科學家發現地震成像無法看見水
這項發現可能使科學家需要重新詮釋地球內部的地震波速圖
Jennifer Chu
當地震來襲，附近的地震儀可以把地震造成的晃動以地震波的形式記錄下來。地震波除了可以透露出地震的震央在哪，科學家也能用地震波繪製地球內部構造的分布圖像，跟醫生可以利用電腦斷層掃瞄來得到身體內部的影像一樣。
透過測量地震波在不同深度傳播的速度，科學家可以辨識地表之下的岩石和物質種類。物質種類不同對地震波的波速有不一樣的影響，而震波分布圖的精準度有多高，取決於科學家在這方面的瞭解程度。
美國麻省理工學院和澳洲國立大學的研究人員最近發現地震波基本上是無法看到一種在地球內部隨處可見的物質：水。
他們的這項發現今日刊登於期刊《自然》(Nature)，不同於一般的假設中認為地震可以捕捉深埋在地球上部地函的水所代表的訊號；事實上，團隊發現即便有少量的水分存在，對地震波行進的速度還是毫無影響。
這項結果或許有助於科學家重新解讀地球內部――比方說中洋脊之類的地方――呈現出的地震波速圖。在中洋脊，形成於地球深部的岩漿穿過海床上的巨大裂縫噴發出來，往洋脊兩側擴散出去，最後固化而形成新的海洋地殼。
在地球深處的岩石仍可以發現微量的水分，但地底下數十公里之處發生的熔融過程會去除這些水分。科學家以往認為地震波速圖顯示了這種「乾溼變化」，也就是從堅硬的板塊轉變成下方具有可塑性的地函。然而，團隊的發現指出地震成像捕捉到的訊號或許不是水，而是熔融物質――也就是範圍極小的熔化岩石。
「如果我們看見(地震波速出現)非常強烈的變化，原因比較有可能是由熔融物質造成。」美國麻省理工學院地球、大氣與行星科學系的研究員Ulrich Faul表示，「我們根據這些實驗結果所得出的想法之中，水不再具有重要意義。這會改變我們對地球內部影像的解讀方式。」
和Faul共同進行研究的包括來自澳洲國立大學的主要作者Christopher
Cline，以及Emmanuel
David、Andrew
Berry與Ian
Jackson。
以扭轉產生地震波
Faul、Cline和其他共同研究人員原先是打算測出水對地震波速的影響有多大。他們就跟大部分研究人員假設的一樣，認為地震成像能「看見」水。水會以氫氧基的形式存在於岩石的個別礦物顆粒中，或者是以分子等級的水團被包圍在礦物顆粒之間。科學家知道即便是如此微量的水，還是可以弱化地球內部深處的岩石。
「我們知道相當少量的水就能對岩石的性質有強烈影響。」Faul表示，「因此，我們過往推論水對地震波的速度也會有重大影響。」
為了測出水對地震波波速的影響程度有多大，團隊製備了許多性質不同的橄欖石樣品。橄欖石為地球上部地函的主要組成礦物，因此決定了上部地函的性質。研究人員讓每個樣品中含有的水量都不同，接著一次將一個樣品置於儀器當中。這個儀器的設計可以緩緩扭轉岩石，就像扭轉橡皮筋一樣。為了模擬地球深處的情況，實驗是在處於高溫高壓環境的爐中進行。
「我們扭轉樣品的其中一端，接著測量另一端產生的應變有多大，以及兩者之間的時間差。」Faul表示，「這可以模擬地震波在地球內部傳遞時的情況。實驗產生的應變大小跟人類頭髮的寬度差不多；在壓力達到大氣壓力2000倍，溫度高到可以熔化鋼鐵的實驗環境下，測量起來是很不容易的。」
團隊預期他們可以發現給定樣品的含水量，則它和地震波在樣品內部傳遞的速度之間會呈現出某種相關性。當最初幾個樣品並未呈現出研究人員預期的行為時，他們先調整樣品成分然後重新測量，但他們得到的結果一直都沒有呈現出對應關係。最後，他們無可避免地得出這項結論：最初的假設是錯誤的。
Faul表示：「在我們(扭轉實驗)的測量結果中，即便我們已經可以清楚分析出岩石確實含有水，它們表現出來的行為就像處於乾燥狀態。此時，我們充分瞭解到水份的影響無足輕重。」
被封裝的岩石
其中一個出人意料的實驗結果顯示地震波波速似乎跟岩石的氧化狀態有關。地球所有岩石都含有某種程度的鐵，就像組裝成車的金屬鐵暴露在一定含量的氧氣之下就會氧化一樣，岩石中的鐵也會處於不同程度的氧化態。研究人員近乎是在無意之間發現橄欖石含有的鐵的氧化狀態會影響地震波在岩石內部的傳遞方式。
Cline和Faul出於必要而重新設置他們的實驗條件之後得到了這項結論。要進行這項實驗，團隊通常是把樣品一個個裝在用鎳和鐵做成的圓柱之中。然而，在測量每個圓柱內部的含水量時，它們發現水裡面的氫原子很容易透過金屬容器而跑到岩石外面。為了將氫原子封在裡面，他們轉而使用以鉑做成的封套。
讓他們驚訝的是，他們發現包裹在樣品周圍的金屬類型會影響到地震波的性質。從另外的獨立實驗顯示真正的變因是橄欖石中的Fe3+有多少。正常來說橄欖石中鐵的氧化態是2+，實驗證明Fe3+的出現會在橄欖石內部造成缺陷而影響地震波波速。
Faul表示團隊的發現顯示地震波或許可以用來繪出某些區域氧化程度的分布圖，比方說隱沒帶，也就是地球的海洋板塊沉入地函之處。然而，根據他們的結果，地震成像無法用來顯示地球內部水的分布情形。有些科學家解讀為含水的區域，事實上可能是熔融岩石。這項見解或許會使我們對地球的板塊運動是如何隨著時間而變化有所改觀。
「有個根本問題是使地球的板塊運動得以運作的潤滑劑是什麼。」Faul表示，「我們的成果指出板塊底部出現的微量熔融物質才是關鍵，而非乾燥板塊之下的含水地函。整體來說，這些結果或許能幫助科學家闡明揮發性物質在地球表面和內部之間的循環過程。」
