Geoscientists from the University of Southern California, Los Angeles, the Institute of Geology and Geophysics at the Chinese Academy of Sciences, Cornell University and the University of Utah say they have detected structural changes near the planet’s center.
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The Earth’s inner core was previously considered to be solid. Image credit: USC Graphic / Edward Sotelo.
“We didn’t set out to define the physical nature of the inner core,” said Professor John Vidale, a researcher at the University of Southern California, Los Angeles.
“What we ended up discovering is evidence that the near surface of Earth’s inner core undergoes structural change.”
Located 5,000 km (3,000 miles) below the Earth’s surface, the inner core is anchored by gravity within the molten liquid outer core. Until now the inner core was widely thought of as a solid sphere.
“The original aim was to further chart the slowing of the inner core. But as I was analyzing multiple decades’ worth of seismograms, one dataset of seismic waves curiously stood out from the rest,” Dr. Vidale said.
“Later on, I’d realize I was staring at evidence the inner core is not solid.”
In the study, the authors analyzed seismic waves recorded by the Eielson and Yellowknife receiver-array stations in northern North America from 121 repeating earthquake pairs between 1991 and 2023 in Antarctica’s South Sandwich Islands.
One dataset of seismic waves from the latter station included uncharacteristic properties the researchers had never seen before.
“At first the dataset confounded me,” Dr. Vidale said.
It wasn’t until the team improved the resolution technique did it become clear the seismic waveforms represented additional physical activity of the inner core.
The physical activity is best explained as temporal changes in the shape of the inner core.
The new study indicates that the near surface of the inner core may undergo viscous deformation, changing its shape and shifting at the inner core’s shallow boundary.
The clearest cause of the structural change is interaction between the inner and outer core.
“The molten outer core is widely known to be turbulent, but its turbulence had not been observed to disrupt its neighbor the inner core on a human timescale,” Dr. Vidale said.
“What we’re observing in this study for the first time is likely the outer core disturbing the inner core.”
“The discovery opens a door to reveal previously hidden dynamics deep within Earth’s core, and may lead to better understanding of Earth’s thermal and magnetic field.”
The study was published in the journal Nature Geoscience.
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J.E. Vidale et al. Annual-scale variability in both the rotation rate and near surface of Earth’s inner core. Nat. Geosci, published online February 10, 2025; doi: 10.1038/s41561-025-01642-2
