significant variability in amplitude, consistent high-frequency content and stable travel times with respect to a standard Earth model4. We infer that the data are best explained by a mosaic structure of the inner core's surface. Such a mosaic may be composed of patches in which the transition from solid inner to liquid outer core includes a thin partially liquid layer interspersed with patches containing a sharp transition.