地震可以讓逆衝斷層猛烈張開後再迅速閉合

原文網址：http://www.geologypage.com/2017/05/earthquakes-can-make-thrust-faults-open-violently-snap-shut.html

地震可以讓逆衝斷層猛烈張開後再迅速閉合

實驗揭露一種新機制可以解釋2011年日本東北大地震的破壞力為何如此強大

災難片中有種老套的場景是這樣的：一場地震突然襲來，地面應聲裂開，將人群和車子全數吞沒。地面裂開也許只是為了製造戲劇效果，不過地震學家長久以來也確信實際上這不會發生。

但根據加州理工學院新研究的實驗結果，在某種例子中會發生這種景象。

5月1日刊登在期刊《自然》(Nature)的研究，闡述了沿著逆衝斷層(thrust fault)的地震發生時，地面如何分裂開來，然後再迅速閉合回去的過程。

逆衝斷層一直以來便是世界上某些最大型地震的發生地點，像是2011年發生在日本外海，破壞福島核電廠的東北大地震。它們發生在一塊岩體跟另一塊互相擠壓而在地殼中產生的脆弱帶，當地震發生時，其中一方會往上滑動。

由加州理工學院和巴黎高等師範學院的工程師與科學家組成的團隊，發現沿著逆衝斷層產生的破裂可以迅速往上傳遞至地表，造成斷層其中一側扭離另外一側，而張開寬達數公尺的裂縫，接著又迅速闔上。

逆衝斷層地震通常發生於當兩塊岩體互相擠壓對方，而壓力超出將兩者固定在原處的摩擦力時。科學家長久以來都假設在地殼淺處，岩盤只會相對於彼此滑動一小段距離，不會產生張裂。

然而，調查東北大地震的研究人員發現斷層的滑移不僅只在淺處發生，在某些地方的滑動距離甚至可以達到50公尺。僅在外海不遠處發生這股劇烈運動，產生的海嘯破壞了日本沿岸的建築物，包括福島第一核電廠在內。

在刊於《自然》(Nature)的論文中，團隊提出的假說認為東北大地震的破裂係沿著斷層往上傳遞。一接近地表，破裂會促使其中一側的岩石扭離另外一側，因而打開一道裂口並暫時完全消除存在於兩岩盤間的摩擦力。這使得斷層的滑動距離可以長達50公尺。

過往認為逆衝斷層不可能會產生裂口。

「事實上，現在模擬地震用的電腦模型大多已經內建此設定。模型被程式規定為斷層的上下盤不能分開彼此。」這篇刊於《自然》論文的主要作者之一，加州理工學院航空與機械工程學系的教授Ares Rosakis表示。「此發現彰顯出實驗與觀察的價值所在。電腦模型的真實程度只能到達內建的先決條件所允許的樣貌。」

該國際團隊在位於加州理工學院，非正式名稱為「地震學風洞」(Seismological Wind Tunnel)的設施中模擬地震而發現了扭轉現象。設施一開始是由Rosakis 和Hiroo Kanamori共同主持。Rosakis是研究物質如何破壞的工程師，而Hiroo Kanamori則是探討地震物理機制的地震學家，同時也是此篇刊於《自然》研究的共同作者。「加州理工學院的研究環境很大程度地幫助我們跟其他不同領域的科學家有密切合作機會。」Kanamori表示，「我們這些地震學家在跟Rosakis教授團隊的合作中獲益良多，因為要實際進行實驗來驗證我們在地震學中的概念通常是很困難的。」

在此設施中，研究人員利用先進的高速光學量測技術來研究地震的破裂如何產生。為了在實驗室中模擬逆衝斷層地震，研究人員先將力學性質跟岩石類似的透明塑膠塊一分為二。接著他們將兩斷塊施予壓力重新拼合，藉此來模擬斷層線承受的構造應力。然後他們安裝小型的鎳-鉻保險絲在他們想讓地震震央發生的位置。當他們點燃保險絲，其所在位置的摩擦力會減小，使得破裂可以沿此微型斷層迅速傳遞。由於實驗用的材料具有光彈性(photoelastic)，意謂著透過光線經過透明材料時產生的干涉行為，可以從視覺上看出應力波在其中如何傳遞。他們利用高速攝影機來記錄地震模擬過程，並以雷射測速儀(粒子速度感應器)捕捉其造成的運動。

共同作者，巴黎高等師範學院的研究科學家Harsha Bhat說：「這是一個地震學家、構造學者和工程師良好合作的絕佳案例。直白來說，也是美/法的合作成果。」Bhat之前曾在加州理工學院進行博士後研究。

團隊驚訝地看到隨著破裂到達地表，斷層會旋開一道裂縫接著迅速閉合。隨後的電腦模擬――此修改過的模型將不允許斷層開裂的人為規則移除――證實了團隊在實驗中觀察到的現象：其中一個岩塊可以劇烈扭轉而遠離另一岩塊。在陸上或水下的逆衝斷層都可以發生此現象，代表該機制可能會改變我們對於海嘯如何產生的理解。

Earthquakes can make thrust faults open violently and snap shut

Experiments reveal a new mechanism that could explain the source of a destructive feature of the 2011 Tohoku earthquake

It is a common trope in disaster movies: an earthquake strikes, causing the ground to rip open and swallow people and cars whole. The gaping earth might make for cinematic drama, but earthquake scientists have long held that it does not happen.

