It may surprise you to learn how little we know about the Earth’s inner workings. While we have a solid understanding of how the upper layers move, forming mountains and causing earthquakes, the deeper we go, the more mysterious it gets.
One area of fierce debate over recent decades has centred on how the Earth’s inner core is moving. Forwards? Backwards? Side to side? No one really knew. But now a new study published in Nature may have settled the debate once and for all.
The paper suggests that the core is backtracking relative to the surface – findings that support a controversial study from last year by Peking University researchers, published in Nature Geoscience.
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The Earth’s inner core is a solid, crystallised iron sphere roughly the size of the Moon. It floats almost 5,000km beneath our feet in a sea of liquid iron, nickel and other metals known as the outer core.
“Because the inner core is solid and it’s floating in the outer core, nothing holds it in place,” Prof John Vidale, a researcher at the University of Southern California (USC) and co-author of the work, told BBC Science Focus.
“We’ve known that the inner cores movement has been changing in some combination since the 1990s … and we’ve been arguing about how it’s changing,” he said.
According to a USC press release, the new study provides “unambiguous evidence” that the inner core began to slow down around 2010, moving slower than the Earth’s surface.
This creates the appearance of the core moving backwards relative to the surface, similar to how a car that slows down appears to move backwards to a driver moving at a constant speed.
If correct, this would be the first observed slowdown in 40 years and supports the theory that the core’s speed changes in 70-year cycles.
To uncover these changes, the team studied repetitive earthquakes, specifically 121 quakes in the South Sandwich Islands between 1991 and 2023, using seismographs in Canada and Alaska. They also used data from Soviet-era nuclear tests.
Vidale and his team searched for matching seismic waveforms from different times. If the inner core rotates independently of the rest of the Earth, waves from recurring quakes would pass through different regions of the core, creating unique waveforms due to its uneven structure.
Conversely, if the core had reversed its rotation, some waveforms should match those from before the reversal, indicating the core had realigned with its previous path.
“When I first saw the seismograms that hinted at this change, I was stumped,” Vidale said. “But when we found two dozen more observations signalling the same pattern, the result was inescapable.
“The inner core had slowed down for the first time in many decades. Other scientists have recently argued for similar and different models, but our latest study provides the most convincing resolution.”
At the surface level, the effects are uncertain, although Vidale noted it could slightly alter the length of our day by about a thousandth of a second, almost imperceptible amid the noise of the churning oceans and atmosphere.
Future work for the team includes measuring more waveforms from different locations and paths through the planet. “Part of it is just waiting. The core is moving back through some positions where it did some weird things around 2001, so we’ll try figuring out what that was,” Vidale concluded.
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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