The Earth’s inner core is 3,000 miles below the surface and was thought to be a solid sphere held by gravity in the molten outer core. Recent studies show that the inner core’s rotation rate or shape can change over years or decades. This was found through variations in seismic waves from repeated earthquakes.
A new study by USC scientists shows that the inner core’s surface may be changing. Their main goal was to study the inner core’s slowing rotation.
John Vidale, a professor at USC, was analyzing decades of seismograms and noticed an unusual dataset that suggested the inner core might not be solid. His team discovered that the surface of the inner core undergoes structural changes influenced by topographical activity. These changes affect the rotation of the inner core and can slightly alter the length of a day.
The study used seismic waveform data from 121 earthquakes near Antarctica’s South Sandwich Islands, recorded between 1991 and 2024. When analyzing data from stations in Fairbanks, Alaska, and Yellowknife, Canada, they found unusual seismic waves in the Yellowknife data, which led to their discoveries.
“At first, the dataset confounded me,” Vidale said. It wasn’t until his research team improved the resolution technique that it became clear the seismic waveforms represented additional physical activity of the inner core.
The inner core’s physical activity is best explained by changes in its shape over time. The study suggests that the near surface of the inner core may deform and change shape due to interactions with the outer core. This causes shifts at the shallow boundary of the inner core. The interaction between the inner and outer core is the most likely cause of these structural changes.
“The molten outer core is widely known to be turbulent, but its turbulence had not been observed to disrupt its neighbor, the inner core, on a human timescale,” Vidale said. “What we’re observing in this study for the first time is likely the outer core disturbing the inner core.”
Vidale said the discovery opens a door to revealing previously hidden dynamics deep within Earth’s core and may lead to a better understanding of its thermal and magnetic fields.
Journal Reference:
- Vidale, J.E., Wang, W., Wang, R. et al. Annual-scale variability in both the rotation rate and near-surface of Earth’s inner core. Nat. Geosci. (2025). DOI: 10.1038/s41561-025-01642-2
