古代海嘯留下來的痕跡預警了當今的潛在危險

原文網址：www.sciencedaily.com/releases/2015/10/151002144903.htm

Signs of ancient mega-tsunami could portend modern hazard

古代海嘯留下來的痕跡預警了當今的潛在危險

Evidence of an 800-foot wave in the Cape Verde Islands

於維德角群島發現的證據顯示曾有高達240公尺的海嘯發生

Scientists working off west Africa in the Cape Verde Islands have found evidence that the sudden collapse of a volcano there tens of thousands of years ago generated an ocean tsunami that dwarfed anything ever seen by humans. The researchers say an 800-foot wave engulfed an island more than 30 miles away. The study could revive a simmering controversy over whether sudden giant collapses present a realistic hazard today around volcanic islands, or even along more distant continental coasts. The study appears today in the journal Science Advances.

於非洲西部外海的維德角(Cape Verde)群島進行研究的科學家，發現的證據顯示數萬年前由火山崩塌造成的海嘯，讓人類從古至今見過的所有海嘯相形見絀。研究人員說這道240公尺高的波浪吞沒了30哩之外的島嶼。此篇研究重燃了對於今日可能突然發生的大型火山崩塌事件，是否真的會對周遭地區，甚至更遠處的大陸海岸造成損害的爭論。這篇研究發表於今日(10/2)的《科學前緣》(Science Advances)期刊當中。

"Our point is that flank collapses can happen extremely fast and catastrophically, and therefore are capable of triggering giant tsunamis," said lead author Ricardo Ramalho, who did the research as a postdoctoral associate at Columbia University's Lamont-Doherty Earth Observatory, where he is now an adjunct scientist. "They probably don't happen very often. But we need to take this into account when we think about the hazard potential of these kinds of volcanic features."

「我們的論點是火山側翼崩塌(flank collapse)可能會發生得相當迅速且極具規模，因而引發巨型海嘯。」第一作者Ricardo Ramalho說，他於哥倫比亞大學的拉蒙特多爾蒂地球觀測站從事博士後研究員的期間進行了此研究，目前他是該單位的兼任研究員。「也許它們的發生頻率並不頻繁。然而，當我們關注這種火山地貌的潛在災害時，我們確實需要將這類事件列入考量。」

The apparent collapse occurred some 73,000 years ago at the Fogo volcano, one of the world's largest and most active island volcanoes. Nowadays, it towers 2,829 meters (9,300 feet) above sea level, and erupts about every 20 years, most recently last fall. Santiago Island, where the wave apparently hit, is now home to some 250,000 people.

約73,000年前，這起重大的崩塌事件發生於福戈火山(Fogo volcano)，其為全球最大且最活躍的火山島之一。現今它的海拔高約2,829公尺，每20年左右便會噴發一次，最近的一次就在去年秋天。而事件發生時被超級海嘯襲擊的聖地牙哥島(Santiago Island)，現在則是250,000人的住所。

There is no dispute that volcanic flanks present a hazard; at least eight smaller collapses have occurred in Alaska, Japan and elsewhere in the last several hundred years, and some have generated deadly tsunamis. But many scientists doubt whether big volcanoes can collapse with the suddenness that the new study suggests. Rather, they envision landslides coming in gradual stages, generating multiple, smaller tsunamis. A 2011 French study also looked at the Fogo collapse, suggesting that it took place somewhere between 124,000-65,000 years ago; but that study says it involved more than one landslide. The French researchers estimate that the resulting multiple waves would have reached only 45 feet--even at that, enough to do plenty of harm today.

