研究人員對於深層地震有新的瞭解

原文網址：http://www.uh.edu/news-events/stories/2018/july-2018/0730earthquake.php

研究人員對於深層地震有新的瞭解

研究結果提出深層地震發源於非均向性岩石中

深層地震是指發生在地底60公里以下的地震。數十年來，研究人員知道這些深層地震將能量發散出去的方式跟起源於地表附近的地震有所不同，卻一直缺乏有系統的方式來瞭解其中緣由。

最近一組休士頓大學的研究團隊發表一種新方法來分析深層地震的震波輻射模式，並提出全世界的深層地震係起源於非均向性的岩石，這是先前從未有人做到的。岩石的非均向性表示地震波在岩石中的傳播方向不同會造成測量出來的速度也不同。

他們的結果於7月30日星期一發表在期刊《自然―地質科學》(Nature Geoscience)。

根據美國地質調查局統計大多數的地震發生在淺部，而它們造成的災害通常也比深層地震更加嚴重。然而，關於深層地震的成因仍然有本質上的問題需要解答。

在地底深處因為高溫的緣故，岩石一般會呈現韌性，也就是可以彎曲，因此無法突然破裂而產生深層地震。深層地震發生於隱沒帶下方，此處為兩個板塊相撞形成海溝的地點，而被拖往地球內部的板塊稱作隱沒板塊。由於深層地震只會發生在隱沒板塊內部，顯示其中必然發生了某些不尋常的作用。

此篇論文的通訊作者，休士頓大學自然科學學院的助理教授Yingcai Zheng是震波成像的專家，他說自1926年發現深層地震之後，地震學家就一直在試圖瞭解此現象。過往提出的理論包含流體造成的影響、runaway thermal heating或者礦物結晶構造瞬間破壞所造成的固態相變。

參與此研究的研究人員除了Zheng之外，還包括第一作者地球與大氣科學系的博士候選人Jiaxuan Li、地球物理學研究教授Leon Thomsen、地質學教授Thomas J. Lapen，以及深圳南方科技大學地副教授暨休士頓大學的兼任教授Xinding Fang。

「過去50年來，有越來越多的證據顯示大部分的深層地震並不遵守在多數淺層地震中看到的雙偶極輻射模式(double-couple radiation pattern)。」Zheng表示，「我們努力思索為什麼會有這樣的現象發生。」雙偶極模式是由已存在的斷層發生剪力破裂而造成。

這項由國家科學基金會補助的研究探討了不同輻射模式產生的可能原因。Zheng表示過往理論提出深層地震源自於不同的破裂機制，因此其中的物理化學作用可能和那些引發淺層地震的機制不同。

但研究人員在研究世上六個不同隱沒帶中發生的1057個深層地震的輻射模式之後，建構出另外一種解釋。他們發現包覆在深層地震源頭周遭的岩石結構，將其震波輻射轉變成非雙偶極模式。Li說：「常見的雙偶極輻射模式以及深層地震的特殊輻射模式，都能用層狀岩石中的剪力破裂來一併解釋。」

在隱沒板塊進入海溝之前，它會吸收海水而形成非均向性的水合礦物。隨著隱沒板塊沉入地函，這些水分會因為高溫高壓環境而被驅趕出來，稱為脫水作用(dehydration)。沿著隱沒板塊和地函交界發生的脫水作用與強力的剪動，可以使岩石變成脆性並破裂，引起中層地震(發生於地下60公里至300公里深)。

Li表示：「雖然隱沒板塊的方向在不同地點會有很大的差異，但我們發現在此深度岩石的非均向性結構總是平行於隱沒板塊的表面。」

研究人員表示即使到了深度更深的地方，還是能發現岩石的非均向性，代表菱鎂礦或排列整齊的碳酸岩質熔融囊(melt pocket)之類的物質，可能參與了產生深層破裂的過程。研究人員提出的非均向性程度相當高――大約有百分之25；而被廣為接受的亞穩定固態相變(meta-stable solid phase change)機制，則無法造成研究人員提出的，造成深層地震輻射模式所需的非均向性。

Researchers report new understanding of deep earthquakes

Work suggests deep earthquakes are hosted in anisotropic rocks

Researchers have known for decades that deep earthquakes - those deeper than 60 kilometers, or about 37 miles below the Earth's surface - radiate seismic energy differently than those that originate closer to the surface. But a systematic approach to understanding why has been lacking.

