從阿拉斯加海床得到的影像顯示高海嘯風險

原文網址：http://blogs.ei.columbia.edu/2017/07/31/new-map-of-alaska-seafloor-suggests-high-tsunami-danger/

從阿拉斯加海床得到的影像顯示高海嘯風險

巨浪可以跨過整個太平洋

 Kevin Krajick

科學家勘查阿拉斯加外海的海床後繪出了當地地質構造，他們表示此構造是種警訊，顯示通常被認為安全無虞的地區有可能產生大海嘯。他們認為此特徵跟2011年形成日本外海東北大海嘯的構造非常相似，那次海嘯奪走了20000條人命還造成三座核反應器熔毀。科學家表示許多未知的類似構造或許還潛伏在世上其他地區。這項發現明天將會刊登於期刊《自然―地質科學》(Nature Geoscience)的印刷版。

主持這項研究，哥倫比亞大學拉蒙特-多爾蒂地球觀測站的地震學家Anne Bécel對此發現表示：「它顯示阿拉斯加的部分地區特別容易產生海嘯。對全世界來說，重要的是這類構造的分布範圍可能十分廣泛。」她說大浪襲擊的範圍可能不只是阿拉斯加，還會影響更南邊的北美海岸、夏威夷和太平洋的其他區域。

當海洋地殼(棕色地區)潛入大陸地殼(橘色)下方，造成海床突然移動時就有可能產生海嘯。在阿拉斯加外海地區，研究人員發現到一條大型斷層，加上其他證據顯示大陸地殼的前緣已經分裂出來，造成此區域的海床更容易移動而有利海嘯的產生。(Anne Becel)

當有大片海洋地殼往下潛至鄰近的大陸地殼下方，又稱「隱沒」的作用發生時就有可能產生海嘯。直到彼此突然錯動之前，部分板塊會卡在一起長達數十年或者數百年，造成張力持續累積。當板塊開始錯動就會產生大地震，同時海床可能像鬆開來的彈簧一樣往上跳或下沉。這種動作造成上方的海水位移，而在表面形成波浪。

2011年的日本海嘯完全出乎人們的意料，因為它有部分是產生自海床的「潛移」區段，此處的板塊會持續移動，將張力以頻繁發生的小地震釋放出來，所以應該能避免張力持續累積而發生大地震。但研究人員現在曉得它的運作方式可能並非總是如此。在日本外海，上覆的大陸板塊前緣稍微脫離了主要塊體。當有相對大型的地震讓此脫離斷塊產生位移，它就會往上彈起而在該地引發高達40公尺的巨浪。由此可以反推具有危險的徵兆：在兩座板塊原本交會的區域――「海溝」靠近陸地的那側海床出現可以劃分脫離區塊邊界的斷層。科學家先前就已經知道該斷層的存在，但沒人曉得其箇中含意。

進行此篇新研究的學者現在也於舒馬金地震空白帶(Shumagin Gap)找出了類似系統，此隱沒帶中的潛移區域大約位在阿拉斯加半島尾端，距安克拉治(Anchorage，阿拉斯加最大城)大約有900多公里。該區段是跨越阿拉斯加半島和阿留申群島的弧形隱沒帶中的一部份。科學家在裝載特殊配備的研究船上，利用新型科技產生的強力聲脈波穿透海床深處。透過解讀傳回的回音，他們可以建立像是電腦斷層掃描攝影的影像來看出海床表面和底下的構造。在海床上可以標出高約5公尺的斷崖，顯示一邊的海床下沉而另一邊則隆升。斷層往下繼續延伸超過30公里，直到兩座板塊彼此相對移動的地方。

團隊也分析了該區域中的小型地震。他們發現在新辨識出來的斷層跟板塊邊界的交會處有群震產生。他們說這證明了它可能是條活斷層。地震模式也指出斷層靠海那一側的摩擦特性跟靠陸地那側不同。這種差異可能讓該區域緩緩從板塊主體分裂出來而形成此斷層；另一種可能是過去板塊突然位移而留下此斷層。共同作者，拉蒙特-多爾蒂的地震學家Donna Shillington說不管它是經由哪種過程產生，它都象徵了潛在的危機。

Shillington表示：「板塊外側因為有那條大斷層而可以獨立移動，造成海嘯更容易發生。若該部分往海洋表層移動，會產生更大的垂直位移量。」一個簡單的比喻是：想像從一個餐盤敲出一塊小碎片，把兩塊破片一起放到桌子上，然後從下方用力敲桌子。比較小的碎片可能會跳得比盤子完整的情況要高，因為將它往下拉的力量比較小。

