火山爆發前的寧靜

原文網址：www.sciencedaily.com/releases/2016/06/160623150201.htm

Volcanoes get quiet before they erupt

火山爆發前的寧靜

When dormant volcanoes are about to erupt, they show some predictive characteristics--seismic activity beneath the volcano starts to increase, gas escapes through the vent, or the surrounding ground starts to deform. However, until now, there has not been a way to forecast eruptions of more restless volcanoes because of the constant seismic activity and gas and steam emissions. Carnegie volcanologist Diana Roman, working with a team of scientists from Penn State, Oxford University, the University of Iceland, and INETER has shown that periods of seismic quiet occur immediately before eruptions and can thus be used to forecast an impending eruption for restless volcanoes. The duration of the silence can indicate the level of energy that will be released when eruption occurs. Longer quiet periods mean a bigger bang.

當處於休眠狀態的火山即將噴發時，它們會先展現出一些前兆，像是火山之下的地震開始增加、噴氣孔冒出氣體，或者周遭的地面逐漸變形。然而到目前為止，對於那些靜不下來的火山，我們尚未有方法可以預測它們是否即將噴發，這是因為它們附近總是不斷地發生地震，且持續冒出蒸氣和各種氣體。卡內基科學研究所的火山學家 Diana Roman與來自賓州州立大學、牛津大學、冰島大學以及尼加拉瓜地球科學研究所(INETER)的科學家團隊通力合作，發現緊接在噴發之前那些活躍的火山會出現一段地震活動停止期，因此可以利用此特性來預測即將到來的火山爆發，而緘默的時間長短可以指示之後噴發時釋放出的能量多寡。沉寂的越久，規模就越是浩大。

The research is published in Earth and Planetary Science Letters.

這項研究發表在期刊《地球與行星科學學報》(Earth and Planetary Science Letters)之上。

The team monitored a sequence of eruptions at the Telica Volcano in Nicaragua in 2011. It is a so-called stratovolcano, with a classic-looking cone built up by many layers of lava and ash. They started monitoring Telica in 2009 with various instruments and by 2011 they had a comprehensive network within 2.5 miles (4 kilometers) of the volcano's summit.

研究團隊監測了尼加拉瓜的特利卡(Telica)火山於2011年發生的一系列噴發事件。特利卡火山是座「層狀火山」，由火山灰和熔岩層層堆疊而成並呈現出典型的錐狀外觀。他們從2009年開始便利用各種儀器來監測特利卡火山，到2011時他們已經在火山頂端2.5哩的範圍中架設好一座十分全面的觀測網。

The 2011 eruptive event was a month-long series of small to moderate ash explosions. Prior to the eruption, there was a lack of deep seismicity or deformation, and small changes in sulfur dioxide gas emissions, indicating that the eruption was not driven by fresh magma. Instead, the eruption likely resulted from the vents being sealed off so that gas could not escape. This resulted in an increase in the pressure that eventually caused the explosions.

2011年的噴發事件是一連串小至中規模的火山灰噴發，約莫持續了一個月之久。在噴發之前，深層地震活動及變形曾一度停止，二氧化硫的噴氣情形也發生了微幅改變，顯示這起噴發事件並非由新鮮的岩漿驅使。反之，噴發可能是因為火山內部的管道被阻塞造成氣體無法逸散。這使得壓力持續累積最終導致火山爆發。

Of the 50 explosions that occurred, 35 had preceding quiet periods lasting 30 minutes or longer. Thirteen explosions were preceded by quiet intervals of at least five minutes. Only two of the 50 did not have any quiet period preceding the explosion.

在發生的50次噴發中，35次在之前有持續超過30分鐘或更長的靜止期，另外13次先行發生的靜止期持續了至少5分鐘，只有2次在之前從未有靜止期出現。

"It is the proverbial calm before the storm," remarked Roman. "The icing on the cake is that we could also use these quiet periods to forecast the amount of energy released."

「這就像俗話說的『暴風雨前的寧靜。』」 Roman如此評論他們的研究。「更妙的是我們還可以用靜止期來預測釋放出的能量多寡。」

The researchers did a "hindsight" analysis of the energy released. They found that the longer the quiet phase preceding an explosion, the more energy was released in the ensuing explosion. The quiet periods ranged from 6 minutes before an explosion to over 10 hours (619 minutes) for the largest explosion.

研究人員事後分析了釋放出的能量，他們發現在噴發之前的靜止期持續越久，隨後發生的噴發就會釋放出越多能量。靜止期的持續時間從某次噴發之前的六分鐘，至最劇烈噴發之前的10小時以上(619分鐘)不等。

The researchers were also able to forecast a minimum energy for impending explosions based on the data from the previous quiet/explosion pairs and the duration of the particular quiet period being analyzed. The correlation between duration of quiet periods and amount of energy released is tied to the duration of the gas pathways being blocked. The longer the blockage, the more pressure builds up resulting in more energy released. Sealing might be occurring due to mineral precipitation in cracks that previously acted as gas pathways, or due to the settling of the rock near the volcano's surface.

根據過往數據中的靜止期和噴發能量之間的成對關係，研究人員可以分析特定靜止期的持續時間來推算即將發生的噴發事件釋放出的最小能量。靜止期的持續時間會和釋放出的能量多寡之間成正比跟氣體通道被阻塞了多久有關。通道被堵住越久，就會累積越高的壓力而釋放出更多能量。堵塞可能是因為以往作為氣體通路的裂縫中有礦物沉澱出來，或者是火山表面附近的岩石滾入裂縫而導致。

"What is clear is that this method of careful monitoring of Telica or other similar volcanoes in real time could be used for short-term forecasts of eruptions," Roman said. "Similar observations of this phenomenon have been noted anecdotally elsewhere. Our work has now quantified that quiet periods can be used for eruption forecasts and that longer quiet periods at recently active volcanoes could indicate a higher risk of energetic eruptions."

「明顯地，詳細監測特利卡或者其他類似火山即時活動的方法可以用來當作火山爆發的短期預測。」 Roman說。「之前在其他地方觀察到的類似現象多半被當作奇聞軼事處理。現在我們的研究利用定量化的方式顯現靜止期確實可以用來預測噴發，且近期處於活躍狀態的火山平息的時間越久，代表劇烈噴發的風險也越高。」

The paper's other authors are Mel Rodgers of Oxford University, Peter LaFemina of Penn State University, Halldor Geirsson of the University of Iceland, and Virginia Tenorio of the Instituto Nicaraguense de Estudios Territoriales.

此篇論文的其他作者尚有牛津大學的Mel Rodgers、賓州大學的Peter LaFemina、冰島大學的Peter LaFemina以及尼加拉瓜地球科學研究所的 Virginia Tenorio。

This work was supported by the National Science Foundation and the Nicaraguan Institute of Earth Sciences (INETER).

這項研究由美國國家科學基金會與尼加拉瓜地球科學研究所資助。

引用自：Carnegie Institution for Science. "Volcanoes get quiet before they erupt." ScienceDaily. ScienceDaily, 23 June 2016.