It's not unusual for the Earth's core to change its rate of rotation and even its shape over time – though not simultaneously. But new research reveals that something unexpected is happening under the radar.
Scientists have long debated the cause of strange changes to seismic waves, triggered by earthquakes, when they ripple through the core. One side argued the rotation rate delays or accelerates the travel time of the waves, while the other argument is that the inner core's shape deforming changes the waves. In the new study, published in Nature Geoscience, scientists from China and the US reveal that it's likely to be both.
The research shows that in 2010 the Earth's inner core went from rotating faster to slower than the rest of the planet, which – along with changes near the surface of the inner core – likely interrupted the seismic waves. Like X-rays, these waves allow scientists to 'see' what's happening inside the planet – so the researchers think the discovery could help us unlock more information about the core's properties and structure.
“For now, these findings simply provide an observable change that might lead us to get a clearer picture of how the inner core is flexing on a timescale of years. Further surprises may await,” Prof John Emilio Vidale, the lead author of the study, told BBC Science Focus.
The Earth’s core is almost as hot as the surface of the Sun. It is also roughly 6,500km (4,000 miles) beneath the planet's surface and has a higher pressure than the deepest part of the ocean. In other words, there is no way to directly access the core and study it.
Instead, scientists utilise seismic waves caused by earthquakes. By examining how these waves travel through different layers of the planet, and more specifically the core, scientists can better understand the structure, movement and behaviour of the core.
In this latest set of research, the team looked at seismic waves from 121 repeating earthquake pairs (two earthquakes that occur in the same place and produce nearly identical seismic waves) in the South Sandwich Islands between 1991 and 2023. These earthquakes were recorded at two separate seismic arrays: Eielson (ILAR) in Alaska and Yellowknife (YKA) in Canada.
By looking at the arrival times and waveform changes of these signals over multiple decades, the team could detect minor shifts in the core’s movement. If the inner core was rotating steadily, the arrival times of seismic waves would shift consistently over time.
This showed a few noticeable trends in the Earth’s inner core. It rotated faster than the Earth’s mantle and crust for several decades, before slowing down around 2010. But some earthquakes showed no significant time shift, suggesting that the rotation paused or reversed at times.
The study had a secondary finding: due to differences in the signals coming from the two seismic arrays, the team was able to detect that something other than rotation was affecting the inner core.
They think viscous deformation near the inner-core boundary might be the culprit. Though the inner core is made of solid metal, variations in the shape of the outer core or changes in the mantle's density can change the inner core's structure.
While this behaviour is seemingly erratic, there currently isn't enough data to confirm whether this is normal or not – and therefore get a clear picture of how the Earth’s core is really behaving.
According to Vidale, the simplest explanation is that the outer core's movements initiate the inner core's rotation, effectively realigning the inner core over decades – but the exact mechanism by which this happens is uncertain.
“If this was the case, the rotation of the cores would be mostly harmonic (predictable)," he said. Alternatively, "the inner core motion could be mostly chaotic, just going in the direction the outer core goes.”
For now, this research remains an interesting analysis of the behaviour of the Earth’s core – but it could lead to more discoveries. Vidale notes that further analysis could show when and where outer core convection is most vigorous, or where the inner core is more or less soft.
He also believes there's a chance it could reveal a link between inner core changes and unpredictable jerks in the Earth's magnetic field. While these don't affect our everyday lives, they can affect satellites and compasses.
About our expert
John Vidale is a Dean's Professor of Earth Sciences at the University of Southern California. His research focuses on earthquakes, Earth's structure, volcanoes and the hazards of strong shaking. Since 2017, Vidale has been 2017 a professor at USC and was director of the Southern California Earthquake Center from 2017 to 2018. He has also served on the National Earthquake Prediction Evaluation Council (NEPEC) from 2007-2017 and the Earthquake Early Warning External Working Group since 2018.
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