WASHINGTON, D.C. — Earth’s inner core, a solid metal ball gyrating within the molten outer core, may be both slowing down and changing shape.
Recent analyses of earthquake waves have suggested that around 15 years ago, the inner core’s rotation may have slowed so much that it appeared to pause or reverse direction relative to the surface. But a new analysis suggests something more must be changing at Earth’s center.
The most probable explanation is that the inner core is not only rotating differently — its surface is probably also morphing, geophysicist John Vidale of the University of Southern California in Los Angeles reported December 9 at a meeting of the American Geophysical Union. The finding could help resolve a long-standing debate over what is changing at the inner core.
No instrument can physically probe Earth’s core. So, researchers study it using seismic waves from earthquakes. Scientists typically use quakes that occur in the South Sandwich Islands near Antarctica, which repose on the opposite side of the planet from instrument arrays in Alaska. The earthquake waves travel through the planet like sonar waves through water, with some passing through the inner core on their way to Alaska. Instruments there then record the waves as squiggly signatures called waveforms, which contain information about what the waves encountered on their journey through Earth.
For robust detections of changes in the inner core, researchers compare similar-sized quakes that occurred in the same place but at different times. Such twin temblors, known as doublets, should generate the same waveforms if their journeys through Earth were identical. But researchers have observed that some doublets in the South Sandwich Islands generate different waveforms in Alaska, indicating that something in the inner core had changed between the times the two quakes in those doublets occurred.
In 2023, geophysicists reported that the…
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