科學家已經知道阿留申隱沒帶的其他部分具有危險性。1946年西方遠處產生的地震和海嘯造成160人罹難，大都位在夏威夷。1964年離岸地震產生的山崩和海嘯造成將近140人喪生，主要是在阿拉斯加，其中19人是在奧勒岡州和加州；而它產生的波浪遠在巴布紐幾內亞甚至南極洲都有偵測到。在2017年7月，於阿留申群島西端附近外海的地震讓整個太平洋地區發布海嘯警報，所幸它最後只在當地形成僅有15公分高的波浪。

至於舒馬金地震空白帶，1788年居住在該區附近昂加島(Unga Island)上的俄國殖民者紀錄了一場大地震，產生的海嘯橫掃了海岸建物並帶走了許多阿留申原住民的性命。研究人員表示這場地震或許起源於舒馬金地震空白帶，但沒有方法可以證明。美國地質調查局的地質學家Rob Witter一直在搜索該區海岸有關海嘯的證據，但他表示目前為止得到的證據卻令他更加困惑。他說潛在威脅「在此處仍然是個謎團。」「我們對於隱沒帶造成的災害實在所知甚少。包括此篇新論文的發現在內，任何一點可以讓我們對其運作方式有更多啟發的新訊息都彌足珍貴。」

作者表示除了日本之外，類似斷層構造只在阿留申群島東邊的俄羅斯千島群島有明確紀錄。但是Shillington說：「我們還有許多地方的影像尚未取得。如果我們能夠搜遍全世界，或許可以看到更多的類似斷層構造。」研究阿留申群島，已退休的美國地質調查局科學家John Miller表示他自身的研究指出，該島弧中還有其他區段具有跟舒馬金和日本外海都很相似，具有威脅性的特徵。他說：「像這樣的危險區域才正要被大量辨認出來。」

拉蒙特的地震學家從1960年代就開始研究阿留申群島的地震，不過早期研究主要是在陸地進行。Bécel表示雖然1980年代美國地質調查局已經可以蒐集到跟這篇新研究同類型的資料，但現在的地震儀器讓他們產生的海床底下圖像變得精緻許多，使他們可以得到這項最新發現。她和其他研究人員在美國執行聲納研究的旗艦Marcus G. Langseth號上進行影像研究。這艘船隸屬於美國國家科學基金會，由拉蒙特―多爾蒂地球觀測站代表美國其他研究機構和大學使用。

New images from under Alaska seafloor suggest high tsunami danger

Waves could travel far across pacific

Scientists probing under the seafloor off Alaska have mapped a geologic structure that they say signals potential for a major tsunami in an area that normally would be considered benign. They say the feature closely resembles one that produced the 2011 Tohoku tsunami off Japan, killing some 20,000 people and melting down three nuclear reactors. Such structures may lurk unrecognized in other areas of the world, say the scientists. The findings will be published tomorrow in the print edition of the journal Nature Geoscience.

The discovery “suggests this part of Alaska is particularly prone to tsunami generation,” said seismologist Anne Bécel of Columbia University’s Lamont-Doherty Earth Observatory, who led the study. “The possibility that such features are widespread is of global significance.” In addition to Alaska, she said, waves could hit more southerly North American coasts, Hawaii and other parts of the Pacific.

A tsunami can occur as ocean crust (brown area) dives under continental crust (orange), causing the ocean floor to suddenly move. In a region off Alaska, researchers have found a large fault and other evidence indicating that the leading edge of the continental crust has split off, creating a tsunami-prone area where the floor can move more efficiently. (Anne Becel)

Tsunamis can occur as giant plates of ocean crust dive under adjoining continental crust, a process called subduction. Some plates get stuck for decades or centuries and tension builds, until they suddenly slip by each other. This produces a big earthquake, and the ocean floor may jump up or down like a released spring. That motion transfers to the overlying water, creating a surface wave.

The 2011 Japan tsunami was a surprise, because it came partly on a “creeping” segment of seafloor, where the plates move steadily, releasing tension in frequent small quakes that should prevent a big one from building. But researchers are now recognizing it may not always work that way. Off Japan, part of the leading edge of the overriding continental plate had become somewhat detached from the main mass. When a relatively modest quake dislodged this detached wedge, it jumped, unleashing a wave that topped 130 feet in places. The telltale sign of danger, in retrospect: a fault in the seafloor that demarcated the detached section’s boundary landward of the “trench,” the zone where the two plates initially meet. The fault had been known to exist, but no one had understood what it meant.

