The inner core began to slow down around 2010, moving more slowly than the Earth’s surface. Credit: University of Southern California
A new study provides clear evidence that Earth’s inner core began to slow down around 2010.
University of Southern California 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 major shift has been revealed through analysis of detailed seismic data from earthquakes and nuclear tests. The deceleration is affected by the dynamics of the surrounding liquid outer core and the gravitational pull of the Earth’s mantle, which may affect the Earth’s rotation slightly.
Inner core dynamics
Scientists at the University of Southern California 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 is rotating faster than the Earth’s surface. However, recent research from the University of Southern California shows conclusively that starting around 2010, the inner core slowed down, and is now moving at a slower pace than the planet’s surface.
“When I first saw the seismograms that hinted at this change, I was mystified,” said John Vidal, the dean’s professor of geosciences in the USC Dornsife College of Letters, Arts and Sciences. “But when we found twenty other observations pointing to the same pattern, the conclusion was inevitable. The inner core had slowed for the first time in several decades. Other scientists have recently argued in favor of similar and different models, but our latest study provides the most convincing solution.”
Relative decline and slowdown
The inner core is in a state of inversion and retraction relative to the planet’s surface due to it moving slightly slower instead of faster than Earth’s mantle for the first time in about 40 years. Compared to its speed in previous decades, the inner core is slowing down.
The inner core is a solid ball of iron and nickel surrounded by an outer core of liquid iron and nickel. Roughly the size of the Moon, the inner core lies more than 3,000 miles below our feet and presents a challenge to researchers: it cannot be visited or viewed. Scientists must use seismic waves of earthquakes to create visualizations of the motion of the inner core.
A new approach to the iterative approach
Vidal and Wei Wang of the Chinese Academy of Sciences used repeated waveforms and earthquakes unlike other research. Repeated earthquakes are seismic events that occur at the same location to produce identical seismograms.
In this study, researchers compiled and analyzed seismic data recorded around the South Sandwich Islands from 121 repeat earthquakes that occurred between 1991 and 2023. They also used data from dual Soviet nuclear tests between 1971 and 1974, as well as repeated French and American tests. Nuclear experiments from other studies of the inner core.
Vidal said that the slowdown in the speed of the inner core was caused by the swaying of the outer core of liquid iron that surrounds it, which generates the Earth’s magnetic field, in addition to gravitational forces from dense areas of the overlying rocky mantle.
Impact on the Earth’s surface
The effects of this change in the movement of the inner core of the Earth’s surface can only be speculated on. Vidal said the retreat of the inner core could change the length of a day by fractions of a second: “It is very difficult to notice that, on the order of a thousandth of a second, it is almost lost in the noise of the oceans and atmosphere.”
Future research by USC scientists hopes to chart the path of the inner core in more detail to reveal exactly why it is shifting.
“The inner dance of the heart may be more vibrant than we know so far,” Vidal said.
Reference: “Inner Core Retraction by Seismic Waveform Reflections” by Wei Wang, Jun E. Fidel, Guanying Pang, Keith D. Cooper, and Ruyan Wang, June 12, 2024, nature.
doi: 10.1038/s41586-024-07536-4
In addition to Vidal, other study authors include Ruian Wang of the University of Southern California Dornsife, Wei Wang of the Chinese Academy of Sciences, Guanying Pang of Cornell University, and Keith Cooper 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).
