1994的波利維亞大地震顯示出我們腳下660公里處有巨大的山脈
Liz Fuller-Wright
大部分的學生都學過地球可以分成三或四層：地殼、地函和地核，有時候會再把地核分成內核和外核。這種分法並沒有錯，但遺漏了科學家在地球內部辨識出來的其他層，包括地函當中的過渡帶。
普林斯頓大學的地震學家Jessica Irving、她之前的研究生Wenbo Wu、以及其他研究人員共同以散射之後的地震波，測定地函中的過渡帶底部與頂部的起伏程度。他們發現位在410公里深的過渡帶頂部大體而言相當平滑；然而，位在660公里深的過渡帶底部，某些地方的起伏程度卻比地表的平均還要高。Wu說：「換句話說，660公里邊界有些地方的地形變化比洛磯山脈或是阿帕拉契山還要崎嶇。」註：本圖並非顯示出真實比例。圖片來源：Kyle McKernan
地函中的過渡帶位在地下660公里，隔開了上部地函和下部地函。在本周發表於期刊《科學》(Science)的研究中，普林斯頓大學的地球物理學家Jessica
Irving和Wenbo
Wu，以及中國科學院測量與地球物理研究所的倪四道，運用波利維亞大地震的資料，發現過渡帶的底部有像是山脈以及其他地形一般的上下起伏。(過渡帶目前尚無正式名稱，研究人員以「660公里邊界」來稱呼。)
科學家必須運用地球上最強力的震波才能看到地球深部的樣貌，而這種震波得由大地震產生。普林斯頓大學的地質科學助理教授Irving說：「要在非常深的地方發生大地震，才能撼動整個地球。」
芮氏規模每增加一，地震的能量就會增加30幾倍，因此大地震的能量比小地震高出許多。此外，Irving說：「深處發生的地震不會耗費太多能量在地殼中，因此可以通過整個地函。」最好的數據來自於規模7.0以上的地震，因為震波會往所有方向傳遞，通過地核到達地球另一端後再傳回來。這項研究最關鍵的地震波數據取自1994年發生於波利維亞的地震，這場規模8.2的地震是紀錄以來第二大的深層地震。
「規模這麼大的地震很少發生。」Irving說：「幸運的是我們現在擁有的地震儀比20年前多出許多。20年來地震學已經變成完全不同的領域，不論是儀器或計算資源都有很大的改變。」
地震學家和數據科學家利用功能強大的電腦，包括普林斯頓大學的超級電腦叢集「老虎」在內，模擬地震波在地球深處散射時的複雜行為。
模擬的原理是依據地震波的基本性質：地震波會轉彎與反彈。就像光波碰到鏡子會反彈回去(反射)，通過稜鏡時會彎折(折射)一樣，地震波在均質的岩石中是直線行進，但碰到邊界或是不平整的地方就會反射或折射。
論文主要作者Wu甫於普林斯頓大學完成地質科學博士學位，目前在加州理工大學擔任博士後研究員。他說：「我們知道幾乎所有的物體表面都會有不平整的地方，因此可以散射光線，使我們看到這些東西，而散射的波就攜帶了物體表面粗糙程度的資訊。這項研究中，我們探討地震波在地球內部傳播時發生的散射現象，由此界定地球660公里邊界的粗糙度。」
研究人員對660公里邊界的粗糙程度相當訝異――這道邊界比我們生活的地表更加崎嶇不平。Wu表示：「換句話說，660公里邊界有些地方的地形變化比洛磯山脈或是阿帕拉契山還要崎嶇。」他們的統計模型無法精確計算出這些「山脈」的高度，但很有可能比地表所有的山脈都還高大。此外，它們的分佈也不平均――就像地殼表面有平滑的海床和巨大的山脈一樣，660公里邊界也有崎嶇跟平坦的區塊。過渡帶的頂部位在410公里深的地函中層，研究人員也檢視了這個地方，結果發現沒有類似的凹凸起伏。
東京工業大學的助理教授，專長為地震學的Christine
Houser並未參與此研究。他說：「他們發現地球的深部層位，就跟我們在地表觀察到的地形一樣，擁有複雜的變化。利用傳遍整個地球之後再回傳的震波，在深達660公里的邊界上發現只有1-3公里高的凹凸起伏，是項極具啟發的成就。……這項發現顯示隨著地震研究的儀器越來越進步，並且開拓出新的領域，我們還是可以利用地震波找出新的小尺度訊號，進而發現地球內部各層的新性質。」。
研究結果的意義
在瞭解地球如何形成以及持續運作的方式時，660公里邊界的起伏具有很重要的意義。地函佔了地球體積的84%，而660公里邊界將其分成上部地函跟下部地函。地質學家長久以來對這道邊界的重要性有很多討論，特別是關於熱在地函傳播的方式：核幔邊界(位在地下2900公里)的高溫岩石上升時是一路順暢地通往地函頂部，或者會在660公里邊界受到阻擋。有些地質化學和礦物學的證據指出上部地函和下部地函的化學成分有所差異，支持這兩個區塊從熱學或物理學的角度來看並未混合的說法。其他觀測結果則顯示兩者的化學性質沒有差異，使得某些研究人員提出「混合均勻地函」的假說，認為上部地函跟下部地函處在同一個傳熱循環之中。
Wu說：「我們的發現對這項問題提供了一些見解。」他們的數據顯示雙方可能都對了一部份。660公里邊界平坦的地方可能是因為此處的縱向混合較為均勻，而比較崎嶇、像是山脈一般的地區則形成於上部地函跟下部地函混合不均的地方。
研究人員發現的凹凸起伏存在於從大到小的各種尺度。理論上來說，熱異常和化學成分不均都能造成這種現象。不過，Wu解釋熱在地函中傳輸的方式，會讓小尺度的熱異常在數百萬年內都被抹除，所以只剩下化學成分的差異才能解釋他們發現到的小尺度起伏。
是什麼造成了如此顯著的化學成分差異？答案是過去屬於地殼，如今靜靜躺在地函中的岩石。太平洋周圍以及世上其他地方有很多隱沒帶，板塊在此相撞後海床會被推入地函。科學家長久以來對這些岩石進入地函後的遭遇有諸多討論。Wu和Irving的研究結果顯示隱沒板塊殘留下來的岩石，可能就位在660公里邊界的附近。
