海邊洞穴保存了5000年來關於海嘯的簡史

原文網址：http://news.rutgers.edu/research-news/sea-cave-preserves-5000-year-snapshot-tsunamis/20170623#.WXW8eIiGPct

海邊洞穴保存了5000年來關於海嘯的簡史

歷史紀錄呈現出我們仍然無法準確地預測會引發海嘯的地震

By Ken Branson

一組國際科學家團隊挖掘印尼的海邊洞穴而發現世上最完整的海嘯紀錄。這段由沉積物紀錄的5000年簡史首次顯示我們對引發巨浪的地震的發生時機幾乎是一無所知。

「2004年印度洋極具破壞力的大海嘯襲捲了數以百萬計的海岸居民，但科學界對此卻毫無防備。」共同作者，羅格斯大學―新布朗斯維克分校海洋與海岸科學系的教授Benjamin Horton表示，「我們從洞穴挖掘出來的地質紀錄顯示我們仍然無法預測下次地震會在何時發生。」

研究主要作者，任職於部分隸屬南洋理工大學的地球觀測站的Charles Rubin表示：「海嘯發生的時間並非平均分布。我們的發現呈現出海嘯的重現時間相當紊亂，這副圖象令人憂心。海嘯之間的間隔可以十分漫長，卻也可能僅僅相差數十年就發生另一次大海嘯。」

這項發表在當期《自然通訊》(Nature Communications)的發現立下了多項世上第一的紀錄：這是首度在海邊洞穴內發現的古代海嘯活動紀錄；在印度洋首度發現如此長時間的海嘯紀錄；也是世上最完好如初的海嘯紀錄。

研究人員於印尼蘇門答拉西岸的一座海邊洞窟做出了這項發現，正好位於2004年12月被大海嘯摧毀的班達亞齊南方。地層記錄顯現出一層層不斷堆疊的沙子、蝙蝠排泄物以及其他由海嘯沉積下來的碎屑，年代位於7900年至2900年前。從2900年前開始形成的地層紀錄則被2004年發生的大海嘯沖走。

這座L型的洞穴入口有一圈岩石而可以將層層堆疊的沙子關在內部。研究人員於洞穴中挖出六道壕溝並利用放射性碳定年來分析交互出現的砂層和其他類型的碎屑。研究人員確定這些地層的狀態為「完好如初」，相當清楚且易於解讀。Horton表示：「你可以看到這是一層沙子然後再一層包括蝙蝠排泄物的有機物。簡單來說此為可以追溯5000年，由一層沙子和一層蝙蝠糞便以及其他沉積物堆疊出來的紀錄。」

巽他巨型逆衝斷層是條位於印度洋，從緬甸延伸至蘇門答臘長度大約有5500公里的斷層。紀錄指出這段期間巽他斷層周圍的地震總共產生了11次海嘯。研究人員發現在5000年中有2000年完全沒有海嘯發生，但在其中一個世紀之間卻有4次海嘯襲擊這座海岸。科學家總結一般而言，小型海嘯會以相對密集的頻率發生，之後有一段長時間的休止期，接著發生超級地震與海嘯，像是2004年襲擊該地的事件。

2004年引發海嘯浩劫的大地震是由巽他巨型逆衝斷層產生，Rubin、Horton和他們的同事一直在研究巽他斷層的地震史。他們的目標是要找出一個地點，可以讓他們得到呈現完好地層紀錄的岩芯樣品。他們搜尋的對象包括了Horton稱為「沉積環境」的地方，像是海岸平原、沿岸湖湖底，任何可以讓他們插入空心金屬圓柱至沉積物6到7公尺深處，並採出可供解讀樣品的地點。但基於許多因素，他們沒有在蘇門答臘西南岸找到可供進行這類研究的地點。然而，在地球觀測站從事海岸洞穴挖掘工作的考古學家Patrick Daly告訴Rubin和Horton有關此洞穴的消息，並提出它可能就是他們一直在尋找的地點。

Horton從來沒有想過要在海岸洞窟尋找海嘯紀錄。他說是Daly不吝於分享他的專業發現――通常考古學家很注意接近他們考古場址的人員――加上他和Rubin開放接納其他領域的觀點才讓這項研究得以實行。Horton表示還有一項原因使得這篇論文可能是他職業生涯中最重要的研究。

他說；「至今為止我所進行的許多(研究)都是接續前人的成果。是先有一個理論之後，再演繹出其他科學研究來證實該理論。但這項研究是真正的具有原創性，而原創性的事物總是很少出現的。」

Sea Cave Preserves 5,000-Year Snapshot of Tsunamis

Record tells us we don't know much about predicting earthquakes that cause tsunamis

An international team of scientists digging in a sea cave in Indonesia has discovered the world’s most pristine record of tsunamis, a 5,000-year-old sedimentary snapshot that reveals for the first time how little is known about when earthquakes trigger massive waves.

