Imma Perfetto
Cosmos science journalist
Geophysicists using seismic data from two underground nuclear explosions in 1969 and 1971 have found evidence that the Earth’s inner core oscillates.
The new study, published in Science Advances, goes against previously accepted models that suggested the Earth’s core consistently rotates at the same rate as, or faster than, the planet’s surface.
The new model – which is a six-year cycle of rotation in one direction and then the other – can explain why a variation in the length of a day has oscillated fairly steadily over the past decades.
“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?” explains co-author John E. Vidale, a geophysicist at the University of Southern California (USC), US.
“The inner core is not fixed – it’s moving under our feet, and it seems to be going back and forth a couple of kilometres every six years.”
Ogres have layers, Earths have layers
The Earth is made up of several layers. We live upon the thinnest surface layer, the crust, underneath which is the mostly solid mantle. Then there is a liquid iron outer core, and finally at the very centre of Earth is the inner core – a hot, dense ball of solid iron and nickel the size of Pluto.
The Earth rotates in a west to east direction (counter-clockwise if viewed from the North Pole) and takes around 24 hours to complete one rotation – that’s a single day on Earth. However, the length of a day has been shown to vary, following a cyclic oscillation pattern every 5.9 years.
Previous research by Vidale and co-author Wei Wang, a researcher in the Department of Earth Sciences at USC, had found that the inner core rotated slower than previously predicted.
Because it’s impossible to directly observe the inner core, it’s movements and changes must be measured indirectly. They used data from the Large Aperture Seismic Array (LASA), a US Air Force facility in Montana, to analyse the compressional waves generated from Soviet underground nuclear bomb tests from 1971-74.
In the new research, Wang and Vidale applied the same methods to a pair of earlier atomic tests that occurred underneath Amchitka Island at the tip of the Alaskan archipelago in 1969 (Milrow) and 1971 (Cannikin).
“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.”
Using seismic waves to measure the inner core’s rotation
Wang and Vidale investigated the difference in rotation of the Earth’s inner core relative to the mantle using pairs of precisely located nuclear explosions. They measured changes in the backscattering within the inner core of a type of seismic wave (the P wave) produced by the explosion.
Inner core seismic waves change over time if the inner core rotates; waves that are scattered within receding regions of the inner core arrive at sensors progressively later, whereas waves scattered in approaching regions arrive earlier.
By studying the time shifts of the waves generated in pairs of blasts, the rotation of the inner core in the intervening years could be estimated.
“Our latest observations show that the inner core spun slightly slower from 1969-71 and then moved the other direction from 1971-74,” explains Vidale. “We also note that the length of day grew and shrank as would be predicted.
“The coincidence of those two observations makes oscillation the likely interpretation.”