WASHINGTON, D.C. — Recent studies indicate that Earth’s inner core, a solid metal sphere spinning within the molten outer core, might be decelerating and altering its form. Analysis of seismic waves from earthquakes has hinted that approximately 15 years ago, the inner core’s rotation may have slowed to a near halt or even reversed its direction relative to the Earth’s surface. However, a fresh analysis suggests that more profound changes are occurring at the planet’s core.

According to geophysicist John Vidale of the University of Southern California, the most plausible explanation is that the inner core is not only rotating differently but also undergoing surface transformations. Vidale presented these findings on December 9 at a meeting of the American Geophysical Union. This discovery could shed light on a long-standing debate about the nature of changes within the inner core.

Direct observation of Earth’s core is impossible, so scientists rely on seismic waves generated by earthquakes to study it. Researchers typically use earthquakes from the South Sandwich Islands near Antarctica, which are situated opposite to instrument arrays in Alaska. These seismic waves travel through the Earth, with some passing through the inner core on their way to Alaska. Instruments there record the waves as waveform signatures, which provide insights into the conditions encountered during their journey.

To detect changes in the inner core, researchers compare waveforms from similar-sized earthquakes occurring in the same location but at different times. These so-called doublets should produce identical waveforms if the inner core remains unchanged. However, discrepancies in waveforms from some doublets in the South Sandwich Islands suggest that the inner core has altered between the times of these earthquakes.

In 2023, geophysicists reported that the waveform differences were due to the inner core’s rotation slowing to a near stop or reversal around 2009. Earlier this year, Vidale’s team confirmed this reversal by matching waveforms before and after the event, identifying periods when the inner core had resumed a previous orientation.

For the latest study, Vidale and his colleagues analyzed around 200 pairs of earthquakes from 1991 to 2024. They compared waveform pairs recorded before and after the reversal at two separate receiver arrays near Fairbanks, Alaska, and Yellowknife, Canada. Interestingly, 10 doublets showed subtle waveform differences at Yellowknife that were not present in Fairbanks, indicating that the inner core’s shallow surface might be deforming.

Vidale suggests that the inner core could be reshaping like a football, with its ends pointing in new directions, or patches of its surface could be swelling or contracting. These changes might be driven by the gravitational pull of the mantle or material flow in the outer core. This is not the first time researchers have observed surface changes in the inner core, but Vidale’s findings suggest a combination of rotational and surface changes.

While the implications for life on Earth remain unclear, Vidale emphasizes that understanding these core dynamics is crucial for a comprehensive picture of our planet’s interior.

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