New evidence suggests Earth's inner core is rotating

A new study from University of Illinois at Urbana-Champaign has found strong evidence to suggest that the Earth’s inner core is rotating. The research is shedding new light on the systems that control our planet’s magnetic field.

According to the study authors, geologists do not fully understand how the Earth’s magnetic field generator works, but it seems to be closely linked to dynamic processes near the inner core-outer core boundary area.

Irregular seismic data, movement of the magnetic poles, and changes in the strength of the magnetic field are all mysterious clues as to what is actually going on in Earth’s inner core.

“In 1996, a small but systematic change of seismic waves passing through the inner core was first detected by our group, which we interpreted as evidence for differential rotation of the inner core relative to the Earth’s surface,” said study co-author Professor Xiaodong Song. 

“However, some studies believe that what we interpret as movement is instead the result of seismic waves reflecting off an alternately enlarging and shrinking inner core boundary, like growing mountains and cutting canyons.”

The researchers analyzed seismic data from various geographic locations and investigated a phenomenon called doublets, or earthquakes that repeat in the same spot over time. 

“Having data from the same location but different times allows us to differentiate between seismic signals that change due to localized variation in relief from those that change due to movement and rotation,” said study lead author Yi Yang.

The study revealed that some of the earthquake-generated seismic waves penetrate through the iron body below the inner core boundary and change over time. This would not happen if the inner core were stationary, the researchers explained. 

“Importantly, we are seeing that these refracted waves change before the reflected waves bounce off the inner core boundary, implying that the changes are coming from inside the inner core,” said Professor Song.

Previous studies were focused on a small, ambiguous dataset that was obtained using a method that is highly dependent on accurate clock time, said the study authors.

“What makes our analysis different is our precise method for determining exactly when the changes in seismic signals occur and arrive at the various seismic stations across the globe,” said Yang.

“We use a seismic wave that did not reach the inner core as a reference wave in our calculations, which eliminates a lot of the ambiguity.”

Professor Song noted that the precise arrival time analysis, an extensive collection of the best quality data, and careful statistical analysis performed by Yang are what give this study its power. 

“This work confirms that the temporal changes come mostly, if not all, from the body of the inner core, and the idea that inner core surface changes are the sole source of the signal changes can now be ruled out.”

The study is published in the journal Earth and Planetary Science Letters

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By Chrissy Sexton, Earth.com Staff