火山側翼崩塌毫無疑問地會釀成災害。過去數百年來至少有八起較小型的事件發生於阿拉斯加、日本和其他地方，其中有些確實引發了致命的海嘯。但許多科學家懷疑大型火山是否真能如此篇新研究所說，發生如此急遽的崩塌。反之，他們預想山崩會多階段發生，而造成好幾波較小規模的海嘯。一篇2011年的法國研究也探討了福戈火山的崩塌，他們認為這起事件發生於124,000至65,000年前之間，但其中包含了不只一次崩塌。這些法國研究人員估計這數波海嘯到達的高度僅約14公尺，即便如此，若發生於今日也足以造成巨大的損害。

A handful of previous other studies have proposed much larger prehistoric collapses and resulting megatsunamis, in the Hawaiian islands, at Italy's Mt. Etna, and the Indian Ocean's Reunion Island. But critics have said these examples are too few and the evidence too thin. The new study adds a new possible example; it says the estimated 160 cubic kilometers (40 cubic miles) of rock that Fogo lost during the collapse was dropped all at once, resulting in the 800-foot wave. By comparison, the biggest known recent tsunamis, which devastated the Indian Ocean's coasts in 2004 and eastern Japan in 2011, reached only about 100 feet. (Like most other well documented tsunamis, these were generated by movements of undersea earthquake faults--not volcanic collapses.)

過往有些研究提出夏威夷群島、義大利的埃特納火山(Mt. Etna)和印度洋的留尼汪島(Reunion Island)於史前時期曾發生過大型崩塌事件，並造成超級海嘯。但質疑者聲稱這些不過是罕見案例且證據太過薄弱。此篇新研究增添了一件可能的新案例：福戈火山於崩塌時估計有多達160立方公里的岩石在同一時間一起崩落，而造成了72公尺高的巨浪。相較之下，近代所知的海嘯事件中最大的兩起：2004年摧毀印度洋海岸地區的海嘯，和2011年的東日本海嘯，所達到的高度也不過9公尺而已。(如同其他大多數有詳細記錄的海嘯，這兩起事件由海底地震斷層的移動造成，而非火山崩塌。)

Santiago Island lies 55 kilometers (34 miles) from Fogo. Several years ago, Ramalho and colleagues were working on Santiago when they spotted unusual boulders lying as far as 2,000 feet inland and nearly 650 feet above sea level. Some are as big as delivery vans, and they are utterly unlike the young volcanic terrain on which they lie. Rather, they match marine-type rocks that ring the island's shoreline: limestones, conglomerates and submarine basalts. Some weigh up to 770 tons. The only realistic explanation the scientists could come up with: A gigantic wave must have ripped them from the shoreline and lofted them up. They derived the size of the wave by calculating the energy it would have taken to accomplish this feat.

聖地牙哥島坐落於距福戈火山55公里遠處。在數年前，Ramalho和他的同僚於聖地牙哥島上進行研究時，發現於距海600公尺遠，高度約海拔200公尺的內陸地區散布著一些不尋常的礫石。這些礫石中有些可以跟卡車一樣大，重達770噸；它們的種類也跟它們腳下的火山體截然不同，反而跟位於島嶼海岸週遭，於海洋環境下生成的岩石如出一轍，包括了石灰岩、礫岩和海底玄武岩。科學家能想到的唯一解釋便是：必然有一道巨浪從海岸颳走了這些石頭，並把它們帶往高處。他們藉由計算達成此種壯舉需要的能量來估計這道巨浪的規模。

To date the event, in the lab Ramalho and Lamont-Doherty geochemist Gisela Winckler measured isotopes of the element helium embedded near the boulders' surfaces. Such isotopes change depending on how long a rock has been lying in the open, exposed to cosmic rays. The analyses centered around 73,000 years--well within the earlier French estimate of a smaller event. The analysis "provides the link between the collapse and impact, which you can make only if you have both dates," said Winckler.

