A new study provides clear evidence that Earth’s inner core began to slow down around 2010.
U.S.C. Scientists have discovered that the Earth’s inner core is slowing down compared to the Earth’s surface. The phenomenon began around 2010 after decades of a reverse trend. The major change was detected through analysis of detailed seismic data from earthquakes and nuclear tests. The slowdown is influenced by the dynamics of the surrounding liquid outer core and gravitational forces from the Earth’s mantle, and may slightly affect the Earth’s rotation.
Inner Core Dynamics
Scientists from the University of Southern California have proven that Earth’s inner core is recessing (slowing down) relative to the planet’s surface, as shown in a new study published June 12 in the journal Nature. Nature.
The scientific community has long debated the movement of the inner core, with some studies suggesting that it rotates faster than the Earth’s surface, but a recent study from the University of Southern California clearly shows that since about 2010, the movement of the inner core has slowed, and is now moving at a slower pace than the Earth’s surface.
“When we first saw the seismic records suggesting this change, we were perplexed,” says John Vidale, chair of the Department of Earth Sciences at the University of Southern California’s Doon Seif College of Arts and Sciences. “But when we found 24 more observations showing the same pattern, the result was inevitable: the inner core’s velocity had slowed for the first time in decades. Other scientists have recently argued for similar and different models, but our latest work presents the most compelling conclusion.”
The relativity of backsliding and slowing down
The inner core is thought to be retreating retrogradely relative to the planet’s surface because, for the first time in about 40 years, it is moving slightly slower than Earth’s mantle rather than faster. Compared to its speed over the past few decades, 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, roughly the size of the moon, lies more than 3,000 miles beneath Earth’s feet and poses a challenge to researchers because it cannot be visited or observed. Scientists must use seismic waves from earthquakes to reconstruct the inner core’s movements.
A new take on the iterative approach
In contrast to other studies, Vidale and Wang Wei of the Chinese Academy of Sciences used waveforms and repeating earthquakes – seismic phenomena that occur in the same place and produce identical earthquake records.
For the study, the researchers collected and analyzed recorded seismic data from 121 repeating earthquakes that occurred around the South Sandwich Islands between 1991 and 2023. They also used data from two Soviet nuclear tests conducted between 1971 and 1974, as well as repeating French and American nuclear tests from other studies of the inner core.
Vidale said the inner core’s slowing down was caused by the churning of the liquid iron outer core that surrounds it, which creates Earth’s magnetic field as well as the gravitational pull from the dense region of the overlying rocky mantle.
Impact on the Earth’s surface
We can only guess how changes in the inner core’s motion might affect the Earth’s surface. Vidal says that the recession of the inner core could change the length of a day by just a split second: “Changes of a thousandth of a second would be barely noticeable amid the roaring noise of the oceans and atmosphere.”
Future work by USC scientists aims to chart the inner core’s orbit in even greater detail and determine exactly why it’s moving as it does.
“The inner core dance may be even more active than we previously knew,” Vidale said.
Reference: Wei Wang, John E. Vidal, Guaning Pan, Keith D. Koper, and Luoyang Wang, “Backtracking the Inner Core Through Inversion of Seismic Waveform Changes,” 12 June 2024; Nature.
Publication date: 10.1038/s41586-024-07536-4
In addition to Vidal, other study authors include Luoyang Wang of the University of Southern California, Dawn of California, Wei Wang of the Chinese Academy of Sciences, Guangning Pan 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, Chinese Academy of Sciences (IGGCAS-201904 and IGGCAS-202204).