The researchers in the new study have now mapped a similar system in the Shumagin Gap, a creeping subduction zone near the end of the Alaska Peninsula some 600 miles from Anchorage. The segment is part of a subduction arc spanning the peninsula and the Aleutian Islands. Sailing on a specially equipped research vessel, the scientists used relatively new technology to penetrate deep into the seafloor with powerful sound pulses. By reading the echoes, they created CAT-scan-like maps of both the surface and what is underneath. The newly mapped fault lies between the trench and the coast, stretching perhaps 90 miles underwater more or less parallel to land. On the seafloor, it is marked by scarps about 15 feet high, indicating that the floor has dropped one side and risen on the other. The fault extends down more than 20 miles, all the way to where the two plates are moving against each other.

The team also analyzed small earthquakes in the region, and found a cluster of seismicity where the newly identified fault meets the plate boundary. This, they say, confirms that the fault may be active. Earthquake patterns also suggest that frictional properties on the seaward side of the fault differ from those on the landward side. These differences may have created the fault, slowly tearing the region off the main mass; or the fault may be the remains of a past sudden movement. Either way, it signals danger, said coauthor Donna Shillington, a Lamont-Doherty seismologist.

“With that big fault there, that outer part of the plate could move independently and make a tsunami a lot more effective,” said Shillington. “You get a lot more vertical motion if the part that moves is close to the seafloor surface.” A rough analogy: imagine snapping off a small piece of a dinner plate, laying the two pieces together on a table and pounding the table from below; the smaller piece will probably jump higher than if the plate were whole, because there is less holding it down.

Other parts of the Aleutian subduction zone are already known to be dangerous. A 1946 quake and tsunami originating further west killed more than 160 people, most in Hawaii. In 1964, an offshore quake killed around 140 people with landslides and tsunamis, mainly in Alaska; 19 people died in Oregon and California, and waves were detected as far off as Papua New Guinea and even Antarctica. In July 2017, an offshore quake near the western tip of the Aleutians triggered a Pacific-wide tsunami warning, but luckily it produced just a six-inch local wave.

As for the Shumagin Gap, in 1788, Russian colonists then living on nearby Unga Island recorded a great quake and tsunami that wiped out coastal structures and killed many native Aleut people. The researchers say it may have originated at the Shumagin Gap, but there is no way to be sure. Rob Witter, a geologist with the U.S. Geological Survey (USGS), has scoured area coastlines for evidence of such a tsunami, but so far evidence has eluded him, he said. The potential danger “remains a puzzle here,” he said. “We know so little about the hazards of subduction zones. Every little bit of new information we can glean about how they work is valuable, including the findings in this new paper.”

The authors say that apart from Japan, such a fault structure has been well documented only off Russia’s Kuril Islands, east of the Aleutians. But, Shillington said, “We don’t have images from many places. If we were to look around the world, we would probably see a lot more.” John Miller, a retired USGS scientist who has studied the Aleutians, said that his own work suggests other segments of the arc have other threatening features that resemble both those in the Shumagin and off Japan. “The dangers of areas like these are just now being widely recognized,” he said.

Lamont seismologists have been studying earthquakes in the Aleutians since the 1960s, but early studies were conducted mainly on land. In the 1980s, the USGS collected the same type of data used in the new study, but seismic equipment now able to produce far more detailed images deep under the sea floor made this latest discovery possible, said Bécel. She and others conducted the imaging survey aboard the Marcus G. Langseth, the United States’ flagship vessel for acoustic research. Owned by the U.S. National Science Foundation, it is operated by Lamont-Doherty on behalf the nation’s universities and other research institutions.

原始論文：Anne Bécel, Donna J. Shillington, Matthias Delescluse, Mladen R. Nedimović, Geoffrey A. Abers, Demian M. Saffer, Spahr C. Webb, Katie M. Keranen, Pierre-Henri Roche, Jiyao Li, Harold Kuehn. Tsunamigenic structures in a creeping section of the Alaska subduction zone. Nature Geoscience, 2017; 10 (8): 609 DOI: 10.1038/ngeo2990

引用自：The Earth Institute at Columbia University. "High tsunami danger in Alaska, perhaps elsewhere: Waves could travel far across Pacific."