The 2003 film 'The Core' deals with the disasters that occur when the Earth's core stops rotating. Birds lose their sense of direction and collide with buildings, while the sky suddenly darkens and lightning storms rage. Although it is a sci-fi movie, it has been evaluated as vividly demonstrating how important the Earth's core is.
While the Earth's core does not stop as depicted in the movie, research findings have revealed that the rotation speed of the inner core can change, and even its shape can be altered.
Researchers at the University of Southern California analyzed seismic waves from repeated earthquakes, noting that there is a possibility that the shape of the Earth's inner core has changed over the past 20 years. The research findings were published in the international academic journal 'Nature Geoscience' on the 11th.
The Earth's inner core is composed of solid metal and plays a vital role in maintaining the Earth's magnetic field. As the inner core grows, it helps the outer liquid outer core to move, and in this process, a magnetic field is formed. Previous studies have shown that the inner core can rotate either faster or slower than the Earth as a whole. It has also been confirmed that the inner core changed from a state of rotating faster than the Earth to one of rotating more slowly around 2010.
To analyze this, researchers investigated data from 168 pairs of repeated earthquakes that occurred along the South American coast from 1991 to 2023. Since the inner core cannot be directly observed, scientists study its structure by analyzing the changes in seismic waves as they pass through the inner core. By comparing seismic waves from the same location, researchers found that seismic waves that passed directly through the inner core had little change, while those that only brushed past the inner core were altered.
This is a phenomenon that cannot be explained merely by changes in the rotation speed of the inner core. It suggests a high possibility that the shape of the inner core's surface itself is changing.
The researchers proposed two possible reasons for the shape change of the inner core. First, the inner core may have been pulled due to density differences in the outer layer, the mantle. Alternatively, the liquid flow of the outer core could have pushed or pulled the inner core, changing its shape. John Vidale, a professor at the University of Southern California, explained, "The inner core is often thought of as spherical, but according to this study, its surface height may have changed by more than 100 meters."
The researchers noted, "We have confirmed that the inner core can not only rotate but also change shape," adding, "This will provide an important clue to understanding the complex movements occurring within the Earth." They plan to unveil more changes and causes happening inside the Earth through further research.
Reference material
Nature Geoscience (2025), DOI: https://doi.org/10.1038/s41561-025-01642-2