megathrust earthquake

東日本大震災から五年を迎えました。あの日の災害から何を学べるのでしょう。

Shaken Beliefs: Seismic Lessons from Japan’s Tohoku Earthquake

Japan could well be called the cradle of seismology. It occupies one of Earth’s most precarious tectonic settings, at the nexus of four plates, and its written record of earthquakes extends back to 599 A.D., during the reign of Empress Suiko. Japanese seismologists of the early twentieth century made a number of significant contributions to geophysics, some of which anticipated plate-tectonic theory by decades. In 1899, several years after a powerful tsunami struck the Sanriku Coast, a man named Akitsune Imamura correctly inferred the type of fault slip (now called a megathrust event) that generates such waves. In the nineteen-twenties, another Japanese scientist, Kiyoo Wadati, collected seismic data that became critical to the development of the Richter scale and, eventually, the discovery of the process of subduction, in which old ocean crust sinks back into Earth’s mantle. Today, Japan spends more money per capita than any other country on earthquake research, engineering, and preparedness.

And yet the disaster of five years ago—a magnitude-9.0 megathrust earthquake off Tohoku, followed by a major tsunami and nuclear accident—came as a surprise. Until March 11, 2011, the consensus among seismologists was that a particular stretch of fault would observe certain rules, rupturing at consistent intervals in events of similar size.

以下省略

http://www.newyorker.com/tech/elements/shaken-beliefs-seismic-lessons-from-japans-tohoku-earthquake

“megathrust”を取り上げます。とはいったものの、手元の辞書で索引しても確認できません、最近使われ始めた単語なのでしょうか。語義を探るため、構成要素から探っていきます。基本的な語彙である”thrust”、その語義は様々ありますが、地震に関連するものであると考慮すると、ここでは「(突然、力強く)ぐいと推すこと、突き刺すこと」や「(地殻の)衝上断層」(『G3』より)あたりになるかと考えられます。ここにmega(milliton, great)を組み合わせたもの、「巨大断層、強大な突き上げ（型の衝撃）」といった意味合いになるかと思います。

ここで定義を検索してみると、YourDictionaryでは” A sudden slip a long a fault between a subducting and an overriding plate; results in a major earthquake”、CollinsDictionaryではearthquakeを伴い “Occurs at a subduction zones at destructive plate boundaries 'convergent boundaries' where one tectonic plate is subducted 'forced underneath' by another”と定義されています。ここでは後者と同じく”a megathrust earthquake”「巨大地震、巨大断層地震」といった形だと覚えるのに易しいでしょう。3月1１日を象徴する言葉です、震災から五年を迎えた今日、あえて取り上げさせて頂きました。(あいづ)