Context: Recently, 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.

Key Findings

• 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.
• In 1996, a small but systematic change of seismic waves passing through the inner core was first detected, which was interpreted as evidence for differential rotation of the inner core relative to the Earth’s surface.
• Some studies said that the 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.
• 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. 
• This work confirms that the temporal changes come mostly, 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.