正在變形的地球內核

 原文網址：https://today.usc.edu/earths-inner-core-is-less-solid-than-previously-thought/

By Will Kwong

美國南加州大學今日發表在《自然—地球科學》(Nature Geoscience)的新研究偵測到地球核心附近的結構有所變化，顯示內核的表面可能正在改變。

地球內核的表面附近或許正在改變。圖片來源：USC Graphic/Edward Sotelo

內核的變化長久以來都是科學家之間討論熱烈的議題。然而，大部分研究人員的重點都是放在如何測定內核的轉動。南加大文理學院的地球科學教授John Vidale是此研究的計畫主持人，他說他們原本「並不是要解釋內核的物理性質」。

「結果我們發現的證據顯示，內核表面附近的結構正處在變化當中，」Vidale表示。內核的轉動會讓一天的長度出現幾分鐘的差異，而這項發現闡明了內核表面的活動對於其轉動過程會有什麼影響；不只如此，這或許也和內核轉動速度持漸變慢的現象有所關連。

重新定義內核

內核位在地表之下3000英里深的地方，受到重力作用而被固定在融化的液態外核之內。在此研究之前一般認為內核是一顆穩固的球體。

這群南加大科學家原本的目標為深入繪製內核的減速狀況。「在我分析長度加起來有數十年的震波圖記錄時，有一組奇特的地震波數據跟其餘的相比之下顯得特別突出，」Vidale表示。「之後我才發覺我看到的是內核並非穩固的證據。」

此研究運用地震波形的數據來探討內核內部發生的事物，這些地震為1991至2024年，南極附近的南三明治群島上42個地點發生的121起重複地震。研究人員分析了阿拉斯加的費爾班克斯以及加拿大黃刀鎮附近的地震儀陣列所接收到的地震波形，其中黃刀鎮的一組地震波數據含有團隊從未見過的特殊性質。

「這組資料最初讓我感到十分困惑，」Vidale說。他的研究團隊改進了解析技術之後，才明白這些地震波的波形代表了內核有比已知更多的物理活動。

內核變形

此物理活動最好的解釋方法是內核的形狀會隨著時間變化。新研究暗示了內核表面附近可能會受到黏性變形，使其形狀改變並在內核淺層移動。

造成內核結構變化最明顯的原因便是內外核之間的交互作用。「眾所皆知處於融化狀態的外核會不停翻攪，但是之前從未以人類的時間尺度觀察到外核的亂流對鄰近的內核有造成任何擾動，」Vidale表示。「我們這項研究首度觀察到的現象，可能就是外核對內核造成的擾動。」

Vidale說此發現開啟了一道途徑來觀察深埋在地球核心、之前不為所知的動力學現象，或許還能讓我們更加瞭解地球的熱力學與磁場活動。

 

Earth's inner core is undergoing a transformation

The surface of the Earth's inner core may be changing, as shown by a new study from USC scientists that detected structural changes near the planet's center, published today in Nature Geoscience.

The changes of the inner core has long been a topic of debate for scientists. However, most research has been focused on assessing rotation. John Vidale, Dean's Professor of Earth Sciences at the USC Dornsife College of Letters, Arts and Sciences and principal investigator of the study, said the researchers "didn't set out to define the physical nature of the inner core."

"What we ended up discovering is evidence that the near surface of Earth's inner core undergoes structural change," Vidale said. The finding sheds light on the role topographical activity plays in rotational changes in the inner core that have minutely altered the length of a day and may relate to the ongoing slowing of the inner core.

Redefining the inner core

Located 3,000 miles below the Earth's surface, the inner core is anchored by gravity within the molten liquid outer core. Until now the inner core was widely thought of as a solid sphere.

The original aim of the USC scientists was to further chart the slowing of the inner core. "But as I was analyzing multiple decades' worth of seismograms, one dataset of seismic waves curiously stood out from the rest," Vidale said. "Later on, I'd realize I was staring at evidence the inner core is not solid."

The study utilized seismic waveform data -- including 121 repeating earthquakes from 42 locations near Antarctica's South Sandwich Islands that occurred between 1991 and 2024 -- to give a glimpse of what takes place in the inner core. As the researchers analyzed the waveforms from receiver-array stations located near Fairbanks, Alaska, and Yellowknife, Canada, one dataset of seismic waves from the latter station included uncharacteristic properties the team had never seen before.

"At first the dataset confounded me," Vidale said. It wasn't until his research team improved the resolution technique did it become clear the seismic waveforms represented additional physical activity of the inner core.

Deformed inner core

The physical activity is best explained as temporal changes in the shape of the inner core. The new study indicates that the near surface of the inner core may undergo viscous deformation, changing its shape and shifting at the inner core's shallow boundary.

The clearest cause of the structural change is interaction between the inner and outer core. "The molten outer core is widely known to be turbulent, but its turbulence had not been observed to disrupt its neighbor the inner core on a human timescale," Vidale said. "What we're observing in this study for the first time is likely the outer core disturbing the inner core."

Vidale said the discovery opens a door to reveal previously hidden dynamics deep within Earth's core, and may lead to better understanding of Earth's thermal and magnetic field.

原始論文：John E. Vidale, Wei Wang, Ruoyan Wang, Guanning Pang, Keith Koper. Annual-scale variability in both the rotation rate and near surface of Earth’s inner core. Nature Geoscience, 2025; DOI: 10.1038/s41561-025-01642-2

引用自：University of Southern California. "Earth's inner core is undergoing a transformation."