倫敦大學學院的研究人員參與的新研究指出，格陵蘭偏遠地區發生的一場山崩產生的巨大海嘯在一座峽灣內部來回晃動了九天，造成了傳遍整個地球的震動。
事件之前(上)與過後(下)的實地拍攝照片。黃色為崩落的山體區塊。紅色虛線為濺起的水花(backsplash)與海嘯(tsunami)到達的高度。圖片來源：UCL
2023年九月全世界都偵測到一股持續九天的神祕地震訊號，使得地震學家感到十分困惑。最近這篇發表在期刊《科學》(Science)的研究結論表示，該訊號的成因是水體的移動。
這起無人目睹的最初事件為一座1.2公里高的山頭崩落至下方遠處的迪克森峽灣，濺起了噴到空中200多公尺的水花，並且造成高達110公尺的波浪。這股波浪在峽灣內部傳遞了10公里，經研究人員估算，其在幾分鐘之內高度下降到7公尺，然後在數天之後降到了只有幾公分。
團隊運用精密的數學模型，並且將山崩的角度以及峽灣特別狹窄且曲折的形狀重建出來，藉此證明這道水體為什麼可以在不損失太多能量的情況下，持續晃動九天的時間。
模型預測這些水來回移動一次的時間為90秒，符合全球各地記錄中在地殼傳遞的震動訊號。
研究人員在論文中寫道，山崩發生的原因是山腳的冰河退縮，使其無法支撐上方的岩壁——歸根究柢，原因即為氣候變遷。這是格陵蘭東部首次觀測到此種類型的山崩與海嘯。
倫敦大學學院地球科學系的Stephen
Hicks博士是共同作者之一，他說：「我第一次看到這股地震訊號時完全一頭霧水。就算我們知道地震儀記錄到的訊號來源有可能是地球表面發生的各類事物，但記錄中從來沒有見過這樣持續許久、傳遍全球，而且只有一種震盪頻率的地震波。這促使我和其他人聯合建立起一個大型科學團隊來解開這道謎團。」
「我們對於這起事件的研究突顯出氣候變遷、冰河穩定性遭到破壞、水體的運動這三種分別發生在大氣圈、冰圈、水圈的事件，竟然能以如此錯綜複雜的方式交織在一起，著實令人讚嘆不已。」
「這是第一次有水體的晃動經由地殼的震動傳遍了整個地球，並且持續好幾天而被記錄下來。」
在地殼中傳遞的震動會成為地震訊號。當時全世界的地震儀——從南極到北極——都有偵測到這股神祕的訊號。完全不同於一般地震紀錄中含有各種頻率的「高亢」與「低沉」聲響，它就只有一種震動頻率，像是道沒有高低起伏的嗡嗡聲。
研究作者第一次發現到這股訊號時，他們將其標註為「USO」，意即「不明震動物體」(unidentified seismic object)。
於此同時，在格陵蘭東北部偏遠地區的峽灣發生了一場大海嘯的新聞，也傳到了相關單位以及在該地工作的研究人員。
研究人員聯合起來成立了一個特別的跨領域團隊，包括了68位來自15個國家40所機構的科學家。他們結合了地震儀與次聲波資料、野外測量數據、地面與衛星影像，還有海嘯波的模擬結果。
團隊也運用了丹麥軍方在事件過後沒幾天駛入峽灣所拍下的照片，藉此檢視崩塌的山壁與冰河前緣，以及海嘯留下的滿目瘡痍。
丹麥與格陵蘭地質調查局的(GEUS) Kristian Svennevig博士是主要作者，他說：「當我們發起這趟科學歷險的時候，所有人都帶著滿滿的疑問，對這股訊號的來源毫無頭緒。我們唯一知道的是它跟那場山崩之間有某種關聯。若不是來自各國從事不同領域的科學家傾注心力進行研究，我們也沒辦法解開這道謎題。」
他接著表示：「身為一個探討山崩的科學家，此研究讓我感興趣的另外一點是，這是格陵蘭東部有史以來首次觀察到山崩與海嘯，顯示氣候變遷對於該地已經造成了重大衝擊。」
研究團隊估計崩落到峽灣中的岩石與冰雪總共有2500萬立方公尺，足以填滿10000座奧運規格的游泳池。
他們利用數值模擬以及當地的資料與影像，證實這是近代歷史觀測到規模最大的海嘯之一。
距離山崩發生地點70公里遠的地方，4公尺高的海嘯對Ella
Ø島上的一座研究基地造成損壞，而峽灣系統當中也有多處文化與考古遺址遭到破壞。
迪克森峽灣位在參觀格陵蘭峽灣的旅程中郵輪經常航行的路線上。幸運的是，山崩與海嘯發生的那天，迪克森峽灣附近沒有任何一艘郵輪經過，不然此等規模的海嘯可能會造成十分慘重的後果。
他們的數學模型以超高解析度重建出峽灣各處的寬度與深度，使得他們可以印證水體來回移動的特有節奏確實符合偵測到的地震訊號。
研究結論表示隨著氣候變遷急遽加速，標定出過往認為是穩定的區域並加以監視，同時建立大型山崩與海嘯事件的早期預警系統，這些事項的重要性將會越來越高。
卡爾斯魯爾理工學院的Thomas
Forbriger是共同作者之一，他說：「高保真度的寬頻地震站是唯一可以捕捉到這種特殊訊號的儀器，如果這種儀器所構成的網路沒有遍及全世界，我們也沒辦法發現並分析這起神奇的事件。」
Climate-change-triggered landslide
caused Earth to vibrate for nine days
A landslide in a remote part of Greenland
caused a mega-tsunami that sloshed back and forth across a fjord for nine days,
generating vibrations throughout Earth, according to a new study involving UCL
researchers.
