Contradicting a previously accepted theory, scientists have concluded that the inner core of the Earth oscillates and results in variations in the length of the day. With the Earth’s structure divided into layers, the inner core is found at the centre followed by the outer core, lower mantle, upper mantle crust, and then atmosphere. It was earlier believed that the inner core, which is the hottest part of the Earth, rotates at a speed faster than the planet’s surface. However, now scientists from the University of Southern California (USC) have noted that the inner core oscillates and has changed direction in past decades.

The findings were part of a new study published in Science Advances. “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.

Vidale added that the inner core of the Earth was found to have spun slower between 1969 and 1971. “We also note that the length of a day grew and shrank as would be predicted,” said Vidale. Highlighting the two observations, Vidale said the coincidence indicates that the inner core oscillates.

Vidale, along with researcher Wei Wang, has utilised seismic data from the Large Aperture Array (LASA) and observed that the inner core rotated slower than previously concluded. While a 1996 research predicted the speed to be 1 degree per year, the new study estimated it to be 0.1 degree per year.

Vidale developed a novel beamforming technique and used it to analyse the waves generated from Soviet underground nuclear bomb tests from 1971 to 1974. Wang adopted the same technique for studying waves generated from two atomic tests conducted beneath Amchitka Island.

Scientists further measured the compressional waves from the nuclear explosions and noted that the inner core had begun sub-rotating at speed of around a tenth of a degree per year. These findings also indicated the six-year oscillation through direct seismic observation for the first time. “The inner core is not fixed — it’s moving under our feet, and it seems to go back and forth a couple of kilometres every six years,” added Vidale.