The inner core began to slow down around 2010, moving slower than the Earth’s surface. Credit: University of Southern California
x close
The inner core began to slow down around 2010, moving slower than the Earth’s surface. Credit: University of Southern California
Scientists at the University of Southern California have shown that Earth’s inner core is moving away—slowing down—relative to the planet’s surface, according to new research published in Nature.
The motion of the inner core has been debated by the scientific community for two decades, with some research suggesting that the inner core rotates faster than the planet’s surface. A new USC study provides clear evidence that the inner core began to slow down around 2010, moving slower than Earth’s surface.
“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 inescapable. The inner core slowed down 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 reversing and slowing down
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. The inner core, roughly the size of the Moon, 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.
Seismic ray paths and event locations. a, PKIKP and PKP ray paths from the SSI source region to the two arrays (ILAR and YKA). The sampled IC region with a representative 1.5 Hz Fresnel zone is indicated by dashed circles centered at the PKIKP puncture points on the ICB. Inset, PKP (PKP(AB) and PKP(BC)), PKiKP(CD) and PKIKP(DF) ray paths. b, Map of the SSI region with source locations colored by focus depth. Credit: Nature (2024). DOI: 10.1038/s41586-024-07536-4
x close
Seismic ray paths and event locations. a, PKIKP and PKP ray paths from the SSI source region to the two arrays (ILAR and YKA). The sampled IC region with a representative 1.5 Hz Fresnel zone is indicated by dashed circles centered at the PKIKP puncture points on the ICB. Inset, PKP (PKP(AB) and PKP(BC)), PKiKP(CD) and PKIKP(DF) ray paths. b, Map of the SSI region with source locations colored by focal depth. Credit: Nature (2024). DOI: 10.1038/s41586-024-07536-4
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 two 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 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 alive than we know yet,” Vidale said.
More information:
Wei Wang et al, Inner Core Backtracking Using Seismic Waveform Change Changes, Nature (2024). DOI: 10.1038/s41586-024-07536-4
Information from the diary:
Nature