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As the ball turns: Earth’s inner core is ‘backtracking’

Using seismic data to measure changes in solid core's motion, geologists discover it now turns more slowly relative to surface of Earth.

New research, supported by University of Utah seismologists, shows the rotation of Earth’s inner core is slowing in relation to the planet’s surface.

For the past two decades, the movement of this solid yet searing hot metal sphere, suspended in the liquid outer core, has been studied closely and debated by the scientific community.

Past research has shown that the inner core has been rotating slightly faster than the planet’s surface.

Keith Koper, University of Utah. Banner Illustration by Edward Sotelo, courtesy of the University of Southern California.

But a different picture is emerging under a study led by the University of Southern California and published this week in Nature. The research team, which includes U geology professor Keith Koper, verified with new evidence—built on analyses of seismographic data—that the inner core’s rotation began to ease and synced with Earth’s spin about 14 years ago.

“What we’re seeing is the inner core had been moving just a little bit faster than the mantle for decades as people have been arguing, and around 2010, it has slowed down and stopped and is starting to move back the other way,” said senior author John Vidale, professor of Earth Sciences at USC Dornsife College of Letters, Arts and Science. “A more subtle point is that it doesn’t seem to be just going back and forth like a pendulum. It seems to be going back slower than it was coming forward, implying that the change is not as regular as we thought.”

The inner core is a solid sphere composed of iron and nickel, surrounded by the liquid iron outer core. Roughly the size of Pluto at 2,442 kilometers in diameter, it accounts for only 1% of Earth’s mass, yet it influences the magnetic field enveloping the planet and the length of the day.

But the core’s location, more than 3,000 miles below Earth’s surface, presents a challenge to researchers since it can’t be visited or viewed.

John Vidale, courtesy University of Southern California

For nearly a century, scientists have been using seismic data to probe the inner core. Its motion relative to the rest of the planet was first discovered in 1996.

In the latest study, researchers analyzed seismic data associated with 121 earthquakes that occurred in the South Atlantic between 1991 and 2023.

“We’ve been looking at seismic waves through the inner core that have been changing, and what we noticed is that the waves are starting to look like they did at some time 10 or 20 years ago,” Vidale said. “And it turns out that when the inner core has gone back to the position it had 10 or 20 years ago, then we see the same pattern of waves coming out.”

Among the co-authors is Koper’s former graduate student Guanning Pang, now a postdoctoral researcher at Cornell University. In just the last two years, Pang and Koper published research that found the inner core’s structure is heterogeneous and determined that the inner core experienced a burst of differential rotation, relative to Earth’s mantle, between 2001 and 2003.

“The inner core is just sitting in this fluid outer core, so it’s decoupled a little bit from the rest of the planet. It’s rotating at a different rate,” Koper said. “The angular momentum has to be conserved, so if it’s rotating differently, then that could affect the rotation observed at Earth’s surface. One of the big ideas in this paper is we have basically a new model or new observations about how the inner core is rotating slightly differently than the rest of the planet.”

Past research into the inner core’s movement has relied on data from repeating earthquakes, which occur in the same location to produce identical seismograms. Differences in the time it takes for the waves to pass through Earth indicate how the core’s position changed during the period between two repeater quakes.

Seismic ray paths and event locations. a) Ray paths of PKIKP and PKP from the SSI source region to the two seismograph arrays (ILAR and YKA). The sampled inner core region with a representative 1.5 Hz Fresnel zone is marked with dashed circles centered at the PKIKP pierce points at the ICB. Inset, the ray paths of PKP (PKP(AB) and PKP(BC)), PKiKP(CD) and PKIKP(DF).

“We’re really trying to image and understand the behavior of this planet that’s inside our planet that affects our magnetic field and geodynamics,” Koper said.

The new research goes deeper by examining the seismograms’ waveforms for new clues.

The 121 quakes used in the study resulted in 143 different pairs of earthquakes whose data could be compared. These quakes were centered around the South Sandwich Islands east of South America’s Cape Horn but measured on the other side of the globe in Alaska.

“It’s important to be really far apart between where the earthquakes occurred and where your seismometers are,” Koper said. “If they’re too close, then the waves will just go through the mantle and they won’t go deep enough.”

With the waves passing through the depths of the planet, they contact the inner core which influences how they are recorded.

“There’s some that just go through the outer core and they miss the inner core altogether. There’s some that bounce like an echo or reflection, and there’s some that go through it,” Koper said. “We look at the ones that go through it, and we compare them to the waves that missed it, and that’s how we know that change is happening in the inner core.”

What causes changes in the core’s rotation and when that rotation changes direction remains unanswered questions and the subject of intense debate among scientists.

“It’s locked in some sense to the mantle. It can only get so far out of whack before it wants to go back to its gravitational equilibrium position,” Koper postulated, “but there’s no good answer for what’s exciting these oscillations.”

The study, titled “Inner core backtracking by seismic waveform change reversals”, appears in the June 13 edition of Nature. The lead author is Wei Wang of the Chinese Academy of Sciences and co-authors include USC’s Routan Wang and former Utah graduate student Gunning Pang, now at Cornell University. Author Keith Koper directs the University of Utah Seismograph Stations. Funding came from the National Science Foundation and the Chinese Academy of Sciences’ Institute of Geology and Geophysics.


  • Brian Maffly Science writer, University of Utah Communications
  • Keith Koper Department of Geology & Geophysics, director of University of Utah Seismograph Stations