Earth’s inner core, a solid iron sphere nestled deep within our planet, has slowed its rotation, according to new research. Scientists from the University of Southern California say their discovering challenges previous notions about the inner core’s behavior and raises intriguing questions about its influence on Earth’s dynamics.

The inner core, a mysterious realm located nearly 3,000 miles beneath our feet, has long been known to rotate independently of the Earth’s surface. Scientists have spent decades studying this phenomenon, believing it to play a crucial role in generating our planet’s magnetic field and shaping the convection patterns in the liquid outer core. Until now, it was widely accepted that the inner core was gradually spinning faster than the rest of the Earth, a process known as super-rotation. However, this latest study, published in the journal Nature, reveals a surprising twist in this narrative. “When I first saw the seismograms that hinted at this change, I was stumped,” says John Vidale, Dean’s Professor of Earth Sciences at the USC Dornsife College of Letters, Arts and Sciences, in a statement. “But when we found two dozen more observations signaling the same pattern, the result was inescapable. The inner core had slowed down for the first time in many decades. Other scientists have recently argued for similar and different models, but our latest study provides the most convincing resolution.”

Slowing Spin, Reversing Rhythm

By analyzing seismic waves generated by repeating earthquakes in the South Sandwich Islands from 1991 to 2023, the researchers discovered that the inner core’s rotation had not only slowed down but had actually reversed direction. The team focused on a specific type of seismic wave called PKIKP, which traverses the inner core and is recorded by seismic arrays in northern North America. By comparing the waveforms of these waves from 143 pairs of repeating earthquakes, they noticed a peculiar pattern. Many of the earthquake pairs exhibited seismic waveforms that changed over time, but remarkably, they later reverted to match their earlier counterparts. This observation suggests that the inner core, after a period of super-rotation from 2003 to 2008, had begun to sub-rotate, or spin more slowly than the Earth’s surface, essentially retracing its previous path. The researchers found that from 2008 to 2023, the inner core sub-rotated two to three times more slowly than its prior super-rotation.

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