研究挑戰歷時已久的理論：海嘯的形成過程

原文網址：www.sciencedaily.com/releases/2017/04/170426164703.htm

研究挑戰歷時已久的理論：海嘯的形成過程

由NASA進行的新研究挑戰了歷時已久的理論中，認為海嘯的形成及其能量的獲取大多來自於海床的垂直運動

不可否認的是大部份的海嘯起因於海床的大規模運動，通常是地震時一個板塊隱沒至另一個板塊之下，或是兩者之間產生滑動。1970年代在波浪水槽(water tank)中進行的實驗顯示，垂直抬升水槽底部可以產生類似海嘯的波浪。在接下來的十年中，日本科學家在波浪水槽裡模擬海床的水平運動，並觀察到其產生的能量微不足道。這導致了現今廣為接受的看法，認為海床的垂直運動是形成海嘯的主要因素。

2007年，美國加州帕薩迪納NASA噴射推進實驗室(JPL)的海洋學家Tony Song，分析2004年印度洋的蘇門答臘大地震後對這項理論提出質疑。地震儀和GPS的數據顯示海床垂直運動產生的能量不足以形成如此強烈的海嘯。反之，由Song和他的同事提出的公式指出，一旦把海床水平運動產生的能量列為影響因子之一，就能完全解釋海嘯具有的能量。這些結果跟三顆衛星蒐集到的海嘯數據相當符合，分別是NASA/法國國家太空研究中心的JASON、美國海軍的GFO(Geosat Follow-on)，以及歐洲太空總署的環境衛星。

Song利用了NASA和德國航空太空中心合作的重力回復及氣候實驗(Gravity Recovery and Climate Experiment, GRACE)任務中衛星蒐集到的數據，對2004年蘇門答臘地震進行的後續研究也支持他的主張，即單靠海床垂直抬升產生的能量不足以形成如此巨大的海嘯。

Song說：「雖然我擁有的所有證據都跟傳統理論牴觸，但我仍需要更多證據。」

他從物理層面蒐集更多證據，也就是要找出海床的水平運動究竟產生了多少動能，這跟海洋的深度以及海床的運動速度成正比。在嚴格審視1980年代的波浪水槽實驗之後，Song發現他們用的水槽無法精準呈現出上述兩個參數。那些水槽深度太淺而無法重現海嘯發生時海洋深度跟海床位移量之間的實際比例；另外，模擬海床水平運動時，水槽內壁的移動速度太慢而無法複製地震時板塊移動的真正速度。

Song表示：「我開始認為是這兩項參數失真的表現導致了我們接受許久卻有誤的結論――水平運動僅產生一小部分的動能。」

建造更優良的波浪水槽

為了要實際測試他的理論，Song和美國科瓦利斯奧勒岡州立大學的研究人員，在該大學的波浪研究實驗室中，同時參照直接測量的結果和衛星觀測資料，來模擬2004年蘇門答臘與2011年日本東北的地震。如同1980年代的實驗，他們在兩座不同水槽的水中推動一個直立壁面來模仿海底的水平移動，但他們使用了以活塞為動力的製波器而可以產生更快的速度。同時，他們也更精準地計算出在真實的海嘯中，海水深度跟水平位移量的比例為何。

新的實驗結果顯示產生2004和2011年的海嘯的能量中，有超過一半以上是來自於海床的水平位移。

此篇研究的共同作者，奧勒岡州立大學的營建工程學教授Solomon Yim表示：「我們從這項研究呈現出要推算海床傳遞給海洋的總能量以及預測海嘯時，不只需要觀察海床的垂直運動，也要考量到水平運動。」

此發現也更加精進Song和他的同僚發展出來的初步預警海嘯方法，其利用GPS技術以偵測海嘯的大小與威力。

由JPL管理的差分全球定位系統(Global Differential Global Positioning System, GDGPS)是非常精確的即時GPS處理系統，它可以在地震發生時測量海床的移動量。當陸地搖動時，震央附近的地面接收站也會跟著移動。透過跟衛星陣列的即時通訊，測站每一秒都可以偵測自身的運動，進而推估海底發生的水平及垂直運動的方向和大小。他們發展出的電腦模型將這些數據跟海底地形圖以及其他資訊結合之後，就可以計算海嘯的大小和行進方向。

「藉著辨識出海床水平移動的重要性，我們的GPS方法可以直接估計出地震傳遞了多少能量給海洋。」Song表示，「我們的目標是要達到海嘯的初步預警，甚至能在它形成之前就知道其規模大小。」

這篇研究刊登於《地球物理研究期刊―海洋》(Journal of Geophysical Research -- Oceans)。

Tsunami formation: Study challenges long-held theory

A new NASA study is challenging a long-held theory that tsunamis form and acquire their energy mostly from vertical movement of the seafloor.