Except, it can, according to new experimental research from Caltech.

The work, appearing in the journal Nature on May 1, shows how the earth can split open — and then quickly close back up — during earthquakes along thrust faults.

Thrust faults have been the site of some of the world’s largest quakes, such as the 2011 Tohoku earthquake off the coast of Japan, which damaged the Fukushima nuclear power plant. They occur in weak areas of the earth’s crust where one slab of rock compresses against another, sliding up and over it during an earthquake.

A team of engineers and scientists from Caltech and École normale supérieure (ENS) in Paris have discovered that fast ruptures propagating up toward the earth’s surface along a thrust fault can cause one side of a fault to twist away from the other, opening up a gap of up to a few meters that then snaps shut.

Thrust fault earthquakes generally occur when two slabs of rock press against one another, and pressure overcomes the friction holding them in place. It has long been assumed that, at shallow depths the plates would just slide against one another for a short distance, without opening.

However, researchers investigating the Tohoku earthquake found that not only did the fault slip at shallow depths, it did so by up to 50 meters in some places. That huge motion, which occurred just offshore, triggered a tsunami that caused damage to facilities along the coast of Japan, including at the Fukushima Daiichi Nuclear Power Plant.

In the Nature paper, the team hypothesizes that the Tohoku earthquake rupture propagated up the fault and — once it neared the surface — caused one slab of rock to twist away from another, opening a gap and momentarily removing any friction between the two walls. This allowed the fault to slip 50 meters.

That opening of the fault was supposed to be impossible.

“This is actually built into most computer models of earthquakes right now. The models have been programed in a way that dictates that the walls of the fault cannot separate from one another,” says Ares Rosakis, Theodore von Kármán Professor of Aeronautics and Mechanical Engineering at Caltech and one of the senior authors of the Nature paper. “The findings demonstrate the value of experimentation and observation. Computer models can only be as realistic as their built-in assumptions allow them to be.”

The international team discovered the twisting phenomenon by simulating an earthquake in a Caltech facility that has been unofficially dubbed the “Seismological Wind Tunnel.” The facility started as a collaboration between Rosakis, an engineer studying how materials fail, and Hiroo Kanamori, a seismologist exploring the physics of earthquakes and a coauthor of the Nature study. “The Caltech research environment helped us a great deal to have close collaboration across different scientific disciplines,” Kanamori said. “We seismologists have benefited a great deal from collaboration with Professor Rosakis’s group, because it is often very difficult to perform experiments to test our ideas in seismology.”

At the facility, researchers use advanced high-speed optical diagnostics to study how earthquake ruptures occur. To simulate a thrust fault earthquake in the lab, the researchers first cut in half a transparent block of plastic that has mechanical properties similar to that of rock. They then put the broken pieces back together under pressure, simulating the tectonic load of a fault line. Next, they place a small nickel-chromium wire fuse at the location where they want the epicenter of the quake to be. When they set off the fuse, the friction at the fuse’s location is reduced, allowing a very fast rupture to propagate up the miniature fault. The material is photoelastic, meaning that it visually shows — through light interference as it travels in the clear material — the propagation of stress waves. The simulated quake is recorded using high-speed cameras and the resulting motion is captured by laser velocimeters (particle speed sensors).

“This is a great example of collaboration between seismologists, tectonisists and engineers. And not to put too fine a point on it, US/French collaboration,” says Harsha Bhat, coauthor of the paper and a research scientist at ENS. Bhat was previously a postdoctoral researcher at Caltech.

The team was surprised to see that, as the rupture hit the surface, the fault twisted open and then snapped shut. Subsequent computer simulations — with models that were modified to remove the artificial rules against the fault opening — confirmed what the team observed experimentally: one slab can twist violently away from the other. This can happen both on land and on underwater thrust faults, meaning that this mechanism has the potential to change our understanding of how tsunamis are generated.

原始論文：Vahe Gabuchian, Ares J. Rosakis, Harsha S. Bhat, Raúl Madariaga, Hiroo Kanamori. Experimental evidence that thrust earthquake ruptures might open faults. Nature, 2017; DOI: 10.1038/nature22045

引用自：California Institute of Technology. "Earthquakes can make thrust faults open violently and snap shut: Experiments reveal a new mechanism that could explain the source of a destructive feature of the 2011 Tohoku earthquake."