為了定出此事件的發生年代，Ramalho和拉蒙特多爾蒂的地球化學家Gisela Winckler測量這些礫石表面附近的氦同位素含量。這種同位素會依據岩石出露於開闊地表，接受宇宙射線照射的時間多寡而改變。他們的分析數據集中於73,000年，與法國科學家估算的小規模事件發生區間相當吻合。這個分析結果「連結了崩塌事件與造成的衝擊。唯有得到兩個事件年代才有辦法達成。」Winckler說。

Tsunami expert Bill McGuire, a professor emeritus at University College London who was not involved in the research, said the study "provides robust evidence of megatsunami formation [and] confirms that when volcanoes collapse, they can do so extremely rapidly." Based on his own work, McGuire s says that such megatsunamis probably come only once every 10,000 years. "Nonetheless," he said, "the scale of such events, as the Fogo study testifies, and their potentially devastating impact, makes them a clear and serious hazard in ocean basins that host active volcanoes."

未參與此研究的倫敦大學學院榮譽教授，海嘯專家Bill McGuire說此研究「提供了堅強的證據顯示超級海嘯曾經發生，並且證明了火山的確能以如此迅速的速度崩塌。」根據他自身的研究，McGuire表示這樣的超級海嘯可能每10,000年才會發生一次。「然而，」他說，「正如福戈火山的研究證實的，以這種事件的規模與可能造成的毀滅性影響，對有活火山在的海域來說，是不容忽視且相當重大的潛在災害。」

Ramalho cautions that the study should not be taken as a red flag that another big collapse is imminent here or elsewhere. "It doesn't mean every collapse happens catastrophically," he said. "But it's maybe not as rare as we thought."

Ramalho慎重地說此研究不該被解讀成另一個大型崩塌事件即將在此，或其他地方發生的警訊。「並不是說每次崩塌都會導致大災難，」他說，「但這種事件也許不如我們想像中的那般稀罕。」

In the early 2000s, other researchers started publishing evidence that the Cape Verdes could generate large tsunamis. Others have argued that Spain's Canary Islands have already done so. Simon Day, a senior researcher at University College London has sparked repeated controversy by warning that any future eruption of the Canary Islands' active Cumbre Vieja volcano could set off a flank collapse that might form an initial wave 3,000 feet high. This, he says, could erase more than nearby islands. Such a wave might still be 300 feet high when it reached west Africa an hour or so later he says, and would still be 150 feet high along the coasts of North and South America. So far, such studies have raised mainly tsunamis of publicity, and vigorous objections from other scientists that such events are improbable. A 2013 study of deep-sea sediments by the United Kingdom's National Oceanography Centre suggests that the Canaries have probably mostly seen gradual collapses.

在2000年代早期，其他研究人員開始陸續發表維德角會產生大型海嘯的證據。另外一些則聲稱西班牙加那利群島(Canary Islands)早已發生了這類事件。倫敦大學學院的研究委員Simon Day不斷引起爭議的研究中，警告了加那利群島的別哈峰活火山(Cumbre Vieja)未來的每一次噴發，都有可能引起側翼崩塌，造成的海嘯可能一開始會高達900公尺。他說這不只會夷平週遭地區。如此大的海浪當它在一個小時或之後到達西非沿岸時仍有90公尺高；而抵達南北美沿岸時還有45公尺高。目前為止，這類研究讓大眾更加關注海嘯，而來自其他科學家的反對聲浪則批評這類事件顯得不切實際。英國國家海洋學中心於2013年發表的一篇海底沉積物研究中，認為加那利群島的崩塌更有可能是逐漸發生的。

Part of the controversy hangs not only on the physics of the collapses themselves, but on how efficiently resulting waves could travel. In 1792, part of Japan's Mount Unzen collapsed, hitting a series of nearby bays with waves as high as 300 feet, and killing some 15,000 people. On July 9, 1958, an earthquake shook 90 million tons of rock into Alaska's isolated Lituya Bay; this created an astounding 1,724-foot-high wave, the largest ever recorded. Two fishermen who happened to be in their boat that day were carried clear over a nearby forest; miraculously, they survived.

