研究認為2018年的基拉韋厄火山噴發是由降雨引起

原文網址：https://www.nasa.gov/feature/goddard/2020/study-suggests-rainfall-triggered-2018-k-lauea-eruption

研究認為2018年的基拉韋厄火山噴發是由降雨引起

By Samson Reiny

2018年五月夏威夷的基拉韋厄火山開始噴發，引起了接下來數個月的強烈活動。直到八月為止的這段期間，熠熠生輝的岩漿從裂隙中往上噴出數百公尺，隨之翻騰而起的塵埃更是竄升至六英里高的高空。大量熔岩流在這座太平洋島嶼的東南岸四處流動，摧毀了數百棟房舍。

Landsat-9衛星在2018年5月14日捕捉到一道蒸氣從基拉韋厄火山的頂端冒出。在數周之前的5月3日，基拉韋厄火山在東部裂谷區爆發，開啟了之後長達數月的火山活動，島上東南岸的數百棟房屋因而遭到摧毀。圖片來源：NASA Earth Observatory images by Joshua Stevens, using Landsat data from the U.S. Geological Survey

當熔化的岩石，也就是岩漿從地底上升到地表時就會發生火山爆發。影響火山什麼時候爆發的因素有很多，像是火山的形狀與岩漿的成分等。4月22日發表在期刊《自然》(Nature)，由NASA提供經費的研究指出在基拉韋厄火山這項案例當中，是由另外一種因素決定了噴發時間：事件發生之前的數個月內，長期且時而大量的降雨。

「我們知道地殼淺處含有的水可以引發地震及山崩，現在我們了解到這還能導致火山爆發。」共同作者，邁阿密大學羅森斯蒂爾海洋與大氣學院的地球物理教授Falk Amelung表示。「在火山內部比起乾燥的岩石來說，含水的岩石更容易受到岩漿造成的壓力而破裂，這反過來又會為岩漿製造出來到地表的途徑。」

對於2018年的基拉韋厄火山爆發，研究人員首先排除了一種常見的原因：岩漿庫內部的壓力增加。當岩漿庫內部的壓力高到某種程度，周圍的岩石會破裂，造成火山爆發。而科學家可以觀察圍岩的膨脹或者抬升來推估岩漿造成的壓力。「壓力增加時地表會抬升數十公分。」Amelung如此解釋。「由於我們在噴發前的一年之內並未看到任何顯著的抬升，於是我們開始思考是否還有別的解釋，這促成我們對降雨展開研究。」

火山內部的液體壓力會直接影響到岩漿破壞圍岩，最終導致火山活動的可能性。研究人員整合了地面測站和NASA衛星量到的雨量，接著模擬長期降雨在火山內部累積的液體壓力如何演變。根據先前已有的實驗室數據和數值模擬結果，他們的模型顯示液體的壓力在2018年前半達到了半個世紀以來的最高值，造成火山體強度下降。作者認為這讓岩漿可以突破火山下方的圍岩，引起隨後的噴發。

研究主要作者，羅森斯蒂爾海洋與大氣學院的博士後研究員Jamie Farquharson表示：「我們在研究基拉韋厄火山的噴發歷史紀錄時發現了一個有趣的現象：在當年雨量最高的時候，岩漿侵入與紀錄到噴發的機率幾乎是平常的兩倍。」他主張在基拉韋厄火山，當地的降雨模式可能會對這些現象的出現時間與頻率有重大影響，而其他火山或許也是如此。

雖然之前已經有人提出滲入地下的雨水會和小型的蒸氣爆發及火山地震有所關連，但這是科學家首次在解釋地下數公里的岩漿作用時，歸因於長達數月且高於平均的雨量。以本次基拉韋厄火山的噴發來說，當年前三個月在火山地區降下的雨量高達2.25公尺，相比之下，過去20年該地在同個期間的雨量平均為0.9公尺。作者指出如果基拉韋厄火山會發生他們所提的這種作用，那在別的地方也有可能發生。

Farquharson表示氣候或許也會帶來影響，因為科學家預估仍在持續的氣候變遷會讓降雨模式產生改變，特別是多數模型預期地球大部分地方的極端降雨事件都會增加，而此效應在位於山區的火山可能還會更加放大。「結論是我們認為降雨誘發的火山活動可能會變得更為常見。」

