In January 2023, a study published in Nature Geoscience showed that the direction of rotation of the Earth's inner core may have been reversed relative to the rotation of the mantle and surface. According to their model, the core may have an oscillation period of about 70 years, making this transition likely to occur every 35 years or so.
The study has attracted a lot of attention, but some researchers are skeptical of the findings, arguing that the Earth's inner core does not rotate independently, and that the data can also be explained by changes in the surface shape of the inner core.
Now, in a recent study published in the journal Nature, researchers have confirmed last year's controversial finding. New research shows that the Earth's inner core rotated in overspeed between 2003 and 2008 and slowly in the opposite direction between 2008 and 2023. The new study provides the most direct evidence for the oscillation of the Earth's inner core.
Multiple Earthquakes
The Earth's inner core is a solid iron-nickel sphere surrounded by a liquid iron-nickel outer core. The inner core has a rich internal structure and influences the convective patterns of the outer core, which in turn affects the Earth's magnetic field. However, studying the Earth's inner core is not an easy task, and because it is located more than 5,000 kilometers below our feet, scientists cannot directly enter this area to observe, and must rely on seismic waves to map its motion.
In the new study, researchers compiled and analysed seismic data from 121 seismic events that occurred at 42 locations around the South Sandwich Islands near Antarctica between 1991 and 2023. Among them, 16 events included 3~7 multiple seismic events, that is, those seismic events that occurred at different times and in the same place, and finally obtained 143 pairs of different repeated earthquakes.
As the seismic waves produced by these earthquakes pass through the Earth's interior, some of them pass through the Earth's core. When these waves reach the other side of the Earth, instruments on the other side of the Earth can record the ground motion as a zigzag linear pattern, known as a seismic waveform.
If the rotation of the Earth's inner core is independent of the rest of the Earth's layers, then waves from multiple earthquakes should pass through different parts of the inner core. And since the anatomy of the kernel is inhomogeneous, the paths of these different waves should produce different waveforms.
However, if the 2023 study is correct, i.e., the rotation of the Earth's inner core relative to the surface has been reversed, then the waveforms of some multiple earthquakes will revert back to their original waveforms after some time. In other words, researchers can study the movement of the core by looking for these matching waveforms that indicate that the core has reoccupied the same position relative to the mantle.
Look for matching waveforms
In the new study, researchers confirmed the correctness of the 2023 study's conclusions after observing how the waveform changes over time. After comparing a large number of waveforms, they found some matching waveforms. These matching waveform data suggest that sometime around 2008, the rotation of the Earth's inner core relative to the mantle was reversed, after which it rotated less than half as fast.
内核似乎正在减速，其旋转速度比地表和地幔更慢。 （图/USC Graphic/Edward Sotelo via usc.edu）
In an absolute sense, the core is still rotating in the same direction as the mantle and surface. It's like a bus and a truck traveling side by side in the same direction, and the truck slows down and the bus keeps going. From the perspective of the bus, the truck appears to be moving backwards; But for pedestrians on the side of the road, both cars are still moving forward. Similarly, if one could stand on the surface of the earth and observe the movement of the Earth's inner core, one would be able to observe that the inner core seemed to be moving in the opposite direction to the mantle and surface.
Wait for further confirmation
The movement of the Earth's inner core has been a controversy in the scientific community for a long time. Now, the new study appears to provide even more definitive evidence that the Earth's inner core is indeed decelerating.
This slowdown may change the length of the day, but the effect will occur on the order of thousandths of a second that is difficult to notice. Next, the researchers hope to map out the trajectory of the inner core in more detail to reveal the exact reason behind this phenomenon.
However, the controversy has not completely subsided. There are still scientists who insist that these waveform data can be explained by the expansion and contraction of parts of the inner core's surface. In response, the researchers say they may be able to get some more definitive confirmation in the coming years. If the rotation of the kernel does oscillate at a frequency of about 70 years, it may soon re-enter the active part of the cycle. By looking at the next 5 or 10 years, we may be able to find out.
#创作团队：
Compilation: Xiaoyu
Typesetting: Wenwen
#参考来源：
https://today.usc.edu/usc-study-confirms-the-rotation-of-earths-inner-core-has-slowed/
https://www.sciencenews.org/article/earths-inner-core-has-weird-rotation
https://www.sciencenews.org/article/earth-inner-core-reverse-rotation
https://www.nature.com/articles/s41586-024-07536-4
https://www.nature.com/articles/s41561-022-01112-z
#图片来源：
封面图 & 首图：USC Graphic/Edward Sotelo via usc.edu）
Source: Principle
Edit: Stop clouds
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