Research confirms that the rotation of the Earth’s inner core has slowed

Scientists from the University of Southern California have proven that the Earth’s inner core is receding – slowing down – relative to the Earth’s surface, as new research published in Nature.

The motion of the inner core has been under debate in the scientific community for two decades, with research showing that the inner core rotates faster than the planet’s surface. The new USC study provides unequivocal evidence that the inner core began to slow its speed and move more slowly than the Earth’s surface around 2010.

“When I first saw the seismograms indicating this change, I was astounded,” said John Vidale, Dean’s Professor of Earth Sciences at the USC Dornsife College of Letters, Arts and Sciences. ‘But when we found another twenty observations that signaled the same pattern, the result was inevitable. 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 compelling resolution.”

The relativity of going backwards and slowing down

The inner core is thought to be inverting and receding relative to the planet’s surface, as it moves slightly slower rather than faster than Earth’s mantle for the first time in about 40 years. Compared to the speed in previous decades, the inner core is slowing down.

The inner core is a solid iron-nickel sphere surrounded by the liquid iron-nickel outer core. The inner core, which is about the size of the moon, is more than 5,000 kilometers beneath our feet and poses a challenge for researchers: it cannot be visited or viewed. Scientists must use the seismic waves from earthquakes to create representations of the movement of the inner core.

A new look at a repetitive approach

Vidale and Wei Wang of the Chinese Academy of Sciences used waveforms and repeating earthquakes in contrast to other research. Repeated earthquakes are seismic events that occur at the same location and produce identical seismograms.

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

Vidale said the inner core’s decreasing velocity was caused by the churning of the outer core of liquid iron that surrounds it, generating Earth’s magnetic field, as well as gravitational pulls from the dense regions of the overlying rocky mantle.

The impact on the earth’s surface

The implications of this change in the motion of the inner core for the Earth’s surface can only be speculated upon. Vidale said that the retreat of the inner core can change the length of a day by fractions of a second: “It’s very difficult to notice, on the order of a thousandth of a second, that you’re almost lost in the sound of the swirling oceans and atmosphere. “

The USC scientists’ future research aims to map the trajectory of the inner core in even greater detail to reveal exactly why it shifts.

“The dance of the inner core may be even more alive than we know yet,” Vidale said.