Scientists find seismic
imaging is blind to water
Findings may lead scientists
to reinterpret seismic maps of the Earth's interior
When an earthquake strikes, nearby seismometers pick up its
vibrations in the form of seismic waves. In addition to revealing the epicenter
of a quake, seismic waves can give scientists a way to map the interior
structures of the Earth, much like a CT scan images the body.
By measuring the velocity
at which seismic waves travel at various depths, scientists can determine the
types of rocks and other materials that lie beneath the Earth’s surface. The
accuracy of such seismic maps depends on scientists’ understanding of how
various materials affect seismic waves’ speeds.
Now researchers at MIT and
the Australian National University have found that seismic waves are
essentially blind to a very common substance found throughout the Earth’s
interior: water.
Their findings, published
today in the journal Nature,
go against a general assumption that seismic imaging can pick up signs of water
deep within the Earth’s upper mantle. In fact, the team found that even trace
amounts of water have no effect on the speed at which seismic waves travel.
The results may help
scientists reinterpret seismic maps of the Earth’s interior. For instance, in
places such as midocean ridges, magma from deep within the Earth erupts through
massive cracks in the seafloor, spreading away from the ridge and eventually
solidifying as new oceanic crust.
The process of melting at
tens of kilometers below the surface removes tiny amounts of water that are
found in rocks at greater depth. Scientists have thought that seismic images
showed this “wet-dry” transition, corresponding to the transition from rigid
tectonic plates to deformable mantle beneath. However, the team’s findings
suggest that seismic imaging may be picking up signs of not water, but rather,
melt — tiny pockets of molten rock.
“If we see very strong
variations [in seismic velocities], it’s more likely that they’re due to melt,”
says Ulrich Faul, a research scientist in MIT’s Department of Earth, Atmospheric,
and Planetary Sciences. “Water, based on these experiments, is no longer a
major player in that sense. This will shift how we interpret images of the
interior of the Earth.”
