撼動氣候科學的海底地震

 原文網址：https://www.caltech.edu/about/news/undersea-earthquakes-shake-climate-science

撼動氣候科學的海底地震

By Emily Velasco

儘管冬天變暖、四季如夏以及冰河不停融化是氣候變遷帶給人類最明顯的感受，但是被溫室氣體困在地球的熱量其實有多達百分之95還儲藏在海洋裡面。因為如此，測量海水的溫度對於氣候學家來說是首要之務。而加州理工學院的研究人員最近發現了一種新的測量工具：海底地震造成的低鳴。

在發表於《科學》(Science)的新論文中，研究人員說明了他們如何運用現有的地震觀測儀以及過去的地震數據，測出地球海洋的溫度過去與現在變化了多少，這種方法甚至可以用在一般工具無法進行測量的深度。

加州理工學院環境科學與工程學系的助理教授Jörn Callies是研究共同作者，他說他們的方法是聆聽海底規律發生的地震產生的聲響。由於這些地震發出的聲音非常響亮，而且在海中傳遞很遠之後強度仍然不會明顯下降，因此監測起來非常容易。

加州理工學院的地球物理博士後研究員Wenbo Wu是論文主要作者。他解釋當地震發生在海底的時候，大部分的能量會透過地球來傳播，但有一部份的能量會以聲音的形式在海裡傳播。這些聲波就跟行經地底的地震波一樣會從震央散發出去，不過它們的速度慢了許多。因此地震波會先到達地震測站，之後聲波才會抵達，看起來就像是同一次事件產生的第二股訊號。這種效應就像是我們通常看到閃電幾秒之後才會聽到雷聲一樣。

「這些海洋裡的聲波可以比雷聲傳到更加遙遠的地方，甚至連數千公里外的地震儀都能清楚記錄下來，」Wu表示。「有趣的是，這些聲波比地震學家經常利用的在固體地球深處傳遞的震動訊號還要更加『響亮』。」

由於聲波在水中的行進速度會隨著水溫增加而變快，因此研究團隊領悟到海水裡的聲波行經一段距離所花費的秒數可以用來推出海水的溫度。

「研究的關鍵是我們運用了重複地震，也就是同個地點一再發生的地震，」他說。「在這項案例中我們把重點放在印尼蘇門答臘外海的重複地震，並且測量地震產生的聲波抵達印度洋中部所花的時間。聲波要走完這段距離大概需要半個小時，而水溫的變化只會產生十分一秒左右的差異。雖然這是非常少量的改變，但我們有辦法測量出來。」

Wu進一步提到由於他們利用的印度洋中部地震儀自2004年以來就一直設置在那，因此以蘇門答臘為例，他們可以往前追溯這裡發生地震時蒐集到的每筆數據，藉此推斷出地震當時的海洋溫度。

「我們運用的地震規模相當微小因此不會造成任何損害，甚至不會讓人們產生一絲感覺，」Wu表示。「但是地震儀可以從相當遙遠的地方偵測到這些地震，因此我們只要進行一次測量，就能得到特定路徑上海水溫度的大尺度變化。」

Callies說如同其他方法蒐集到的數據所指出的結果，他們的分析證實印度洋的溫度正在升高――不過速度可能比之前估計的還快。

「海洋對於氣候變遷的速度有相當重要的影響，」他說。「海洋在氣候系統當中是能量的主要儲藏地點，而在進行監測時深海是特別需要關注的地方。我們的方法優點之一在於聲波可以取樣至深度2000公尺以下的地方，這是慣用的測量方法很少測得的數據。」

他們的結果和先前的不同數據相比，海洋暖化的速度最多可以比之前認為的快上69%。不過Callies謹慎地表示他們還需要蒐集並分析更多數據，因此還不能立即下定論。

Callies表示全世界的海底都會發生地震，因此他和同僚開發出的這套系統應該可以推廣出去，藉此監測全球海洋的水溫。Wu也表示由於這項技術是利用已經架設好的儀器設施，所以價格也相對低廉。

「我們認為還有很多地方也能進行這類觀測，」Callies總結。「藉此我們希望可以貢獻新的數據來讓我們了解海洋的暖化速度有多快。」

 

Undersea earthquakes shake up climate science

Despite climate change being most obvious to people as unseasonably warm winter days or melting glaciers, as much as 95 percent of the extra heat trapped on Earth by greenhouse gases is held in the world's oceans. For that reason, monitoring the temperature of ocean waters has been a priority for climate scientists, and now Caltech researchers have discovered that seismic rumblings on the seafloor can provide them with another tool for doing that.