「由於我們偵測到的地震波只能顯示傳輸當下地球所處的狀態，因此人們可能會輕易推論，地震學家無法幫助人類瞭解地球內部過去45億年來經歷的變化。」Irving說，「然而，這項研究的結果可以使我們更加瞭解沒入地函內部的古老板塊會有什麼樣的經歷，以及年代久遠的地函物質可能存在於什麼地方，這相當令人振奮。」
她繼而表示：「當地震學讓我們可以從時間和空間尺度上更加了解地球內部時，是我感到最興奮的時刻。」
Massive 1994 Bolivian earthquake reveals mountains 660 kilometers
below our feet
Most schoolchildren learn that the Earth has three (or four) layers: a
crust, mantle and core, which is sometimes subdivided into an inner and outer
core. That’s not wrong, but it does leave out several other layers that
scientists have identified within the Earth, including the transition zone
within the mantle.
In a study published this week in Science, Princeton geophysicists Jessica
Irving and Wenbo Wu, in collaboration with Sidao Ni from the Institute of
Geodesy and Geophysics in China, used data from an enormous earthquake in
Bolivia to find mountains and other topography on the base of the transition
zone, a layer 660 kilometers (410 miles) straight down that separates the upper
and lower mantle. (Lacking a formal name for this layer, the researchers simply
call it “the 660-km boundary.”)
To peer deep into the Earth, scientists use the most
powerful waves on the planet, which are generated by massive earthquakes. “You
want a big, deep earthquake to get the whole planet to shake,” said Irving, an
assistant professor of geosciences.
Big earthquakes are vastly more powerful than small
ones — energy increases 30-fold with every step up the Richter scale — and deep
earthquakes, “instead of frittering away their energy in the crust, can get the
whole mantle going,” Irving said. She gets her best data from earthquakes that
are magnitude 7.0 or higher, she said, as the shockwaves they send out in all
directions can travel through the core to the other side of the planet — and
back again. For this study, the key data came from waves picked up after a
magnitude 8.2 earthquake — the second-largest deep earthquake ever recorded —
that shook Bolivia in 1994.
“Earthquakes this big don’t come along very often,”
she said. “We’re lucky now that we have so many more seismometers than we did
even 20 years ago. Seismology is a different field than it was 20 years ago, between
instruments and computational resources.”
Seismologists and data scientists use powerful
computers, including Princeton’s Tiger supercomputer cluster, to simulate the
complicated behavior of scattering waves in the deep Earth.
