A new study shows that the inner core oscillates

USC researchers have identified a six-year cycle of super and sub-rotation in the Earth’s inner core, contradicting previously accepted models that suggested it consistently rotates at a faster rate than the planet’s surface. Credit: Edward Sotelo/USC

USC scientists have found evidence that the Earth’s inner core wobbles, contradicting previously accepted models that suggested it consistently rotates at a faster rate than the planet’s surface.

Their study, published today in advances in science, shows that the inner core changed direction in the six-year period from 1969 to 1974, according to the seismic data analysis. Scientists say their model of inner core motion also explains the variation in day length, which has been shown to oscillate persistently over the past few decades.

“From our findings, we can see changes to the Earth’s surface compared to its inner core, as people have claimed for 20 years,” said John E. Vidale, study co-author and professor of Earth Sciences at USC Dornsife College. of Letters, Arts and Sciences. “However, our latest observations show that the inner core rotated a little slower from 1969 to 1971 and then shifted in the other direction from 1971 to 1974. We also noticed that day length grew and shrank as would be expected.

“The coincidence of these two observations makes the wobble the likely interpretation.”

Atomic test analysis identifies rotation speed and direction

Our understanding of the inner core has expanded dramatically 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 is also impossible to observe directly, which means that researchers scramble for indirect measurements to explain the pattern, speed, and cause of motion and changes.

Research published in 1996 was the first to propose that the inner core rotates faster than the rest of the planet – also known as super-rotation – by approximately 1 degree per year. Subsequent findings by Vidale reinforced the idea that the inner core super-rotates, albeit at a slower rate.

Using data from the Large Aperture Seismic Array (LASA), a US Air Force facility in Montana, researchers Wei Wang and Vidale found that the inner core was rotating more slowly than previously predicted, approximately 0.1 degrees per year. The study analyzed waves generated from Soviet underground nuclear bomb tests from 1971 to 1974 in the arctic Novaya Zemlya archipelago using a new beamforming technique developed by Vidale.

The new findings came when Wang and Vidale applied the same methodology to a pair of earlier atomic tests under Amchitka Island at the tip of the Alaskan archipelago – Milrow in 1969 and Cannikin in 1971. Measuring the compression waves resulting from nuclear explosions, they found the inner core reversed direction, sub-rotating at least a tenth of a degree per year.

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

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

Future research to dig deeper into why the inner core formed

Vidale and Wang noted that future research would depend on finding observations accurate enough to match these results. Using seismological data from atomic tests in previous studies, they were able to pinpoint the exact location and time of the seismic event very simply, says Wang. However, Montana LASA closed in 1978 and the era of US underground atomic testing is over, meaning researchers would need to rely on comparatively 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 geomagnetic fields, both of which are compatible with the theory in amplitude and phase. Vidale says the findings provide a compelling theory for many questions posed by the research community.

“The inner core isn’t fixed — it’s moving under our feet and seems to go back and forth a few kilometers every six years,” Vidale said. “One of the questions we’re trying to answer is, is the inner core progressively moving or is it locked up compared to everything else in the long term? We’re trying to understand how the inner core formed and how it moves over time – that’s a important step to better understand this process.”

Earth’s inner core: A mixture of solid Fe and liquid-like light elements

More information:
Wei Wang et al, Seismological Observation of Earth’s Oscillating Inner Core, advances in science (2022). DOI: 10.1126/sciadv.abm9916. www.science.org/doi/10.1126/sciadv.abm9916

Provided by the University of Southern California

Quote: Earth moves a lot under our feet: New study shows inner core wobbles (2022, June 10) retrieved June 10, 2022 from https://phys.org/news/2022-06-earth- feet-core-oscillates .html

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