Scientists at the University of Southern California have proven that the Earth’s inner core is moving backwards (slowing down) relative to the planet’s surface. Nature.

The motion of the inner core has been debated in the scientific community for two decades; Some studies show that the inner core rotates faster than the planet’s surface. A new USC study provides definitive evidence that the inner core began to slow down around 2010, moving more slowly than the Earth’s surface.

“When I first saw the seismograms showing this change, I was confused,” said John Vidale, a professor in the Department of Earth Sciences in the USC Dornsife College of Letters, Arts and Sciences. “But when we discovered two dozen more observations pointing to the same pattern, the conclusion was inevitable. The inner core slowed down for the first time in decades. Other scientists have recently debated similar and different models, but our latest study offers the most convincing solution.”

Reverse motion and deceleration relationship

The inner core is thought to be rotating back and forth relative to the planet’s surface, moving slightly slower rather than faster than the Earth’s mantle for the first time in nearly 40 years. Compared to its speed in previous years, the inner core is slowing down.

The inner core is a solid iron-nickel sphere surrounded by a liquid iron-nickel outer core. The inner core, which is about the size of the Moon, lies more than 3,000 miles beneath our feet, and that’s a problem for researchers: it can’t be visited or seen. Scientists must use seismic waves from earthquakes to enable visualization of the movement of the inner core.

A new look at the iterative approach

Unlike other studies, Vidale and Wei Wang of the Chinese Academy of Sciences used waves and repeated earthquakes. Recurrent earthquakes are seismic events that occur in the same location and produce the same seismograms.

In this study, researchers collected and analyzed seismic data recorded around the South Sandwich Islands from 121 repeat earthquakes that occurred between 1991 and 2023. They also used data from two Soviet nuclear tests conducted between 1971 and 1974, as well as from repeated French and American tests. Nuclear tests from other studies of the inner core.

Vidale said the slowing of the inner core is due to the churning of the liquid iron outer core that surrounds it and creates the Earth’s magnetic field, and the gravitational forces of dense regions in the overlying rocky mantle.

Impact on Earth’s surface

One can only speculate about the consequences of this change in the motion of the inner core for the Earth’s surface. Vidale said backtracking the inner core could change the length of a day by fractions of a second: “It’s very difficult to see, about a thousandth of a second, almost lost in the noise of the turbulent oceans and atmosphere.”

Future research by USC scientists aims to map the inner core’s orbit in more detail to determine exactly why it changes.

“The dance of the inner core may be much more vibrant than we know so far,” Vidale said.