火山內部的「膨脹」可以用來預測火山噴發

原文網址：http://www.cam.ac.uk/research/news/bulges-in-volcanoes-could-be-used-to-predict-eruptions

火山內部的「膨脹」可以用來預測火山噴發

劍橋大學的研究團隊發展出新方法來測量火山內部的壓力，並且發現這可以當作未來火山噴發的可靠指標。

利用一種稱作「地震噪訊干涉測量法」(seismic noise interferometry)的技術和其他地球物理觀測方法，研究人員可以測得在火山內部傳遞的能量。他們發現能量傳遞的速度，跟觀察到岩石的膨脹收縮量之間有很好的關係，因此這項技術可以用來更精準地預測火山噴發的時間。他們的結果發表在期刊《科學進展》(Science Advances)。

研究用的數據係由美國地質調查局於夏威夷基拉韋厄火山各處蒐集而來。基拉韋厄火山是座相當活躍的火山，在其山頂下方就有一座翻騰著岩漿的熔岩湖。研究人員在為期四年的研究期間，利用儀器來偵測地震波行經火山的速度隨著時間的相對變化。然後他們再把結果跟第二組資料，同時期火山坡度的微小變化測量結果比對。

基拉韋厄火山是座相當活躍的火山，隨著山頂之下的岩漿庫壓力增加或減少，整座山體也會持續出現膨脹收縮的現象。基拉韋厄火山至今仍在進行的噴發活動始於1983年，幾乎是毫不間斷地噴濺出熔岩。在今年稍早有一大塊山體崩落，形成的開口讓一道熔岩「瀑布」出現，直通下方的海洋。由於基拉韋厄火山具有如此大量的岩漿活動，使其成為世上有最多研究進行的火山之一。

劍橋大學的研究人員利用地震噪訊來找出控制基拉韋厄火山運動的因素是什麼。地震噪訊是地球不斷發出的微幅震動，從地震到海中的波浪都是成因之一。在單一儀器中通常會將噪訊解讀為隨機訊號，但研究人員將儀器成對分析之後，他們可以觀察通過兩個儀器之間的能量，從而將來自火山的地震噪訊獨立出來。

研究第一作者，劍橋大學地球科學系的博士生Clare Donaldso表示：「我們感興趣的是儀器之間傳遞的能量如何改變，有沒有發生加速或減速的現象。我們想要知道地震波速率的變化是否反映了噴發之前因為火山膨脹所導致的內部壓力增加。這對火山噴發的預報來說相當重要。」

在基拉韋厄火山熔岩湖之下一至兩公里處存在著一座岩漿庫。當這座地下岩漿庫中的岩漿有所增減時，整個火山頂峰也會隨之膨脹收縮。於此同時，地震波的速率也會跟著改變。岩漿庫逐漸填滿的時候會讓壓力增加，造成圍岩內部的裂縫閉合使得地震波速率變快，反之亦然。

Donaldson表示：「這是我們首度能比較長時間下地震噪訊和火山變形之間的關係，兩者之間呈現出來的強烈關聯性意謂此或許可以當作預測火山噴發的新方式。」

火山地震學傳統上是在測量火山地區發生的小型地震。當岩漿在地下移動而在岩層中鑿出裂縫時，通常會引發微小的地震。因此偵測這些地震成為一種非常良好的噴發預測方法。但是岩漿有時會靜悄悄地流經先前就存在的裂隙，造成地震可能不會發生。不過這種新技術仍然可以偵測到由岩漿流動產生的變化。

地震噪訊具有持續發生的特性，且對其它方法會遺漏的變化相當敏感。研究人員期盼他們的新研究可以讓此方法運用到全世界數百座的活火山。

'Bulges' in volcanoes could be used to predict eruptions

A team of researchers from the University of Cambridge have developed a new way of measuring the pressure inside volcanoes, and found that it can be a reliable indicator of future eruptions.

Using a technique called ‘seismic noise interferometry’ combined with geophysical measurements, the researchers measured the energy moving through a volcano. They found that there is a good correlation between the speed at which the energy travelled and the amount of bulging and shrinking observed in the rock. The technique could be used to predict more accurately when a volcano will erupt. Their results are reported in the journal Science Advances.

Data was collected by the US Geological Survey across Kīlauea in Hawaii, a very active volcano with a lake of bubbling lava just beneath its summit. During a four-year period, the researchers used sensors to measure relative changes in the velocity of seismic waves moving through the volcano over time. They then compared their results with a second set of data which measured tiny changes in the angle of the volcano over the same time period.

As Kīlauea is such an active volcano, it is constantly bulging and shrinking as pressure in the magma chamber beneath the summit increases and decreases. Kīlauea’s current eruption started in 1983, and it spews and sputters lava almost constantly. Earlier this year, a large part of the volcano fell away and it opened up a huge ‘waterfall’ of lava into the ocean below. Due to this high volume of activity, Kīlauea is also one of the most-studied volcanoes on Earth.

The Cambridge researchers used seismic noise to detect what was controlling Kīlauea’s movement. Seismic noise is a persistent low-level vibration in the Earth, caused by everything from earthquakes to waves in the ocean, and can often be read on a single sensor as random noise. But by pairing sensors together, the researchers were able to observe energy passing between the two, therefore allowing them to isolate the seismic noise that was coming from the volcano.

“We were interested in how the energy travelling between the sensors changes, whether it’s getting faster or slower,” said Clare Donaldson, a PhD student in Cambridge’s Department of Earth Sciences, and the paper’s first author. “We want to know whether the seismic velocity changes reflect increasing pressure in the volcano, as volcanoes bulge out before an eruption. This is crucial for eruption forecasting.”

One to two kilometres below Kīlauea’s lava lake, there is a reservoir of magma. As the amount of magma changes in this underground reservoir, the whole summit of the volcano bulges and shrinks. At the same time, the seismic velocity changes. As the magma chamber fills up, it causes an increase in pressure, which leads to cracks closing in the surrounding rock and producing faster seismic waves – and vice versa.

 “This is the first time that we’ve been able to compare seismic noise with deformation over such a long period, and the strong correlation between the two shows that this could be a new way of predicting volcanic eruptions,” said Donaldson.

Volcano seismology has traditionally measured small earthquakes at volcanoes. When magma moves underground, it often sets off tiny earthquakes, as it cracks its way through solid rock. Detecting these earthquakes is therefore very useful for eruption prediction. But sometimes magma can flow silently, through pre-existing pathways, and no earthquakes may occur. This new technique will still detect the changes caused by the magma flow.

Seismic noise occurs continuously, and is sensitive to changes that would otherwise have been missed. The researchers anticipate that this new research will allow the method to be used at the hundreds of active volcanoes around the world.

原始論文：Clare Donaldson, Corentin Caudron, Robert G. Green, Weston A. Thelen and Robert S. White. Relative seismic velocity variations correlate with deformation at Kīlauea volcano. Science Advances, 2017 DOI: 10.1126/sciadv.1700219

引用自：University of Cambridge. "'Bulges' in volcanoes could be used to predict eruptions."