Faul’s co-authors are lead
author Christopher Cline, along with Emmanuel David, Andrew Berry, and Ian
Jackson, of the Australian National University.
A seismic twist
Faul, Cline, and their
colleagues originally set out to determine exactly how water affects seismic
wave speeds. They assumed, as most researchers have, that seismic imaging can
“see” water, in the form of hydroxyl groups within individual mineral grains in
rocks, and as molecular-scale pockets of water trapped between these grains.
Water, even in tiny amounts, has been known to weaken rocks deep in the Earth’s
interior.
“It was known that water
has a strong effect in very small quantities on the properties of rocks,” Faul
says. “From there, the inference was that water also affects seismic wave
speeds substantially.”
To measure the extent to
which water affects seismic wave speeds, the team produced different samples of
olivine — a mineral that constitutes the majority of Earth’s upper mantle and
determines its properties. They trapped various amounts of water within each
sample, and then placed the samples one at a time in a machine engineered to
slowly twist a rock, similar to twisting a rubber band. The experiments were
done in a furnace at high pressures and temperatures, in order to simulate
conditions deep within the Earth.
“We twist the sample at one
end and measure the magnitude and time delay of the resulting strain at the
other end,” Faul says. “This simulates propagation of seismic waves through the
Earth. The magnitude of this strain is similar to the width of a thin human
hair — not very easy to measure at a pressure of 2,000 times atmospheric
pressure and a temperature that approaches the melting temperature of steel.”
The team expected to find a
correlation between the amount of water in a given sample and the speed at
which seismic waves would propagate through that sample. When the initial
samples did not show the anticipated behavior, the researchers modified the
composition and measured again, but they kept getting the same negative result.
Eventually it became inescapable that the original hypothesis was incorrect.
“From our [twisting]
measurements, the rocks behaved as if they were dry, even though we could
clearly analyze the water in there,” Faul says. “At that point, we knew water
makes no difference.”
A rock, encased
Another unexpected outcome
of the experiments was that seismic wave velocity appeared to depend on a rock’s
oxidation state. All rocks on Earth contain certain amounts of iron, at various
states of oxidation, just as metallic iron on a car can rust when exposed to a
certain amount of oxygen. The researchers found, almost unintentionally, that
the oxidation of iron in olivine affects the way seismic waves travel through
the rock.
Cline and Faul came to this
conclusion after having to reconfigure their experimental setup. To carry out
their experiments, the team typically encases each rock sample in a cylinder
made from nickel and iron. However, in measuring each sample’s water content in
this cylinder, they found that hydrogen atoms in water tended to escape out of
the rock, through the metal casing. To contain hydrogen, they switched their
casing to one made from platinum.
To their surprise they
found that the type of metal surrounding the samples affected their seismic
properties. Separate experiments showed that what in fact changed was the
amount of Fe3+ in olivine. Normally the oxidation state of iron in
olivine is 2+. As it turns out, the presence of Fe3+ produces
imperfections which affect seismic wave speeds.
Faul says that the group’s
findings suggest that seismic waves may be used to map levels of oxidation,
such as at subduction zones — regions in the Earth where oceanic plates sink
down into the mantle. Based on their results, however, seismic imaging cannot
be used to image the distribution of water in the Earth’s interior. What some
scientists interpreted as water may in fact be melt — an insight that may
change our understanding of how the Earth shifts its tectonic plates over time.
“An underlying question is
what lubricates tectonic plates on Earth,” Faul says. “Our work points toward
the importance of small amounts of melt at the base of tectonic plates, rather
then a wet mantle beneath dry plates. Overall these results may help to
illuminate volatile cycling between the interior and the surface of the Earth.”
原始論文：C. J. Cline II, U. H. Faul, E. C. David, A. J. Berry & I.
Jackson. Redox-influenced seismic properties of upper-mantle olivine. Nature,
2018 DOI: 10.1038/nature25764
引用自：Massachusetts
Institute of Technology. “Scientists find seismic imaging is blind to water”
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