“The devastating 2004 Indian Ocean tsunami caught millions of coastal residents and the scientific community off-guard,” says co-author Benjamin Horton, a professor in the Department of Marine and Coastal Sciences at Rutgers University-New Brunswick.“Our geological record from a cave illustrates that we still cannot predict when the next earthquake will happen.”

“Tsunamis are not evenly spaced through time,” says Charles Rubin, the study’s lead author and a professor at the Earth Observatory of Singapore, part of Nanyang Technological University. “Our findings present a worrying picture of highly erratic tsunami recurrence. There can be long periods between tsunamis, but you can also get major tsunamis that are separated by just a few decades.”

The discovery, reported in the current issue of Nature Communications, logs a number of firsts: the first record of ancient tsunami activity found in a sea cave; the first record for such a long time period in the Indian Ocean; and the most pristine record of tsunamis anywhere in the world.

The discovery was made in a sea cave on the west coast of Sumatra in Indonesia, just south of the city of Banda Aceh, which was devastated by the tsunami of December 2004. The stratigraphic record reveals successive layers of sand, bat droppings and other debris laid down by tsunamis between 7,900 and 2,900 years ago. The stratigraphy since 2,900 years ago was washed away by the 2004 tsunami.

The L-shaped cave had a rim of rocks at the entrance that trapped successive layers of sand inside. The researchers dug six trenches and analyzed the alternating layers of sand and debris using radio carbon dating. The researchers define “pristine” as stratigraphic layers that are distinct and easy to read. “You have a layer of sand and a layer of organic material that includes bat droppings, so simply it is a layer of sand and a layer of bat crap, and so on, going back for 5,000 years,” Horton says.

The record indicates that 11 tsunamis were generated during that period by earthquakes along the Sunda Megathrust, the 3,300-mile-long fault running from Myanmar to Sumatra in the Indian Ocean. The researchers found there were two tsunami-free millennia during the 5,000 years, and one century in which four tsunamis struck the coast. In general, the scientists report, smaller tsunamis occur relatively close together, followed by long dormant periods, followed by great quakes and tsunamis, such as the one that struck in 2004.

Rubin, Horton and their colleagues were studying the seismic history of the Sunda Megathrust, which was responsible for the 2004 earthquake that triggered the disastrous tsunami. They were looking for places to take core samples that would give them a good stratigraphy. This involves looking for what Horton calls “depositional places” – coastal plains, coastal lake bottoms, any place to plunge a hollow metal cylinder six or seven meters down and produce a readable sample. But for various reasons, there was no site along the southwest coast of Sumatra that would do the job. But Patrick Daly, an archaeologist at EOS who had been working on a dig in the coastal cave, told Rubin and Horton about it and suggested it might be the place they were looking for.

Looking for tsunami records in a sea cave was not something that would have occurred to Horton, and he says Daly’s professional generosity – archaeologists are careful about who gets near their digs – and his own and Rubin’s openness to insights from other disciplines made the research possible. Horton says this paper may be the most important in his career for another reason.

“A lot of (the research) I’ve done is incremental,” he says. “I have a hypothesis, and I do deductive science to test the hypothesis. But this is really original, and original stuff doesn’t happen all that often.”

原始論文：Charles M. Rubin, Benjamin P. Horton, Kerry Sieh, Jessica E. Pilarczyk, Patrick Daly, Nazli Ismail, Andrew C. Parnell. Highly variable recurrence of tsunamis in the 7,400 years before the 2004 Indian Ocean tsunami. Nature Communications, 2017; 8: 16019 DOI: 10.1038/NCOMMS16019

引用自：Rutgers University. "Sea cave preserves 5,000-year snapshot of tsunamis: Record tells us we don't know much about predicting earthquakes that cause tsunamis." 

原文網址：http://news.rutgers.edu/research-news/sea-cave-preserves-5000-year-snapshot-tsunamis/20170623#.WXW8eIiGPct