The inner core began to slow down around 2010, moving more slowly than the Earth’s surface. Credit: USC
A new study provides clear evidence that Earth’s inner core began slowing down around 2010.
USC scientists have discovered 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 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, which potentially slightly affects the Earth’s rotation.
Inner core dynamics
USC scientists have proven that Earth’s inner core is recoiling — 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 convincingly shows that since about 2010, the inner core has slowed down, now moving at rates slower than the planet’s surface.
“When I first saw the seismograms that hinted at this change, I was stunned,” said John Vidale, a professor of earth sciences in the USC Dornsife College of Letters, Arts and Sciences. “But when we found two dozen more sightings signaling the same pattern, the result was inescapable. The inner core 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 convincing solution.
Relativity of retrogression and delay
The inner core is thought to be moving back and forth relative to the planet’s surface due to it moving slightly slower instead of faster than Earth’s mantle for the first time in approximately 40 years. Compared to its speed in 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 seen. Scientists must use the seismic waves of earthquakes to create images of the movement of the inner core.
A new look at an iterative approach
Vidale and Wei Wang of the Chinese Academy of Sciences used waveforms and repeated earthquakes unlike other studies. Repeated earthquakes are seismic events that occur at the same location to produce identical seismograms.
In this study, the researchers collected 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 multiple French and American nuclear tests from other studies of the inner core.
Vidale said the slowing of the inner core is caused by the stirring of the liquid iron outer core that surrounds it, which generates Earth’s magnetic field, as well as the gravitational pull from dense regions of the overlying rocky mantle.
The impact on the earth’s surface
The consequences of this change in the motion of the inner core for the Earth’s surface can only be speculated. Vidale said that inner core backtracking can change the length of 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 turbulent oceans and atmosphere. “
Future research by the USC scientists aims to map the trajectory of the inner core in even greater detail to reveal exactly why it is shifting.
“The dance of the inner core may be even more lively than we know so far,” Vidal said.
Reference: “Inner core backtracking by seismic waveform inversion” 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 of the Chinese Academy of Sciences (IGGCAS-201904 and IGGCAS-202204).
