USC researchers prove that Earth’s core is slowing down

Earth's inner core slows down

The inner core began to slow down around 2010, moving slower than the Earth’s surface. Credit: USC

A new study provides clear evidence that Earth’s inner core began to slow down around 2010.

USC Scientists have discovered that Earth’s inner core is slowing down compared to the planet’s surface, a phenomenon that began around 2010 after decades of the opposite trend. This significant change was discovered through detailed analysis of seismic data from earthquakes and nuclear tests. The deceleration is affected by the dynamics of the surrounding liquid outer core and the gravitational pull from the Earth’s mantle, potentially slightly affecting the Earth’s rotation.

Inner core dynamics

Scientists at USC have proven that Earth’s inner core is receding—slowing down—relative to the planet’s surface, as shown in new research published June 12 in the journal Nature.

The scientific community has long debated the motion of the inner core, with some studies suggesting that it rotates faster than the Earth’s surface. However, recent research from USC conclusively shows that starting in 2010, the inner core has slowed down, now moving at a slower rate than the planet’s surface.

“When I first saw the seismograms that hinted at this change, I was stunned,” said John Vidale, Dean’s Professor of Earth Sciences in the USC Dornsife College of Letters, Arts and Sciences. “But when we found two dozen other observations that signaled the same pattern, the result was inescapable. The inner core had slowed 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 solution.”

Relativity of retrogression and deceleration

The inner core is considered to be moving backwards and forwards relative to the planet’s surface due to moving slightly slower instead of faster than the Earth’s mantle for the first time in about 40 years. Relative to its 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. About the size of the moon, the inner core sits more than 3,000 miles below our feet and presents a challenge for researchers: it can’t be visited or seen. Scientists must use seismic waves from earthquakes to create representations of the movement of the inner core.

A new attitude to an iterative approach

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

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

Vidale said the inner core’s slowing rate was caused by the outflow of the liquid iron outer core surrounding it, which generates Earth’s magnetic field, as well as gravitational pulls from dense regions of the rocky mantle.

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. Vidale said that the recoil of the inner core can change the length of a day by 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. “

Future research by USC scientists aims to describe the trajectory of the inner core in even greater detail to find out exactly why it is shifting.

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

Reference: “Development of the inner core from the variation of the seismic waveform” by Wei Wang, John E. Vidale, Guanning Pang, Keith D. Koper, and Ruoyan Wang, 12 June 2024, Nature.
DOI: 10.1038/s41586-024-07536-4

In addition to Vidale, other study authors include Ruoyan Wang of USC Dornsife, Wei Wang of the Chinese Academy of Sciences, Guanning Pang of Cornell University and Keith Koper of the University of Utah.

This research was supported by the National Science Foundation (EAR-2041892) and the Institute of Geology and Geophysics of the Chinese Academy of Sciences (IGGCAS-201904 and IGGCAS-202204).

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