As much as we have explored and modeled our planet, there are a number of mysteries that surround the Earth’s inner core. This isn’t that surprising, as it’s pretty difficult to study a region over 5,100 kilometers (3,170 miles) beneath our feet when the furthest we have physically drilled into the Earth is a measly 12,263 meters (40,230 feet). But we can learn about the center by looking at seismic waves traveling through the Earth, as well as the magnetic field lines of the planet, the result of conditions in the core.

Scientists have gotten pretty good at this, and found out a few surprising things along the way. In 2022, for instance, one team used data collected during nuclear tests in the 1960s and 70s. They found that the inner core had reversed direction with respect to the surface, sub-rotating (i.e. going slower than the surface) a tenth of a degree at least once per year. Rather than being relatively fixed, the core appeared to be oscillating.

“The inner core is not fixed – it’s moving under our feet, and it seems to [be] going back and forth a couple of kilometers every six years,” study author John E. Vidale, Professor of Earth Sciences at USC, said in a press release at the time.  “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.”

Subsequent studies showed further evidence that the rotation of the core has been slowing down compared to the rest of the Earth since around 2010. In the new study, Vidale and his team attempted to investigate changes to seismic waves as the Earth’s core rotated at different speeds.

“Recent work confirmed that the inner core rotated faster and then slower than the rest of Earth in the last few decades; this work analysed inner-core-traversing (PKIKP) seismic waves recorded by the Eielson (ILAR) and Yellowknife (YKA) arrays in northern North America from 121 repeating earthquake pairs between 1991 and 2023 in the South Sandwich Islands,” the team explained in their study. “Here we extend this set of repeating earthquakes and compare pairs at times when the inner core re-occupied the same position, revealing non-rotational changes at YKA but not ILAR between 2004 and 2008.”

Looking at 168 repeating pairs of earthquakes from before and after the core had returned to the same position, they were able to analyze whether changes to seismic waves were due to rotation rate or other factors. According to the team, part of the change to seismic waves was due to changes in the shape of the core.

“The waveform changes observed here, and the changes cited in other studies, would arise from local deformation or material changes,” the team concluded. “The most likely explanation of the observed changes in YKA, PKIKP waves is viscous deformation of the [inner-core boundary] and the shallow IC, driven by [outer core] tractions and the [core–mantle boundary topography] coupling.”

Other more “exotic” changes have not been ruled out by the team, such as the expulsion of melt, and further study is needed.

“Here we simply argue that change is present in addition to the more dominant signal of differential rotation of the entire IC,” the team stated. “We thus present a resolution of a long-standing debate—both rotation and non-rotational changes are present, and the latter merit careful investigation to see what is stirring in the core.”

The study is published in Nature Geoscience.