New study reveals inner core oscillates

USC scientists have found evidence that Earth’s inner core is oscillating, contradicting previously accepted models that suggested it is consistently rotating faster than the planet’s surface.

Their study, published today in Science Advances, shows that the inner core changed direction over the six-year period from 1969-74, according to the analysis of seismic data. The scientists say their model of inner core motion also explains the variation in the length of the day, which has been shown to fluctuate continuously over the decades.

“From our findings, we can see that the Earth’s surface is shifting compared to the inner core, as people have argued for 20 years,” said John E. Vidale, co-author of the study and Dean’s Professor of Earth Sciences at USC Dornsife College of Letters, Arts and Sciences. “However, our latest observations show that the inner core rotated slightly more slowly from 1969-71 and then moved the other way from 1971-74. We also note that the length of the day grew and shrank as would be predicted.

“The coincidence of those two observations makes oscillation the likely interpretation.”

Analysis of atomic tests pinpoints rotational speed and direction

Our understanding of the inner core has expanded tremendously over the past 30 years. The inner core – a hot, dense ball of solid iron the size of Pluto – has been shown to move and/or change over decades. It’s also impossible to observe directly, meaning researchers struggle with indirect measurements to explain the pattern, speed, and cause of the movement and changes.

Research published in 1996 was the first to propose that the inner core rotates faster than the rest of the planet — known as superrotation — at about 1 degree per year. Subsequent findings from Vidale reinforced the idea that the inner core is superrotating, albeit at a slower rate.

Using data from the Large Aperture Seismic Array (LASA), a United States Air Force facility in Montana, researcher Wei Wang and Vidale found that the inner core was spinning slower than previously predicted, about 0.1 degrees per year. The study analyzed waves generated by underground Soviet nuclear bomb tests from 1971-74 in the Arctic archipelago Nova Zemlya, using a new beamforming technique developed by Vidale.

The new findings emerged when Wang and Vidale applied the same methodology to some previous nuclear tests under Amchitka Island at the tip of the Alaskan archipelago – Milrow in 1969 and Cannikin in 1971. They measured the compression waves resulting from the nuclear explosions. , they discovered the inner core had changed direction and rotated at least a tenth of a degree a year.

This latest study marked the first time the known six-year oscillation was indicated by direct seismological observation.

“The idea that the inner core oscillates was a model out there, but the community was divided on whether it was viable,” Vidale says. “We went into this expecting to see the same rotational direction and speed in the previous pair of atomic tests, but instead we saw the opposite. We were quite surprised to find that it was moving the other way.”

Future research to delve deeper into the formation of the inner core

Vidale and Wang both noted that future research would depend on finding sufficiently accurate observations to compare these results. Using seismic data from atomic tests in previous studies, they have been able to determine the exact location and time of the very simple seismic event, Wang says. However, the Montana LASA closed in 1978 and the era of underground nuclear testing in the US is over, meaning the researchers would have to rely on relatively inaccurate earthquake data, even with recent advances in instrumentation.

The study supports speculation that the inner core oscillates based on variations in day length — plus or minus 0.2 seconds over six years — and Earth’s magnetic fields, both of which agree with the theory in both amplitude and phase. Vidale says the findings provide a compelling theory for many questions posed by the research community.

“The inner core isn’t solid — it moves under our feet and it seems to go back and forth a few miles every six years,” Vidale said. “One of the questions we tried to answer is: does the inner core move gradually or is it largely locked in the long run compared to everything else? We try to understand how the inner core is formed and how it moves into the moves over time – this is an important step to better understand this process.”

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