Inner core of Earth appears to be spinning

Earth's interior
Michael Vincent

The Earth’s inner core, which is the innermost layer of the planet, seems to be rotating approximately 0.05-0.1 degrees per year, according to a new study. The research team at the University of Illinois in Urbana-Champaign analysed seismic data from earthquakes and published their findings in Earth and Planetary Science Letters.

Geologists understand little of how the Earth’s magnetic field is generated. They suspect that dynamic processes which are at play between the border of the inner and outer core are responsible. 

Using seismic data and novel processing techniques, the team found that the Earth’s inner core is spinning. Researchers consider this detail, “the best evidence yet,” as it could help clarify the mechanisms underlying Earth’s magnetic field. 

The team examined seismic data from various geographic locations, especially areas where doublets—closely spaced earthquakes—occur. They had noticed that some seismic waves pierced into the inner core and fluctuated with time, a sign that the inner core was not immobile.

“In 1996, our group first detected a small but systematic change in the seismic waves passing through the inner core, which we interpreted as evidence of the differential rotation of the inner core relative to the surface of the Earth,” says co-author Xiaodong Song. “Importantly, we are seeing that these refracted waves change before the reflected waves bounce off the inner core boundary, implying that the changes come from inside the inner core,” adds Song.

Earlier studies have used an uncertain method that depended on the exact time of the clock to come up with little data. “What makes our analysis different is our precise method of determining exactly when changes in seismic signals occur and reach the various seismic stations around the world,” says co-author and graduate student Yi Yang. “We use a seismic wave that did not reach the inner core as a reference wave in our calculations, which removes a lot of ambiguity.” 

Remarkably, the inner core of the Earth’s rotation speed is less than one degree every million years as it grows, even as the outer core solidifies. 

“It is this careful analysis of arrival time, an extensive collection of the best-quality data, and careful statistical analysis by Yang, that gives this study its power. This work confirms that temporal changes come primarily, if not entirely, from the inner core body, and the idea that changes on the surface of the inner core are the only source of signal changes can now be ruled out.”

Reference: Yi Yang & Xiaodong Song. 2020. Origin of temporal changes of inner-core seismic waves. Earth and Planetary Science Letters 541: 116267; doi: 10.1016 / j.epsl.2020.116267

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