How does the inner core rotate relative to the mantle?
In plain words
Earthquake waves passing through the inner core change slightly over decades, suggesting it spins a little faster or slower than the rest of Earth. Whether it speeds ahead, oscillates back and forth, or changes shape instead is disputed.
Precise statement
Using temporal changes in PKIKP waveforms from repeating earthquakes and explosions since the 1960s, determine the differential rotation rate history $\omega_{\mathrm{ic}}(t)$ of the inner core relative to the mantle and whether it is a steady super-rotation, a multidecadal oscillation (period near 70 yr), a shorter oscillation (about 6 yr), or an artifact of inner-core surface deformation. An answer is $\omega_{\mathrm{ic}}(t)$ with uncertainties that jointly fits seismic, length-of-day and geodetic data.
What would settle it
A global, consistent analysis of repeating-earthquake doublets that separates rotation from local inner-core boundary changes.
Status in the literature
Unverified note
Yang and Song 2023 and Wang and coauthors 2024 report a recent slowdown or backward motion relative to the mantle; Vidale and coauthors 2025 (Nature Geoscience, https://doi.org/10.1038/s41561-025-01642-2) report annual-scale changes of both rotation and the inner-core near surface, so rotation and boundary deformation are not yet separated.