研究人員用衛星測得的雨量資料來自於NASA和日本宇宙航空研究開發機構 (JAXA) 主持的國際計畫——全球降雨觀測計畫，以及之前NASA和JAXA進行的熱帶降雨觀測計畫。歐洲太空總署和歐盟委員會的衛星哨兵一號，則提供了研究人員和地表變形有關的觀測數據。研究經費來自於NASA的地球科學部門。

Study suggests rainfall triggered 2018 Kīlauea eruption

In May 2018 Kīlauea volcano on the island of Hawaii erupted, touching off months of intense activity. Through August, incandescent lava from fissures spewed hundreds of feet in the air, and billowing ash clouds reached as high as six miles into the atmosphere. Huge lava flows inundated land up and down the Pacific island’s southeast coast, destroying hundreds of homes.

Volcanoes erupt when molten rock called magma rises to the surface, and many factors, from the shape of the volcano to the composition of the magma, factor into the timing of eruptions. In the case of Kīlauea, a new, NASA-funded study published April 22 in the journal Nature points to another eruption factor: prolonged, sometimes heavy rainfall in the months leading up to the event.

“We knew that changes to water content in Earth’s shallow crust can trigger earthquakes and landslides, and now we know that it can also trigger eruptions,” said Falk Amelung, professor of geophysics at the University of Miami Rosenstiel School of Marine and Atmospheric Science and co-author of the study. “Under pressure from magma, wet rock breaks easier than dry rock inside the volcano. That, in turn, forges pathways for magma to travel to Earth’s surface.”

First, for the 2018 Kīlauea eruption researchers ruled out a common cause: increased pressure in the magma chamber, which, when it becomes great enough, is able to break through the surrounding rock. Scientists can infer increased magma pressure by observing the inflation, or rise, of the surrounding rock. “This pressurization causes the ground to inflate by a few tens of centimeters,” Amelung explained. “As we did not see any significant inflation in the year prior to the eruption, we started to think about alternative explanations, which led us to investigating precipitation.”

Using a combination of ground-based and NASA satellite measurements of rainfall, the researchers modeled the evolution of fluid pressure caused by sustained rainfall that accumulated in the volcano’s interior—a factor that can directly influence the propensity for magma to break through the surrounding rock, ultimately driving volcanic activity. Based on pre-existing laboratory data and numerical simulations, their model results suggest that, in early 2018, fluid pressure had been at its highest in almost half a century, weakening the volcanic edifice, which the authors propose enabled magma to break through confining rock beneath the volcano and lead to the subsequent eruption.

“Interestingly, when we investigate Kīlauea’s historical eruption record, we see that magmatic intrusions and recorded eruptions are almost twice as likely to occur during the wettest parts of the year,” said Jamie Farquharson, a postdoctoral researcher at the Rosenstiel School and lead author of the study. He argues that local rainfall patterns may contribute significantly to the timing and frequency of these phenomena at Kīlauea and perhaps at other volcanoes.

While rainfall infiltration has been linked to small steam explosions and volcanic earthquakes, this is the first time that scientists attribute months of above-average rainfall to explain magmatic processes more than a mile below the surface. In the case of the Kīlauea eruption, the first quarter’s total rainfall over the volcano that year was about 2.25 meters compared to the 0.9-meter average for the area in that timeframe over the past 20 years. The authors note that if this process occurs as proposed at Kīlauea, then it is likely to occur elsewhere as well.

A climatic link may also be at play, Farquharson said, as ongoing climate change is predicted to bring about changes in rainfall patterns. In particular, most models project increases in extreme precipitation over most of the globe, an effect that may be further amplified in mountainous volcanic regions. “As a result, we expect that rainfall-induced volcanic activity could become more common.”

For satellite measurements of rainfall, researchers used data from the Global Precipitation Measurement—an international mission led by NASA and the Japanese Space Exploration Agency (JAXA) — and the earlier NASA/JAXA Tropical Rainfall Measurement Mission. The European Space Agency’s and European Commission’s Sentinel-1 satellite provided researchers with data from which ground deformation observations were derived. NASA’s Earth Science Division provided funding support.

原始論文：Jamie I. Farquharson, Falk Amelung. Extreme rainfall triggered the 2018 rift eruption at Kīlauea Volcano. Nature, 2020; 580 (7804): 491 DOI: 10.1038/s41586-020-2172-5

引用自：NASA/Goddard Space Flight Center. "Excessive rainfall may have triggered 2018 Kilauea eruption."