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Something strange is happening at Earth’s center.
Decades of earthquake data show that Earth’s inner core has been rotating slower than its mantle and surface since around 2010, researchers report June 12 in Nature. The study appears to confirm a controversial finding from last year that the inner core may have reversed its rotation relative to the mantle and surface, a shift that might occur every 35 years or so (SN: 1/23/23).
The new study also suggests that something has been interfering with the most recent turnaround, says geophysicist John Vidale of the University of Southern California in Los Angeles. “It’s going back more slowly than it was coming forward.”
In an absolute sense, the inner core is still rotating in the same direction as the mantle and surface. Imagine a bus and truck driving next to each other in the same direction. The truck decelerates, and the bus moves ahead. From the bus’s perspective, the truck now seems to be moving backward. But to a pedestrian, both vehicles appear to be going forward.
Similarly, the new study suggests that if a person standing on Earth’s surface could see the inner core — akin to the bus driver looking at the truck — it would seem to be turning in the opposite direction as it was a couple decades ago.
The 2023 study was a big hit in the headlines, but less vaunted by other researchers. Some, like seismologist Lianxing Wen of Stony Brook University in New York, countered that the inner core wasn’t rotating on its own, and that the data could be explained by the shifting shape of the inner core’s surface. Others were convinced that the rotation fluctuated over shorter periods of time. Another analysis of the data from the 2023 study suggested a 20-to-30-year oscillation, contrasting with a study coauthored by Vidale from the year before, which suggested that the rotation oscillated over a 6-year period.
For the new study, Vidale and his colleagues looked at repeating earthquakes — those that struck at the same place but at different times — from 1991 to 2023 in the South Sandwich Islands near Antarctica. The seismic waves from those temblors traversed the planet’s interior, with some passing through the inner core. When those waves arrived at the far side of the planet, instruments in Alaska recorded the ground shaking as squiggly line graphs called waveforms.
Vidale and his colleagues searched for waveforms from months or years apart that matched. If the inner core rotates independently from the Earth’s other layers, then waves from repeating quakes should cross different parts of it. And because the inner core’s anatomy is thought to be nonuniform, those different wave paths should produce distinct waveforms. But if the 2023 study was right, and the inner core had reversed its rotation with respect to the surface, there should be some identical waveforms from before and after the turnaround, marking when the inner core had stepped back into an old track.
Out of 200 waveform comparisons, the team found 25 matches. These data suggest the inner core flipped its rotation relative to the mantle sometime around 2008, after which it proceeded to rotate less than half as fast in the new direction.
According to Vidale, the slower backtracking may indicate that the inner core is being deformed by the gravitational pull of the mantle, which contains roughly 70 percent of Earth’s mass. Denser pockets of the mantle may knead the inner core as it churns, distorting the oscillation, he says. “We know the inner core’s surface is right at the melting point, so it’s natural to think it’s soft in the outermost part.”
After observing how the waveforms match up across time, Vidale says he now agrees with the conclusion from the 2023 study: The gyration of the inner core probably oscillates on a roughly 70-year cycle.
As for Wen, “nothing has changed.” He insists that the swelling and contracting of parts of the inner core’s surface can fully explain the data. These patches may rise or subside by a kilometer or more over the course of a few months — changes significant enough to alter the waveforms of repeating quakes, he says.
Geophysicist Hrvoje Tkalčić says, “It is very likely the truth is somewhere in between.” Seismologists seem to be converging upon this idea that the inner core’s rotation is distinct and fluctuates, but “we need more data to find the ultimate truth,” says Tkalčić, of the Australian National University in Canberra. Researchers must make many assumptions about the inaccessible regions of Earth’s interior, he says, hence the diverging perspectives.
Some clarity could emerge in the coming years. If the inner core’s rotation oscillates at the frequency suspected by Vidale’s team, it may soon reenter a vigorous part of the cycle, he says. Around 20 years ago, the inner core appears to have briefly rotated very quickly, and it should soon do that again, Vidale says. “By watching it for the next five or 10 years, we can probably get a better idea of what happened back then.”
