With the help of extensive seismic data from repeating Earthquakes known as doublets, as well as new data-processing methods, researchers have found seismic signals that provide the best evidence yet that Earth’s inner core really is spinning.
A new study of the Earth’s inner core used repetitive seismic data from earthquakes, called doublets, to find that refracted waves rather than reflected waves change over time, providing the best evidence thus far that the Earth’s inner core is spinning.
The new study by researchers at the University of Illinois at Urbana-Champaign is published in the journal Earth and Planetary Science Letters. Geologists do not fully understand how Earth’s magnetic field generator works but speculate it is intimately related to dynamic processes near the border area between the inner core and the outer core, the researchers have revealed. Variations in the location of the magnetic poles, changes in field strength, and anomalous seismic data have prompted researchers to take a closer examination.
Back in 1996, scientists detected for the first time small but systematic variations in the seismic waves passing through the inner core, which was interpreted as evidence of the 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 that are reflected in an inner core boundary that alternately enlarges and contracts, such as the growth of mountains and the cutting of canyons, professor of geology and co-study author Xiaodong Song, from Peking University explained.
The researchers present seismic data from a variety of geographic locations and repetitive earthquakes, called doublets, that occur at the same location over time. “Having data from the same location but at 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 Yi Yang, a graduate student, and lead author of the study.
The team found that some of the seismic waves generated by earthquakes penetrate through the iron body below the inner core boundary and change over time, which would not happen if the inner core were stationary, the researchers revealed.
“More importantly, we are seeing these refracted waves change before the reflected waves bounce off the boundary of the inner core, implying that the changes come from within the inner core,” Song explained.
The basis of the debate lies in the fact that previous studies looked at a relatively small group of somewhat ambiguous data generated from a method that relies heavily on the exact time of the clock, the researchers said.
“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,” explained Yang.
“We use a seismic wave that did not reach the inner core as the reference wave in our calculations, which eliminates a lot of ambiguity,” he added.
This precise analysis, an extensive collection of high-quality data, and careful statistical analysis by Yang are what power this study, Song said in a statement.
“This work confirms that the temporal changes come primarily, if not all, from the body of the inner core, and the idea that changes to the inner core surface are the only source of signal changes can now be ruled out,” he said.