The Earth moves far under our feet: A new study shows that the inner core oscillates
USC scientists found evidence indicating the Earth’s inner core oscillates, contradicting previously held beliefs that it rotates at a higher pace than the planet’s surface.
According to seismic data analysis, their study, published in Science Advances, demonstrates that the inner core changed direction throughout the six-year period from 1969 to 1974. The scientists claim that their model of inner core movement also explains the change in day length, which has been shown to oscillate over several decades.
“From our findings, we can see the Earth’s surface shifts compared to its inner core, as people have asserted 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 spun slightly slower from 1969–71 and then moved the other direction from 1971–74. We also see that the duration of the day expanded and shrank as expected.
“The coincidence of those two observations makes oscillation the likely interpretation.”
Analysis of atomic tests pinpoints rotation rate and direction
Our understanding of the inner core has expanded dramatically in 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, so scientists must rely on indirect measurements to explain the pattern, speed, and cause of the movement and changes.
The first proof that the inner core rotates faster than the rest of the globe, commonly known as super-rotation, was published in 1996. Subsequent findings from 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, Wei Wang and Vidale discovered that the inner core rotated at a slower rate than previously estimated, at around 0.1 degree per year. The waves generated by Soviet underground nuclear bomb tests in the Arctic archipelago of Novaya Zemlya in 1971-74 were examined using a revolutionary beamforming technique developed by Vidale.
The new findings emerged when Wang and Vidale applied the same methodology to a pair of earlier atomic tests beneath Amchitka Island at the tip of the Alaskan archipelago—Milrow in 1969 and Cannikin in 1971. They determined the inner core has reversed direction, sub-rotating at least a tenth of a degree every year, by measuring the compressional waves caused by nuclear explosions.
This current study was the first to show the well-known six-year oscillation by direct seismological observation.
“The idea the inner core oscillates was a model that was out there, but the community has been split on whether it was viable,” Vidale says. “We went into this expecting to see the same rotation direction and rate in the earlier pair of atomic tests, but instead we saw the opposite. We were quite surprised to find that it was moving in the other direction.”
Future research to dig deeper into why inner core formed
Vidale and Wang both noted future research would depend on finding sufficiently precise observations to compare against these results. They were able to establish the exact location and timing of the very simple seismic event by using seismological data from previous studies, according to Wang. However, the Montana LASA closed in 1978 and the era of U.S. underground atomic testing is over, meaning that the researchers would need to rely on comparatively imprecise earthquake data, even with recent advances in instrumentation.
The study does support the speculation that the inner core oscillates based on variations in the length of day—plus or minus 0.2 seconds over six years—and geomagnetic fields, both of which match the theory in both amplitude and phase. According to Vidale, the data give a persuasive hypothesis for many of the scientific community’s questions.
“The inner core is not fixed—moving it’s under our feet, and it seems to going 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.”