Researchers have discovered that Earth’s core oscillates, contradicting the assumption that the core rotates continuously faster than the planet’s surface.
Based on an analysis of seismic data, USC researchers reported in Science Advances that the inner core changed direction in the six-year period between 1969 and 1974.
According to scientists, their inner core motion model also explains persistent oscillations in day length over the past few decades.
In the last 30 years, our understanding of the inner core has greatly expanded. The inner core is a ball of hot and dense iron about the size of Pluto. Research has shown it has been moving and/or changing for decades.
Furthermore, in the absence of direct observation, researchers attempt to explain movement and change patterns, speeds, and causes through indirect measurements.
“From our findings, we can see the Earth’s surface shifts compared to its inner core, as people have asserted for 20 years,” explained 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 spun slightly slower from 1969-71 and then moved the other direction from 1971-74. We also note that the length of a day grew and shrank as would be predicted.”
“The coincidence of those two observations makes oscillation the likely interpretation.”
It was first proposed in 1996 that the core of the planet rotates faster than the rest of the planet – referred to as super-rotation – by approximately one degree per year. Vidal’s subsequent discoveries supported the idea that the inner core rotates at a slower rate.
Research team Wei Wang and Vidale analyzed data from the Large Aperture Seismic Array (LASA), a US Air Force facility in Montana, to find that the inner core is spinning slightly slower than previously estimated, about 0.1 degrees per year.
In the study, a novel beamforming technique developed by Vidale was used to analyze the waves generated by Soviet nuclear bomb tests conducted between 1971 and 1974 in the arctic archipelago of Novaya Zemlya.
Using the same methodology, Wang and Vidale analyzed atomic tests under Amchitka Island, at the tip of the Alaskan archipelago: Milrow in 1969 and Cannikin in 1971.
In measurements of compression waves resulting from nuclear explosions, they discovered that the inner core reversed direction, underrotating by at least a tenth of a degree per year. The latest study represented the first time the well-known six-year oscillation had been directly observed by seismologists.
Vidale and Wang point out that future research is dependent on finding precise observations to compare with the present results. Using seismic data from previous studies on atomic tests, Wang says they were able to pinpoint the exact location and time of the seismic event.
However, with the Montana LASA’s closure in 1978 and the end of underground nuclear testing in the US, researchers could no longer rely on recent advances in instrumentation to measure seismic properties.
This study supports the hypothesis that the inner core oscillates as measured by variations in day length — over a period of six years, plus or minus 0.2 seconds — and by geomagnetic fields, which match theory both in amplitude and in phase. Several questions raised by the research community are addressed with the findings, says Vidale.
“The inner core is not fixed — it’s moving under our feet, and it seems to go back and forth a couple of kilometers every six years,” Vidale said.
“One of the questions we tried to answer is, does the inner core progressively move, or is it mostly locked compared to everything else in the long term? We’re trying to understand how the inner core formed and how it moves over time — this is an important step in better understanding this process.”
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