原文網址：www.sciencedaily.com/releases/2017/05/170525141547.htm
為什麼蘇門答臘大地震如此嚴重
一組國際科學家團隊發現的證據顯示，在海床底下深處的礦物脫水現象，對2004年12月26日發生的蘇門答臘大地震的嚴重程度有相當影響。
這場規模高達9.2的地震以及隨之產生的海嘯，摧毀了印度洋沿岸地區的城鎮，死亡人數總計超過25萬人。
2016年，在一次海洋科學鑽探計畫中，研究人員前往此地區對蘇門答臘大地震進行研究。該航次為國際海洋發現計畫的一部份，由南安普敦大學和科羅拉多礦業學院的科學家主持。
在此次於研究船聯合果敢號上進行的考察期間，研究人員首度從進入蘇門答臘隱沒帶的海洋板塊中取得沉積物和岩石樣品。隱沒帶是地球上兩座板塊互相聚合，其中一方滑入另一方之下的區域。這些地方是世界上規模最大的地震發生之處，且常常伴隨著破壞力極強的海嘯。
日前刊登於期刊《科學》(Science)的新論文中，對海床深處發現的沉積物樣品進行的研究成果有詳細描述，主要作者為德國布萊梅大學海洋環境科學中心的Andre Hüpers博士。
共同主持這次考察隊伍的南安普敦大學教授Lisa McNeill表示：「2004年的印度洋海嘯是由一場極為強烈的地震引起，其破裂範圍十分廣大。我們想要找出如此大型的地震成因為何，以及對擁有類似地質特性的其他區域來說，它可能具有什麼樣的意義。」
科學家將他們的研究重心放在沉積物的礦物於地底深處發生的脫水作用。脫水作用通常發生於隱沒帶內部並會受到溫度及沉積物的組成影響。一般認為它會控制板塊之間產生滑動的位置和距離，因此也決定了地震的嚴重程度。
在蘇門答臘，團隊運用了最先進的海洋鑽探技術來取出海床下方1.5公里深的樣品。他們接著分析了沉積物的礦物組成，以及化學、熱力學和物理性質。然後進行模擬以計算這些沉積物和岩石運往東方250公里處的隱沒帶，接著被埋至相當深處而溫度越來越高的過程中，會發生什麼樣的行為變化。
此處海床上的沉積物是從喜馬拉雅山脈和青藏高原侵蝕下來，經由陸上河流以及海流運輸數千公里才到此處。團隊發現海床沉積物的厚度足以讓溫度升至脫水作用可以進行的範圍，使得沉積物在到達隱沒帶之前就已經徹底脫水。這會形成特別強韌的物質，造成隱沒帶的斷層面能在比較淺的地方產生大範圍滑動，而形成特別強烈的地震――如同我們在2004年見到的。
布萊梅大學的Andre Hüpers博士表示：「我們的發現解釋了2004大地震的性質之一――特別大面積的破裂面是如何產生，並且顯示出其他擁有巨厚高溫沉積物和岩石的隱沒帶，也可能會經歷同樣現象。」
「對於那些僅有少數或根本沒有地震歷史紀錄的隱沒帶來說，這項研究特別重要，因為我們幾乎不瞭解它們發生地震的可能性有多高。由於隱沒帶地震的再現周期通常有數百年到千年以上，因此我們對一些隱沒帶過去發生的地震性質認識相當有限。」
類似的隱沒帶位在加勒比海(小安地列斯群島)、伊朗和巴基斯坦外海(馬克蘭隱沒帶)以及美國和加拿大西部外海(卡斯卡迪亞隱沒帶)。團隊在接下來數年會繼續研究從蘇門答臘鑽探計畫採集到的樣品和數據，方式包括實驗室試驗和更進一步的數值模擬。結果將會用來評估蘇門答臘和其他類似隱沒帶未來發生災害的可能性。
Why
the Sumatra earthquake was so severe
An international team of scientists has found evidence
suggesting the dehydration of minerals deep below the ocean floor influenced
the severity of the Sumatra earthquake, which took place on December 26, 2004.
The earthquake, measuring magnitude 9.2, and the
subsequent tsunami, devastated coastal communities of the Indian Ocean, killing
over 250,000 people.
Research into the earthquake was conducted during a
scientific ocean drilling expedition to the region in 2016, as part of the
International Ocean Discovery Program (IODP), led by scientists from the
University of Southampton and Colorado School of Mines.
During the expedition on board the research vessel
JOIDES Resolution, the researchers sampled, for the first time, sediments and
rocks from the oceanic tectonic plate which feeds the Sumatra subduction zone.
A subduction zone is an area where two of the Earth's tectonic plates converge,
one sliding beneath the other, generating the largest earthquakes on Earth,
many with destructive tsunamis.
Findings of a study on sediment samples found far
below the seabed are now detailed in a new paper led by Dr Andre Hüpers of the
MARUM-Center for Marine Environmental Sciences at University of Bremen -
published in the journal Science.
Expedition co-leader Professor Lisa McNeill, of the University
of Southampton, says: "The 2004 Indian Ocean tsunami was triggered by an
unusually strong earthquake with an extensive rupture area. We wanted to find
out what caused such a large earthquake and tsunami and what this might mean
for other regions with similar geological properties."
The scientists concentrated their research on a
process of dehydration of sedimentary minerals deep below the ground, which
usually occurs within the subduction zone. It is believed this dehydration
process, which is influenced by the temperature and composition of the
sediments, normally controls the location and extent of slip between the
plates, and therefore the severity of an earthquake.
In Sumatra, the team used the latest advances in ocean
drilling to extract samples from 1.5 km below the seabed. They then took
measurements of sediment composition and chemical, thermal, and physical
properties and ran simulations to calculate how the sediments and rock would
behave once they had travelled 250 km to the east towards the subduction zone,
and been buried significantly deeper, reaching higher temperatures.
The researchers found that the sediments on the ocean
floor, eroded from the Himalayan mountain range and Tibetan Plateau and
transported thousands of kilometres by rivers on land and in the ocean, are
thick enough to reach high temperatures and to drive the dehydration process to
completion before the sediments reach the subduction zone. This creates
unusually strong material, allowing earthquake slip at the subduction fault
surface to shallower depths and over a larger fault area - causing the
exceptionally strong earthquake seen in 2004.
Dr Andre Hüpers of the University of Bremen says:
"Our findings explain the extent of the large rupture area, which was a
feature of the 2004 earthquake, and suggest that other subduction zones with
thick and hotter sediment and rocks, could also experience this phenomenon.
"This will be particularly important for
subduction zones with limited or no historic subduction earthquakes, where the
hazard potential is not well known. Subduction zone earthquakes typically have
a return time of a few hundred to a thousand years. Therefore our knowledge of
previous earthquakes in some subduction zones can be very limited."
Similar subduction zones exist in the Caribbean
(Lesser Antilles), off Iran and Pakistan (Makran), and off western USA and
Canada (Cascadia). The team will continue research on the samples and data
obtained from the Sumatra drilling expedition over the next few years,
including laboratory experiments and further numerical simulations, and they
will use their results to assess the potential future hazards both in Sumatra
and at these comparable subduction zones.
原始論文：Andre Hüpers, Marta E. Torres, et al. Sediments
tell a tsunami story. Science, 2017 DOI: 10.1126/science.aal3429
引用自：University of Southampton. "Why the Sumatra earthquake
was so severe." ScienceDaily. ScienceDaily, 25 May 2017.
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