Now a team of researchers from the University of Houston has reported a way to analyze seismic wave radiation patterns in deep earthquakes to suggest global deep earthquakes are in anisotropic rocks, something that had not previously been done. The rock anisotropy refers to differences in seismic wave propagation speeds when measured along different directions.

Their findings were published Monday, July 30, by the journal Nature Geoscience.

Most earthquakes occur at shallow depths, according to the U.S. Geological Survey, and they generally cause more damage than deeper earthquakes. But there are still substantial questions about the causes of deep earthquakes.

Normal rocks are ductile, or pliable, at these great depths because of high temperature and thus aren't able to rupture in an abrupt fashion to produce deep earthquakes, which occur below subduction zones where two tectonic plates collide at ocean trenches. The plate being pushed under is referred to as the subducting slab. The fact that deep earthquakes occur only in these slabs suggests some unusual process is happening within the slab.

Yingcai Zheng, assistant professor of seismic imaging in the UH College of Natural Sciences and Mathematics and corresponding author for the paper, said seismologists have sought to understand deep earthquakes since the phenomenon was discovered in 1926. Hypotheses include the effect of fluids, runaway thermal heating or solid-phase change due to sudden collapse of the mineral crystal structure.

In addition to Zheng, researchers involved in the work include the first author Jiaxuan Li, a Ph.D. candidate in the Department of Earth and Atmospheric Sciences; Leon Thomsen, research professor of geophysics; Thomas J. Lapen, professor of geology; and Xinding Fang, adjunct professor at UH and concurrently associate professor at the Southern University of Science and Technology China.

"Over the past 50 years, there has been growing evidence that a large proportion of deep earthquakes do not follow the double-couple radiation pattern seen in most shallow earthquakes," Zheng said. "We set out to look at why that happens." The double-couple pattern is caused by a shear rupture of a pre-existing fault.

The work, funded by the National Science Foundation, looked at potential reasons for the differing radiation patterns; Zheng said earlier theories suggest that deep earthquakes stem from a different rupture mechanism and possibly different physical and chemical processes than those that spark shallow earthquakes.

But after studying the radiation patterns of 1,057 deep earthquakes at six subduction zones worldwide, the researchers determined another explanation. They found that the surrounding rock fabric enclosing the deep quake alters the seismic radiation into a non-double-couple pattern. "Both the common double-couple radiation patterns and uncommon patterns of deep earthquakes can be explained simultaneously by shear rupture in a laminated rock fabric," Li said.

Before the subducting plate enters the trench, it can absorb sea water to form hydrated anisotropic minerals. As the slab descends in the Earth's mantle, the water can be expelled due to high pressure and high temperature conditions, a process known as dehydration. The dehydration and strong shearing along the slab interface can make the rock brittle and lead to rupture in intermediate-depth earthquakes, defined as those between 60 kilometers and 300 kilometers deep (37 miles to 186 miles).

"We found at these depths that the anisotropic rock fabric is always parallel to the slab surface, although the slab can change directions greatly from place to place," Li said.

Anisotropy is also found in rocks at even greater depths, which suggests materials such as magnesite or aligned carbonatite melt pockets may be involved in generating the deep ruptures, the researchers said. Because the inferred anisotropy is high -- about 25 percent -- the widely believed meta-stable solid phase change mechanism is not able to provide the needed anisotropy inferred by the researchers.

原始論文：Jiaxuan Li, Yingcai Zheng, Leon Thomsen, Thomas J. Lapen, Xinding Fang. Deep earthquakes in subducting slabs hosted in highly anisotropic rock fabric. Nature Geoscience, 2018; DOI: 10.1038/s41561-018-0188-3

引用自：University of Houston. "UH Researchers Report New Understanding of Deep Earthquakes"