Study: Earth's Core Is Slowing Down

Inner Core of Earth Began to Decrease Its Speed around 2010, And It May Change Day's Length, Study Suggests

USC scientists have discovered that the Earth's inner core is decelerating in relation to the planet's surface, according to new research published Wednesday 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 started to reduce its speed around 2010, now moving slower than the Earth's surface.


Earth's Core Is Slowing Down
Earth's Core Is Slowing Down

"When I first saw the seismograms that hinted at this change, I was stumped," said John Vidale, Dean's Professor of Earth Sciences at the USC Dornsife College of Letters, Arts and Sciences. "But when we found two dozen more observations signaling 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."

Inner Core Backtracking: A Slower Pace

The inner core appears to be reversing and backtracking relative to the planet's surface by moving slightly slower than the Earth's mantle for the first time in roughly 40 years. Compared to its speed in previous decades, the inner core is decelerating.

The inner core, a solid iron-nickel sphere, is surrounded by the liquid iron-nickel outer core. This solid sphere, about the size of the moon, is located more than 3,000 miles beneath the Earth's surface. Due to its inaccessibility, scientists rely on seismic waves generated by earthquakes to study its movement.

A Fresh Perspective on Seismic Data

Vidale and Wei Wang of the Chinese Academy of Sciences employed waveforms and repeating earthquakes, a method distinct from other research. Repeating earthquakes are seismic events that occur at the same location, producing identical seismograms.

In their study, the researchers analyzed seismic data from the South Sandwich Islands, gathered from 121 repeating earthquakes that occurred between 1991 and 2023. They also used data from Soviet nuclear tests conducted between 1971 and 1974, along with repeated French and American nuclear tests from other inner core studies.

Vidale explained that the inner core's slowed speed is due to the churning of the surrounding liquid iron outer core, which generates Earth's magnetic field, as well as gravitational forces from the dense regions of the overlying rocky mantle.

Surface Implications of the Inner Core's Movement

The effects of this change in the inner core's movement on Earth's surface are still speculative. Vidale suggested that the inner core's backtracking could alter the length of a day by mere fractions of a second: "It's very hard to notice, on the order of a thousandth of a second, almost lost in the noise of the churning oceans and atmosphere."

USC scientists aim to chart the inner core's trajectory in greater detail in future research to uncover the reasons behind its shifting.

"The dance of the inner core might be even more lively than we know so far," Vidale said.

The study was published in the journal Nature.


W. Wang et al. Inner core backtracking by seismic waveform change reversals. Nature, published online; doi: 10.1038/s41586-024-07536-4


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