The Earth’s core plays a central role in regulating planetary rotation and generating the geomagnetic field. Traditionally, scientists believed the inner core rotated steadily with, if not slightly faster than, the mantle. However, recent evidence suggests that the inner core does not maintain a constant pace. Instead, it may slow down, pause, or even reverse direction relative to the mantle.
This emerging understanding raises an important question: how does this oscillatory motion influence Earth’s overall rotation, the length of a day, and the behavior of its magnetic field?
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What Does “Oscillation” of the Inner Core Mean?
Oscillation refers to the repetitive back-and-forth variation of a physical quantity around an equilibrium state. In the context of Earth’s inner core, this describes periodic changes in its rotation, either shifting speed or even reversing direction, relative to the mantle and crust.
The inner core is a solid iron-nickel sphere ~1,220?km in radius enclosed by the liquid outer core, which mechanically decouples it from the mantle and crust and allows differential rotation. Its motion reflects a balance between electromagnetic torque from the geodynamo, which drives rotation, and gravitational coupling with the laterally heterogeneous mantle, which acts as a restoring force.
When magnetic forcing alters the inner core’s rotation relative to the mantle, gravitational coupling counteracts this change and returns the system toward equilibrium, producing periodic back-and-forth motion.1
Early seismic interpretations suggested superrotation of the inner core, with estimates of up to about one degree per year faster than the mantle and crust. Later studies challenged this view, showing that the relative motion is not constant.
For instance, a 2022 USC study, analyzing nuclear explosion data from 1969–1974, demonstrated periodic reversals in rotation direction, indicating that the inner core’s motion slowed and sometimes reversed.2
More recently, a recent global earthquake doublet analysis by Peking University researchers further revealed a multidecadal oscillation, showing that inner core rotation nearly stopped or slowed around 2009, consistent with variable relative motion.3
These observations collectively support the view that the inner core does not maintain a steady superrotation but instead exhibits time-varying, oscillatory behavior influenced by interactions with the mantle and outer core.
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Evidence: Peering Deep with Seismic Waves
But how do scientists detect motion thousands of kilometers beneath Earth’s surface? They rely on seismic waves generated by earthquakes and historical nuclear tests.
Primary waves (P waves) that travel through (or reflect off) the inner core provide critical insights into its structure. When the inner core shifts relative to the mantle, subtle changes occur in internal scatterers or boundary positions. These shifts alter seismic wave arrival times and affect waveform similarity between repeated seismic events. By carefully comparing these signals, researchers can detect even slight variations in inner core motion.4
Using this approach, USC researchers analyzed seismic records from underground nuclear tests conducted between 1969 and 1974. The results showed reversals in the direction of inner-core rotation over a timescale of roughly 6 years, with superrotation of about 0.1° per year and subrotation averaging 0.05° per year, contradicting models of steady superrotation.
The inferred oscillation was found to correlate with observed variations in Earth’s length of day of 0.01–0.12 milliseconds, consistent with angular momentum exchange between the inner core and the mantle. This agreement strengthened the interpretation that the short-term length-of-day variations are partly driven by inner-core differential rotation.2
More recently, Peking University researchers extended this methodology to global datasets of 172 repeating earthquake doublets spanning several decades from the early 1990s through 2021. This study reported a marked reduction in temporal seismic changes after about 2009, with waveform similarity values rising from below 0.6 to near 1.0 and double-differential times showing a negative trend of −15?±?8 milliseconds per year, suggesting a slowdown or reversal in inner-core motion.
When combined with earlier data, the observations are best explained by a multidecadal oscillation with an approximate 70-year period, featuring turning points in the early 1970s and around 2009. The convergence of results from nuclear explosion data and earthquake doublets, which rely on different seismic phases and analytical techniques, provides strong evidence for long-period oscillatory behavior.3
Additionally, satellite-based and geodetic analyses, including measurements of Earth’s rotation and gravity field variations, reveal shorter-period signals consistent with inner-core oscillations.
Together, these observations reinforce the view that the inner core exhibits time-varying, oscillatory rotation relative to the mantle.5
Points of Contention & Debate
Despite this growing body of evidence, the scientific community remains divided on several key points.
