Scientists at the University of Southern California have confirmed that Earth’s inner core is slowing its rotation relative to the planet’s surface, according to new research published in *Nature*.
For two decades, the scientific community has debated the movement of the inner core, with some studies suggesting it rotates faster than the Earth’s surface. The new USC study provides clear evidence that the inner core began to slow down around 2010, now rotating slower than the Earth’s surface.
“When I first saw the seismograms that hinted at this change, I was stumped,” said John Vidale, Dean’s Professor of Earth Sciences at USC. “But when we found two dozen more observations showing the same pattern, it was undeniable. The inner core had slowed down for the first time in many decades. While other scientists have proposed various models, our latest study offers the most convincing evidence.”
Relative Motion and Backtracking The inner core appears to be reversing and backtracking relative to the planet’s surface because it is now moving slower than the Earth’s mantle for the first time in about 40 years. Compared to its speed in previous decades, the inner core’s rotation has decelerated.
The inner core, a solid iron-nickel sphere roughly the size of the moon, is located over 3,000 miles beneath the Earth’s surface. It is surrounded by the liquid iron-nickel outer core. Due to its inaccessibility, scientists rely on seismic waves from earthquakes to study its movement.
New Methodology and Findings Vidale and Wei Wang of the Chinese Academy of Sciences used waveforms from repeating earthquakes to analyze the inner core’s movement. Repeating earthquakes, which occur at the same location and produce identical seismograms, provided crucial data for their study.
The researchers examined seismic data from 121 repeating earthquakes near the South Sandwich Islands, spanning from 1991 to 2023. They also used data from Soviet nuclear tests (1971-1974) and other nuclear tests conducted by France and the United States.
Vidale explained that the inner core’s slowed rotation is influenced by the churning of the surrounding liquid iron outer core, which generates Earth’s magnetic field, and by gravitational forces from the dense rocky mantle above.
Impact on Earth’s Surface The slowing of the inner core might slightly alter the length of a day by fractions of a second, though this change is minuscule and difficult to detect amid the dynamic activity of Earth’s oceans and atmosphere.
Future research aims to chart the inner core’s trajectory in greater detail to understand the reasons behind its shifting motion. “The dance of the inner core might be even more lively than we know so far,” Vidale said.
