Recent seismic wave analysis indicates that Earth's inner core, a solid metal sphere rotating within the molten outer core, may be experiencing changes in its rotation and surface.
Geophysicist John Vidale from the University of Southern California reported on December 9, 2023, that the inner core's rotation might have slowed down significantly, potentially stopping or even reversing direction relative to the surface approximately 15 years ago. This new analysis suggests further transformations within the Earth's center.
As physical probing of the Earth's core is impossible, researchers utilize seismic waves generated by earthquakes for study. Notably, seismic events from the South Sandwich Islands, located near Antarctica, are often analyzed, as they are diametrically opposite to Alaska's seismic instruments. The seismic waves travel through the Earth akin to sonar waves in water, providing data on their journey through the core.
To detect changes in the inner core, researchers compare seismic events of similar magnitude occurring in the same location at different times. These 'doublets' should produce identical waveforms if their paths through the Earth are the same. However, differences in waveforms from doublets in the South Sandwich Islands observed in Alaska suggest alterations in the inner core.
In 2023, geophysicists noted that the differences in waveform shapes indicated a significant slowdown in the inner core's rotation, seemingly stopping or reversing around 2009. Vidale's team confirmed this inversion by comparing waveforms before and after the event, identifying moments when the inner core resumed its previous orientation.
For their recent study, Vidale and colleagues analyzed around 200 earthquake pairs from 1991 to 2024, focusing on waveforms recorded near Fairbanks, Alaska, and Yellowknife, Canada. They found subtle differences in waveforms in Yellowknife that were absent in Fairbanks, suggesting varying paths through the Earth's interior.
Vidale proposed that the simplest explanation for the observed differences is deformation of the shallow inner core. The entire inner core may be reshaping, akin to a football being adjusted to point in new directions. Alternatively, localized surface bulges or depressions may be forming due to gravitational influences from the mantle or material flow in the outer core.
This is not the first report of surface changes in the inner core. In 2006, geophysicist Lianxing Wen from Stony Brook University noted potential vertical movements of hundreds of meters per decade in certain areas, attributed to cooling material from the inner core. However, Wen and colleagues argue that the inner core does not rotate differently from the Earth.
Geophysicist Xiaodong Song from Peking University, who was among the first to suggest distinct rotation of the inner core, generally aligns with Vidale's conclusions while acknowledging the possibility of other changes occurring simultaneously.
As for the implications of these findings for life on Earth, Vidale stated, 'We do not know if this will affect anything on the surface, but we cannot be certain until we determine what is happening.'