J波顯示地球內核為固態

原文網址：http://www.anu.edu.au/news/all-news/anu-researchers-confirm-earth%E2%80%99s-inner-core-is-solid

J波顯示地球內核為固態

澳洲國立大學進行的新研究可以幫助我們瞭解我們居住的星球如何形成

J波。圖片來源：Than-Son Phạm和Hrvoje Tkalčić

Hrvoje Tkalčić副教授和博士學者Than-Son Phạm確信他們找到了地球內核為固態的直接證據。

他們想出了一種新方法可以偵測內核裡面的剪力波――J波。剪力波是一種只能在固態物體中行進的波。

Tkalčić說：「我們發現內核確實是固體，但它也比我們過去認為的還要軟。」

「如果我們的結論正確無誤，代表內核的彈性性質跟金鉑有幾分類似。內核就像時光膠囊一樣――我們對它瞭解越多，就能瞭解地球如何形成，又是如何演變。」

內核的剪力波十分微弱使我們難以直接觀測。事實上，自1930至40年代科學家預測內核為固體開始，偵測它們的存在就被視為全球地震學的「聖杯」。

因此研究人員必須想出一種全新方式來解決問題。

他們用的「波場對比法」(correlation wavefield method)著眼在大地震發生之後兩個地震站接收到的訊號之間的相似度，而非分析直接到達測站的波。此團隊之前也運用相似的技術來測量南極冰層的厚度。

Tkalčic博士表示：「我們去除了大地震發生之後震波圖最初三小時的紀錄，把重點放在發生後三到十小時。目的是要把那些明顯的訊號去除掉。」

「利用全球地震監測網，我們可以把每次大地震之後每一個測站的紀錄成對分析，這會做出極為繁多的組合。然後再測量震波圖倆倆之間的相似度。這種方式稱作互相關(cross correlation)或是相似度量測。從相似度的測量結果我們可以建立全球相關圖(correlogram)――有點像是地球留下的指紋。」

研究顯示結果接著可以用來證明J波的存在並推算剪力波在內核的速度。

Tkalčić博士說這種可以具體呈現出剪力波存在的資訊相當重要，不過這項研究帶出有關內核的其他資訊甚至令人更加興奮。

「舉例而言，目前我們並不曉得內核的確切溫度、年代與固化速率。不過有了這些全球地震學的嶄新進展，我們正逐步找出答案。」

「瞭解地球的內核也跟地球磁場的產生和維持問題之間有直接關聯。如果沒有地磁，地球表面就不會有生物存在。」

這項研究刊登於期刊《科學》(Science)。

Earth’s inner core is solid, 'J waves' suggest

A new study by researchers at The Australian National University (ANU) could help us understand how our planet was formed.

Associate Professor Hrvoje Tkalčić and PhD Scholar Than-Son Phạm are confident they now have direct proof that Earth's inner core is solid.

They came up with a way to detect shear waves, or "J waves" in the inner core -- a type of wave which can only travel through solid objects.

"We found the inner core is indeed solid, but we also found that it's softer than previously thought," Associate Professor Tkalčić said.

"It turns out -- if our results are correct -- the inner core shares some similar elastic properties with gold and platinum. The inner core is like a time capsule, if we understand it we'll understand how the planet was formed, and how it evolves."

Inner core shear waves are so tiny and feeble they can't be observed directly. In fact, detecting them has been considered the "Holy Grail" of global seismology since scientists first predicted the inner core was solid in the 1930s and 40s.

So the researchers had to come up with a creative approach.

Their so-called correlation wavefield method looks at the similarities between the signals at two receivers after a major earthquake, rather than the direct wave arrivals. A similar technique has been used by the same team to measure the thickness of the ice in Antarctica.

"We're throwing away the first three hours of the seismogram and what we're looking at is between three and 10 hours after a large earthquake happens. We want to get rid of the big signals," Dr Tkalčic said.

"Using a global network of stations, we take every single receiver pair and every single large earthquake -- that's many combinations -- and we measure the similarity between the seismograms. That's called cross correlation, or the measure of similarity. From those similarities we construct a global correlogram -- a sort of fingerprint of the Earth."

The study shows these results can then be used to demonstrate the existence of J waves and infer the shear wave speed in the inner core.

While this specific information about shear waves is important, Dr Tkalčić says what this research tells us about the inner core is even more exciting.

"For instance we don't know yet what the exact temperature of the inner core is, what the age of the inner core is, or how quickly it solidifies, but with these new advances in global seismology, we are slowly getting there.

"The understanding of the Earth's inner core has direct consequences for the generation and maintenance of the geomagnetic field, and without that geomagnetic field there would be no life on the Earth's surface."

The research has been published in the journal Science.

原始論文：Hrvoje Tkalčić, Thanh-Son Phạm. Shear properties of Earth’s inner core constrained by a detection of J waves in global correlation wavefield. Science, 2018; 362 (6412): 329 DOI: 10.1126/science.aau7649

引用自：Australian National University. " ANU researchers confirm Earth’s inner core is solid."