The inner core began to decrease in speed around 2010 and moved more slowly than the Earth’s surface. Credit: USC
A new study provides clear evidence that Earth’s inner core began slowing its speed around 2010.
USC Scientists have discovered that the Earth’s inner core is slowing down relative to the Earth’s surface, a phenomenon that began around 2010 after decades of the opposite trend. This significant shift was detected through detailed seismic data analysis of earthquakes and nuclear tests. The deceleration is influenced by the dynamics of the surrounding liquid outer core and the gravity of the Earth’s mantle, which may slightly influence the Earth’s rotation.
Inner core dynamics
USC scientists 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 shows conclusively that the inner core has slowed down since about 2010 and is now moving at a slower pace than that of the planet’s surface.
“When I first saw the seismograms indicating this change, I was amazed,” said John Vidale, Dean’s Professor of Earth Sciences at the USC Dornsife College of Letters, Arts and Sciences. “But when we found another 20 observations that signaled the same pattern, the result was inescapable. 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 offers the most compelling solution.”
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 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, 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 previously know,” 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, June 12, 2024, Nature.
DOI: 10.1038/s41586-024-07536-4
In addition to Vidale, other authors of the study 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).