爭議點不只在於崩塌本身的物理機制，也有一部分是針對崩塌造成的波浪的傳遞效率。1972年，日本雲仙火山有一部分崩落下來，造成高達90公尺的海浪並席捲了附近的許多港灣，造成約15,000人死亡。1958年7月9日，一場地震將9,000萬噸的岩石震落至阿拉斯加一處封閉的海灣-利圖亞灣當中，引起的巨浪達到驚為天人的525公尺，為歷史最高紀錄。事發當時載有兩名漁夫的漁船被海嘯掃進了附近的森林，奇蹟似地，他們生存了下來。

These events, however, occurred in confined spaces. In the open ocean, waves created by landslides are generally thought to lose energy quickly, and thus to pose mainly a regional hazard. However, this is based largely on modeling, not real-world experience, so no one really knows how fast a killer wave might decay into a harmless ripple. In any case, most scientists are more concerned with tsunamis generated by undersea earthquakes, which are more common. When seabed faults slip, as they did in 2004 and 2011, they shove massive amounts of water upward. In deep water, this shows up as a mere swell at the surface; but when the swell reaches shallower coastal areas, its energy concentrates into in a smaller volume of water, and it rears up dramatically. The 2004 Indian Ocean earthquake and tsunami killed 230,000 people in 14 countries; the 2011 Tohoku event killed nearly 20,000 in Japan, and has caused a long-term nuclear disaster.

然而，這些事件都是發生在封閉地區。在開闊大洋環境中，一般認為由山崩引發的海浪能量會快速消散，因此主要造成的災害為地區性。然而，這些推論大都是根據模擬結果，而非觀察現實世界得來，是故沒有人真正知道致命巨浪要多久才會減弱成一道無害的輕波。無論如何，大多科學家更關注的是較為常見的，由海底地震引發的海嘯。當海底斷層錯動時，正如2004年和2011年發生的那樣，它們會抬起大量上方的海水。在深水處，這只會造成表面稍稍起伏，但這股起伏到達淺水的海岸地區時，所蘊含的能量會集中於一小部分的海水，造成海水急遽升高。2004年印度洋的地震與海嘯奪走了14個國家230,000人的性命；2011年日本的東北大海嘯造成20,000人喪生，而導致的核災影響更為深遠。

James Hunt, a tsunami expert at the United Kingdom's National Oceanography Centre who was not involved in the study, said the research makes it clear that "even modest landslides could produce high-amplitude anomalous tsunami waves on opposing island coastlines." The question, he said, "is whether these translate into hazardous events in the far field, which is debatable."

未參與此研究，任職於英國國家海洋學中心的海嘯專家James Hunt說此研究清楚顯示「即使是規模中等的山崩也能造成異常巨大的海嘯，並襲擊對面島嶼的海岸。」他說問題在於，「這是否會對更遠的地方造成重大災情？仍有許多值得探討之處。」

When Fogo erupted last year, Ramalho and other geologists rushed in to observe. Lava flows (since calmed down) displaced some 1,200 people, and destroyed buildings including a new volcano visitors' center. "Right now, people in Cape Verde have a lot more to worry about, like rebuilding their livelihoods after the last eruption," said Ramalho. "But Fogo may collapse again one day, so we need to be vigilant."

當福戈火山去年噴發時，Ramalho和其他地質學家火速前往當地觀察這起爆發事件。熔岩流(自火山活動平息以來)迫使1,200人搬離家園，並且摧毀了許多建築物，包括一座新的火山遊客中心。「對於現在的維德角人民來說還有許多更值得憂心的事情，像是上次噴發過後該如何重建家園，」Ramalho說，「但福戈火山可能會在某天突然崩塌，所以我們仍需上緊發條保持警戒。」

引用自：The Earth Institute at Columbia University. "Signs of ancient mega-tsunami could portend modern hazard: Evidence of an 800-foot wave in the Cape Verde Islands." ScienceDaily. ScienceDaily, 2 October 2015.