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 found that Earth’s inner core is slowing down relative to the planet’s surface, a phenomenon that began around 2010 after decades of the opposite trend. This significant shift was detected using detailed seismic analysis of earthquake and nuclear test data. The deceleration is affected by the dynamics of the surrounding liquid outer core and the gravitational pull of the Earth’s mantle, potentially slightly affecting the Earth’s rotation.
Inner core dynamics
USC scientists have shown that Earth’s inner core is receding—slowing down—relative to the planet’s surface, according to 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 shows conclusively that starting around 2010, the inner core slowed down and is now moving at a slower pace than the surface of the planet.
“When I first saw the seismograms that indicated this change, I was amazed,” said John Vidale, Dean Professor of Earth Sciences in the USC Dornsife College of Letters, Arts and Sciences. “But when we found two dozen other observations signaling the same pattern, the result was inevitable. The inner core has slowed down for the first time in many decades. Other researchers have recently argued for similar and different models, but our latest study provides the most convincing solution.
Relativity of retrograde motion and deceleration
The inner core is considered to be backing up and tracking back relative to the planet’s surface due to the fact that it is moving slightly slower instead of faster than Earth’s mantle for the first time in about 40 years. Compared to the speed of previous decades, the inner core is slowing down.
The inner core is a solid iron-nickel sphere surrounded by a liquid iron-nickel outer core. About the size of the moon, the inner core lies more than 3,000 miles below our feet and presents a challenge for researchers: it cannot be visited or viewed. Scientists must use the seismic waves of an earthquake to create a representation of the movement of the inner core.
A new look at the iterative approach
Vidale and Wei Wang of the Chinese Academy of Sciences used curves and repeated earthquakes unlike other research. Recurring earthquakes are seismic events that occur at the same location and produce identical seismograms.
In this study, researchers collected and analyzed seismic data recorded around the South Sandwich Islands from 121 recurring earthquakes that occurred between 1991 and 2023. They also used data from the twin Soviet nuclear tests between 1971 and 1974, as well as repeated French and U.S. nuclear tests from other studies of the inner core.
Vidale said the inner core’s slowing speed was caused by the swirling of the liquid iron outer core surrounding it, which generates Earth’s magnetic field, as well as gravitational pulls from dense regions of 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 that tracking back the inner core can change the length of the 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 seeks to map the trajectory of the inner core in even greater detail to reveal exactly why it’s shifting.
“The dance of the inner core may be even more alive than we know yet,” Vidale said.
Reference: “Inner core backtracking by seismic waveform change reversals” by Wei Wang, John E. Vidale, Guanning Pang, Keith D. Koper, and Ruoyan Wang, 12 Jun 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, Chinese Academy of Sciences (IGGCAS-201904 and IGGCAS-202204).