The periodicity of observed oscillations remains unresolved, with studies identifying both a short ~6-year cycle and a longer ~70-year cycle, yet their relationship and physical origins are unclear. Different analyses of seismic data have produced divergent results, leaving researchers to debate whether the observed periodicities point to a single, coherent underlying process or to multiple, distinct phenomena operating within the core.
A central point of contention concerns whether the seismic anomalies reflect true angular rotation of the inner core relative to the mantle or instead result from evolving structural or material properties within the core. Alternative explanations propose that localized growth or melting at the inner core boundary, redistribution of light elements, or changes in crystallographic texture could produce waveform variations that mimic rotation without requiring differential angular motion.
The debate is further complicated by uncertainties surrounding inner core anisotropy and how it evolves over time. P-wave velocities vary depending on direction, likely due to the alignment of iron crystals, but they also display complex hemispherical differences and depth-dependent patterns.
Adding to the challenge, processes such as recrystallization, deformation, and grain growth can gradually alter the core’s internal structure. These changes affect seismic wave speeds and lead to time-varying arrival patterns. As a result, it becomes difficult to separate true rotational motion from shifts caused by structural evolution within the inner core itself.
Resolving these questions requires high-quality seismic data capable of constraining both the spatial and temporal behavior of anisotropy, combined with improved theoretical understanding of inner core microphysics under extreme pressure and temperature conditions.2,3
Links to Earth’s Dynamics
Understanding inner core oscillations carries significant implications for Earth’s broader geodynamic behavior.
The oscillatory motion of Earth’s inner core is directly linked to variations in planetary rotation, as evidenced by correlations with minor length-of-day changes of approximately ±0.2?seconds over multi-year periods. These variations reflect angular momentum exchange between the solid inner core and the overlying mantle through gravitational coupling, with shorter 6-year oscillations matching observations from 2022 and multidecadal 70-year oscillations corresponding to 2023 findings.2,3
Such length-of-day changes provide independent verification of inner core dynamics and the mechanical coupling that connects the deepest interior to surface measurements accessible via satellite geodesy.5
The inner core’s motion interacts with the geodynamo in the liquid outer core, transmitting electromagnetic torques that drive differential rotation and influence convection patterns, thereby shaping Earth’s magnetic field. However, according to Dr. Amoré Nel, Applied Geomagnetic Researcher at the South African National Space Agency, the Earth’s protective magnetic field is generated primarily by the outer liquid core, and changes in inner core rotation would at most contribute minor fluctuations to the inherently dynamic magnetic field.6
Broader Impacts & Future Research
Earth’s inner core displays oscillatory, time-dependent motion rather than continuous superrotation, providing a coherent framework that links seismic observations, variations in the length of day, and changes in the geomagnetic field through angular momentum transfer among the inner core, outer core, and mantle.
This coupling explains how deep interior processes produce measurable surface and satellite-based signals, including subtle changes in planetary rotation and magnetic field structure.
Researchers are now working to distinguish true differential rotation from evolving inner core structure and anisotropy, using longer seismic records, denser global coverage of repeating events, and integrated core–mantle modeling.
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References and Further Reading
- Duan, P., & Huang, C. (2020). On the Mantle-Inner Core Gravitational Oscillation Under the Action of the Electromagnetic Coupling Effects. Journal of Geophysical Research: Solid Earth, 125(2), e2019JB018863. https://doi.org/10.1029/2019JB018863
- Wang, W., & Vidale, J. E. (2022). Seismological observation of Earth’s oscillating inner core. Science Advances. https://doi.org/abm9916
- Yang, Y., & Song, X. (2023). Multidecadal variation of the Earth’s inner-core rotation. Nature Geoscience, 16(2), 182-187. https://doi.org/10.1038/s41561-022-01112-z
- Wang, W., Vidale, J. E., Pang, G., Koper, K. D., & Wang, R. (2024). Inner core backtracking by seismic waveform change reversals. Nature, 631(8020), 340-343. https://doi.org/10.1038/s41586-024-07536-4
- Mandea, M., Dehant, V., & Cazenave, A. (2019). GRACE - Gravity Data for Understanding the Deep Earth’s Interior. Remote Sensing, 12(24), 4186. https://doi.org/10.3390/rs12244186
- Fouche, D. (2023). Changes in Earth’s inner core will not have dramatic effect on magnetic field. https://www.sansa.org.za/2023/01/changes-in-earths-inner-core-will-not-have-dramatic-effect-on-magnetic-field/
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