In a new paper publishing in Science, the researchers show how they are able to make use of existing seismic monitoring equipment, as well as historic seismic data, to determine how much the temperature of the earth's oceans has changed and continues changing, even at depths that are normally out of the reach of conventional tools.

They do this by listening for the sounds from the many earthquakes that regularly occur under the ocean, says Jörn Callies, assistant professor of environmental science and engineering at Caltech and study co-author. Callies says these earthquake sounds are powerful and travel long distances through the ocean without significantly weakening, which makes them easy to monitor.

Wenbo Wu, postdoctoral scholar in geophysics and lead author of the paper, explains that when an earthquake happens under the ocean, most of its energy travels through the earth, but a portion of that energy is transmitted into the water as sound. These sound waves propagate outward from the quake's epicenter just like seismic waves that travel through the ground, but the sound waves move at a much slower speed. As a result, ground waves will arrive at a seismic monitoring station first, followed by the sound waves, which will appear as a secondary signal of the same event. The effect is roughly similar to how you can often see the flash from lightning seconds before you hear its thunder.

"These sound waves in the ocean can be clearly recorded by seismometers at a much longer distance than thunder — from thousands of kilometers away," Wu says. "Interestingly, they are even 'louder' than the vibrations traveling deep in the solid Earth, which are more widely used by seismologists."

The speed of sound in water increases as the water's temperature rises, so, the team realized, the length of time it takes a sound to travel a given distance in the ocean can be used to deduce the water's temperature.

"The key is that we use repeating earthquakes—earthquakes that happen again and again in the same place," he says. "In this example we're looking at earthquakes that occur off Sumatra in Indonesia, and we measure when they arrive in the central Indian ocean. It takes about a half hour for them to travel that distance, with water temperature causing about one-tenth-of-a second difference. It's a very small fractional change, but we can measure it."

Wu adds that because they are using a seismometer that has been in the same location in the central Indian Ocean since 2004, they can look back at the data it collected each time an earthquake occurred in Sumatra, for example, and thus determine the temperature of the ocean at that same time.

"We are using small earthquakes that are too small to cause any damage or even be felt by humans at all," Wu says. "But the seismometer can detect them from great distances , thus allowing us to monitor large-scale ocean temperature changes on a particular path in one measurement."

Callies says the data they have analyzed confirm that the Indian Ocean has been warming, as other data collected through other methods have indicated, but that it might be warming even faster than previously estimated.

"The ocean plays a key role in the rate that the climate is changing," he says. "The ocean is the main reservoir of energy in the climate system, and the deep ocean in particular is important to monitor. One advantage of our method is that the sound waves sample depths below 2,000 meters, where there are very few conventional measurements."

Depending on which set of previous data they compare their results to, ocean warming appears to be as much as 69 percent greater than had been believed. However, Callies cautions against drawing any immediate conclusions, as more data need to be collected and analyzed.

Because undersea earthquakes happen all over the world, Callies says it should be possible to expand the system he and his fellow researchers developed so that it can monitor water temperatures in all of the oceans. Wu adds that because the technique makes use of existing infrastructure and equipment, it is relatively low-cost.

"We think we can do this in a lot of other regions," Callies says. "And by doing this, we hope to contribute to the data about how our oceans are warming."

原始論文：Wenbo Wu, Zhongwen Zhan, Shirui Peng, Sidao Ni, Jörn Callies. Seismic ocean thermometry. Science, 2020; 369 (6510): 1510 DOI: 10.1126/science.abb9519

引用自：California Institute of Technology. "Undersea earthquakes shake up climate science."