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Title: How does the inner core rotate relative to the mantle?
Plain: 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: 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.
Settled by: A global, consistent analysis of repeating-earthquake doublets that separates rotation from local inner-core boundary changes.
Kind: well-posed
Answer type: value
References: (empty)
Literature status: contested
Status 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.
Posed since: 1996