An undisputed fact was that most tsunamis result from a massive shifting of the seafloor -- usually from the subduction, or sliding, of one tectonic plate under another during an earthquake. Experiments conducted in wave tanks in the 1970s demonstrated that vertical uplift of the tank bottom could generate tsunami-like waves. In the following decade, Japanese scientists simulated horizontal seafloor displacements in a wave tank and observed that the resulting energy was negligible. This led to the current widely held view that vertical movement of the seafloor is the primary factor in tsunami generation.

In 2007, Tony Song, an oceanographer at NASA's Jet Propulsion Laboratory in Pasadena, California, cast doubt on that theory after analyzing the powerful 2004 Sumatra earthquake in the Indian Ocean. Seismograph and GPS data showed that the vertical uplift of the seafloor did not produce enough energy to create a tsunami that powerful. But formulations by Song and his colleagues showed that once energy from the horizontal movement of the seafloor was factored in, all of the tsunami's energy was accounted for. Those results matched tsunami data collected from a trio of satellites -the NASA/Centre National d'Etudes Spatiales (CNES) Jason, the U.S. Navy's Geosat Follow-on and the European Space Agency's Environmental Satellite.

Further research by Song on the 2004 Sumatra earthquake, using satellite data from the NASA/German Aerospace Center Gravity Recovery and Climate Experiment (GRACE) mission, also backed up his claim that the amount of energy created by the vertical uplift of the seafloor alone was insufficient for a tsunami of that size.

"I had all this evidence that contradicted the conventional theory, but I needed more proof," Song said.

His search for more proof rested on physics -- namely, the fact that horizontal seafloor movement creates kinetic energy, which is proportional to the depth of the ocean and the speed of the seafloor's movement. After critically evaluating the wave tank experiments of the 1980s, Song found that the tanks used did not accurately represent either of these two variables. They were too shallow to reproduce the actual ratio between ocean depth and seafloor movement that exists in a tsunami, and the wall in the tank that simulated the horizontal seafloor movement moved too slowly to replicate the actual speed at which a tectonic plate moves during an earthquake.

"I began to consider that those two misrepresentations were responsible for the long-accepted but misleading conclusion that horizontal movement produces only a small amount of kinetic energy," Song said.

Building a Better Wave Tank

To put his theory to the test, Song and researchers from Oregon State University in Corvallis simulated the 2004 Sumatra and 2011 Tohoku earthquakes at the university's Wave Research Laboratory by using both directly measured and satellite observations as reference. Like the experiments of the 1980s, they mimicked horizontal land displacement in two different tanks by moving a vertical wall in the tank against water, but they used a piston-powered wave maker capable of generating faster speeds. They also better accounted for the ratio of how deep the water is to the amount of horizontal displacement in actual tsunamis.

The new experiments illustrated that horizontal seafloor displacement contributed more than half the energy that generated the 2004 and 2011 tsunamis.

"From this study, we've demonstrated that we need to look at not only the vertical but also the horizontal movement of the seafloor to derive the total energy transferred to the ocean and predict a tsunami," said Solomon Yim, a professor of civil and construction engineering at Oregon State University and a co-author on the study.

The finding further validates an approach developed by Song and his colleagues that uses GPS technology to detect a tsunami's size and strength for early warnings.

The JPL-managed Global Differential Global Positioning System (GDGPS) is a very accurate real-time GPS processing system that can measure seafloor movement during an earthquake. As the land shifts, ground receiver stations nearer to the epicenter also shift. The stations can detect their movement every second through real-time communication with a constellation of satellites to estimate the amount and direction of horizontal and vertical land displacement that took place in the ocean. They developed computer models to incorporate that data with ocean floor topography and other information to calculate the size and direction of a tsunami.

"By identifying the important role of the horizontal motion of the seafloor, our GPS approach directly estimates the energy transferred by an earthquake to the ocean," Song said. "Our goal is to detect a tsunami's size before it even forms, for early warnings."

The study is published in Journal of Geophysical Research -- Oceans.

原始論文：Y. Tony Song, Ali Mohtat, Solomon C. Yim. New insights on tsunami genesis and energy source. Journal of Geophysical Research: Oceans, 2017; DOI: 10.1002/2016JC012556

引用自：NASA/Jet Propulsion Laboratory. "Tsunami formation: Study challenges long-held theory." ScienceDaily. ScienceDaily, 26 April 2017.