Fig. 4: Schematic model of the inner core (IC) containing an innermost inner core (IMIC) and various body-wave probing methods to the Earth’s center.

A On the left panel, the IC model contains IMIC with two distinct P-wave transversely isotropic patterns in outer inner core (OIC) and IMIC; black and red bars represent the fast and slow anisotropic direction; a vertical gray line represents the Earth’s rotation axis (ERA). On the right panel are cylindrically anisotropic models of OIC (black dashed line) and IMIC (gray line). Schematic views of IMIC-sampling methods using (B) absolute PKIKP waves, (C) PKIKP multiples (this study) – blue and orange ray lines represent ray paths of the second and fourth PKIKP multiples, and (D) correlation feature I2* at the two receivers (blue ray paths) results from cross-correlating high-multiples of PKIKP (orange ray paths). The cross-sections of the Earth in the right column contain ray path segments sampling the OIC and IMIC. Yellow stars are sources, and blue inverted triangles are receivers.