Decades of earthquake data indicate that Earth’s inner core has been rotating more slowly than its mantle and surface since around 2010. This finding supports a controversial claim from last year that the inner core may have reversed its rotation relative to the mantle and surface, potentially happening every 35 years.
The study also suggests that something has interfered with the 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.”
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In absolute terms, the inner core still rotates in the same direction as the mantle and surface. Imagine a bus and truck driving side by side. If the truck slows down, the bus moves ahead. From the bus’s perspective, the truck seems to move backward, although both vehicles are moving forward.
Similarly, if someone on Earth’s surface could see the inner core, it would seem to be turning in the opposite direction compared to a couple of decades ago.
The 2023 study gained media attention but faced skepticism from researchers. Seismologist Lianxing Wen of Stony Brook University in New York argued that the inner core wasn’t rotating independently and that the data could be explained by changes in the inner core’s surface shape. Others believed that the rotation fluctuated over shorter periods. Another analysis suggested a 20-to-30-year oscillation, while Vidale’s previous study suggested a 6-year oscillation.
For the new study, Vidale and his colleagues examined repeating earthquakes from 1991 to 2023 in the South Sandwich Islands near Antarctica. Seismic waves from these earthquakes passed through the inner core, and instruments in Alaska recorded the waveforms. The team searched for matching waveforms from different times. If the inner core rotates independently, waves from repeating quakes should cross different parts of it, producing distinct waveforms. Matching waveforms before and after the supposed reversal would indicate the inner core had retraced an old path.
Out of 200 waveform comparisons, the team found 25 matches. These data suggest the inner core flipped its rotation around 2008 and then rotated less than half as fast in the new direction.
Vidale suggests that the slower backtracking may be due to the inner core being deformed by the gravitational pull of the mantle, which contains 70% of Earth’s mass. Denser mantle pockets may knead the inner core, distorting the oscillation. “The inner core’s surface is right at the melting point, so it’s natural to think it’s soft in the outermost part.”
Vidale agrees with the 2023 study’s conclusion: The inner core’s gyration probably oscillates on a roughly 70-year cycle. Wen, however, maintains that changes in the inner core’s surface shape can fully explain the data.
Geophysicist Hrvoje Tkalčić believes the truth likely lies between these perspectives. Seismologists are converging on the idea that the inner core’s rotation fluctuates, but more data is needed for a definitive answer. Future observations could provide clarity if the inner core’s rotation soon reenters a vigorous part of its cycle. Vidale says, “By watching it for the next five or 10 years, we can probably get a better idea of what happened back then.”