The technology depends on a fundamental property of
waves: their ability to bend and bounce. Just as light waves can bounce (reflect)
off a mirror or bend (refract) when passing through a prism, earthquake waves
travel straight through homogenous rocks but reflect or refract when they
encounter any boundary or roughness.
“We know that almost all objects have surface
roughness and therefore scatter light,” said Wu, the lead author on the new
paper, who just completed his geosciences Ph.D. and is now a postdoctoral
researcher at the California Institute of Technology. “That’s why we can see
these objects — the scattering waves carry the information about the surface’s
roughness. In this study, we investigated scattered seismic waves traveling
inside the Earth to constrain the roughness of the Earth’s 660-km boundary.”
The researchers were surprised by just how rough that
boundary is — rougher than the surface layer that we all live on. “In other
words, stronger topography than the Rocky Mountains or the Appalachians is
present at the 660-km boundary,” said Wu. Their statistical model didn’t allow
for precise height determinations, but there’s a chance that these mountains
are bigger than anything on the surface of the Earth. The roughness wasn’t
equally distributed, either; just as the crust’s surface has smooth ocean
floors and massive mountains, the 660-km boundary has rough areas and smooth
patches. The researchers also examined a layer 410 kilometers (255 miles) down,
at the top of the mid-mantle “transition zone,” and they did not find similar
roughness.
“They find that Earth’s deep layers are just as
complicated as what we observe at the surface,” said seismologist Christine
Houser, an assistant professor at the Tokyo Institute of Technology who was not
involved in this research. “To find 2-mile (1-3 km) elevation changes on a
boundary that is over 400 miles (660 km) deep using waves that travel through
the entire Earth and back is an inspiring feat. … Their findings suggest that
as earthquakes occur and seismic instruments become more sophisticated and
expand into new areas, we will continue to detect new small-scale signals which
reveal new properties of Earth’s layers.”
What it means
The presence of roughness on the 660-km boundary has
significant implications for understanding how our planet formed and continues
to function. That layer divides the mantle, which makes up about 84 percent of
the Earth’s volume, into its upper and lower sections. For years, geoscientists
have debated just how important that boundary is. In particular, they have
investigated how heat travels through the mantle — whether hot rocks are
carried smoothly from the core-mantle boundary (almost 2,000 miles down) all
the way up to the top of the mantle, or whether that transfer is interrupted at
this layer. Some geochemical and mineralogical evidence suggests that the upper
and lower mantle are chemically different, which supports the idea that the two
sections don’t mix thermally or physically. Other observations suggest no
chemical difference between the upper and lower mantle, leading some to argue
for what’s called a “well-mixed mantle,” with both the upper and lower mantle
participating in the same heat-transfer cycle.
“Our findings provide insight into this question,”
said Wu. Their data suggests that both groups might be partially right. The
smoother areas of the 660-km boundary could result from more thorough vertical
mixing, while the rougher, mountainous areas may have formed where the upper
and lower mantle don’t mix as well.
In addition, the roughness the researchers found,
which existed at large, moderate and small scales, could theoretically be
caused by heat anomalies or chemical heterogeneities. But because of how heat
in transported within the mantle, Wu explained, any small-scale thermal anomaly
would be smoothed out within a few million years. That leaves only chemical
differences to explain the small-scale roughness they found.
What could cause significant chemical differences?
The introduction of rocks that used to belong to the crust, now resting quietly
in the mantle. Scientists have long debated the fate of the slabs of sea floor
that get pushed into the mantle at subduction zones, the collisions happening
found all around the Pacific Ocean and elsewhere around the world. Wu and
Irving suggest that remnants of these slabs may now be just above or just below
the 660-km boundary.
“It’s easy to assume, given we can only detect
seismic waves traveling through the Earth in its current state, that
seismologists can’t help understand how Earth’s interior has changed over the
past 4.5 billion years,” said Irving. “What’s exciting about these results is
that they give us new information to understand the fate of ancient tectonic
plates which have descended into the mantle, and where ancient mantle material
might still reside.”
She added: “Seismology is most exciting when it lets
us better understand our planet’s interior in both space and time.”
原始論文：Wenbo Wu, Sidao Ni and Jessica Irving. Inferring
Earth's discontinuous chemical layering from the 660-kilometer boundary
topography. Science, 2019 DOI: 10.1126/science.aav0822
引用自：Princeton University. "Massive Bolivian earthquake
reveals mountains 660 kilometers below our feet."
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