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Earth’s inner core is rotating, study

Growing mountains or shifting ground: What is going on in Earth’s inner core?

Earth’s inner core is rotating, study
Graphic by Michael Vincent

Temporal changes of inner-core (IC) seismic phases have been confirmed with high-quality waveform doublets. However, the nature of the temporal changes is still controversial.

Geologists don’t completely understand how the Earth’s magnetic field generator works; however, presume it is firmly connected to dynamic procedures close to the inner core-outer core boundary area.

A new study based on seismic data from repeating earthquakes and new data-processing methods has revealed that Earth’s inner core is rotating. The findings could give detailed insights into the processes that control the planet’s magnetic field.

Geology professor and study co-author Xiaodong Song, who is now at Peking University said, “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. 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.”

Scientists reviewed seismic data from a range of geographic locations and repeating earthquakes, called doublets, that occur in the same spot over time. This allowed them to categorize between seismic signals that change due to localized variation in relief from those that change due to movement and rotation.

Scientists found that some of the earthquake-generated seismic waves infiltrate through the iron body underneath the inner core boundary and change after some time, which would not occur if the inner core were stationary.

Song said“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.”

Yi Yang, a graduate student and lead author of the study said, “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. We use a seismic wave that did not reach inner core as a reference wave in our calculations, which eliminates a lot of the ambiguity.”

Song said, “This 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.”

Journal Reference:
  1. Yi Yang, Origin of temporal changes of inner-core seismic waves. DOI: 10.1016/j.epsl.2020.116267