The study, published in the journal Science, concluded that this movement of
water was the cause of a mysterious, global seismic signal that lasted for nine
days and puzzled seismologists in September 2023.
The initial event, not observed by human eye, was the
collapse of a 1.2km-high mountain peak into the remote Dickson Fjord beneath,
causing a backsplash of water 200 metres in the air, with a wave up to 110
metres high. This wave, extending across 10km of fjord, reduced to seven metres
within a few minutes, the researchers calculated, and would have fallen to a
few centimetres in the days after.
The team used a detailed mathematical model,
recreating the angle of the landslide and the uniquely narrow and bendy fjord,
to demonstrate how the sloshing of water would have continued for nine days,
with little energy able to escape.
The model predicted that the mass of water would have
moved back and forth every 90 seconds, matching the recordings of vibrations
travelling in the Earth’s crust all around the globe.
The landslide, the researchers wrote, was a result of
the glacier at the foot of the mountain thinning, becoming unable to hold up
the rock-face above it. This was ultimately due to climate change. The
landslide and tsunami were the first observed in eastern Greenland.
Co-author Dr Stephen Hicks, of UCL Earth Sciences,
said: “When I first saw the seismic signal, I was completely baffled. Even
though we know seismometers can record a variety of sources happening on
Earth’s surface, never before has such a long-lasting, globally travelling
seismic wave, containing only a single frequency of oscillation, been recorded.
This inspired me to co-lead a large team of scientists to figure out the
puzzle.
“Our study of this event amazingly highlights the
intricate interconnections between climate change in the atmosphere,
destabilisation of glacier ice in the cryosphere, movements of water bodies in
the hydrosphere, and Earth’s solid crust in the lithosphere.
“This is the
first time that water sloshing has been recorded as vibrations through the
Earth’s crust, travelling the world over and lasting several days.”
The mysterious seismic signal – coming from a
vibration through the Earth’s crust – was detected by seismometers all over the
globe, from the Arctic to Antarctica. It looked completely different to
frequency-rich ‘rumbles’ and ‘pings’ from earthquake recordings, as it
contained only a single vibration frequency, like a monotonous-sounding hum.
When the study’s authors first discovered the signal,
they made a note of it as a “USO”: unidentified seismic object.
At the same time, news of a large tsunami in a remote
northeast Greenland fjord reached authorities and researchers working in the
area.
The researchers joined forces in a unique
multidisciplinary group involving 68 scientists from 40 institutions in 15
countries, combining seismometer and infrasound data, field measurements,
on-the-ground and satellite imagery, and simulations of tsunami waves.
The team also used imagery captured by the Danish
military who sailed into the fjord just days after the event to inspect the
collapsed mountain-face and glacier front along with the dramatic scars left by
the tsunami.
Lead author Dr Kristian Svennevig, from the
Geological Survey of Denmark and Greenland (GEUS), said: “When we set out on
this scientific adventure, everybody was puzzled and no one had the faintest
idea what caused this signal. All we knew was that it was somehow associated with
the landslide. We only managed to solve this enigma through a huge
interdisciplinary and international effort.”
He added: “As a landslide scientist, an additional
interesting aspect of this study is that this is the first-ever landslide and
tsunami observed from eastern Greenland, showing how climate change already has
major impacts there.”
The team estimated that 25 million cubic metres of
rock and ice crashed into the fjord (enough to fill 10,000 Olympic-sized
swimming pools).
They confirmed the size of the tsunami, one of the
largest seen in recent history, using numerical simulations as well as local
data and imagery.
Seventy kilometres away from the landslide,
four-metre-high tsunami waves damaged a research base at Ella Ø (island) and
destroyed cultural and archaeological heritage sites across the fjord system.
The fjord is on a route commonly used by tourist cruise
ships visiting the Greenland fjords. Fortunately, no cruise ships were close to
Dickson Fjord on the day of the landslide and tsunami, but if they had been,
the consequences of a tsunami wave of that magnitude could have been
devastating.
Mathematical models recreating the width and depth of
the fjord at very high resolution demonstrated how the distinct rhythm of a
mass of water moving back and forth matched the seismic signal.
The study concluded that with rapidly accelerating
climate change, it will become more important than ever to characterise and
monitor regions previously considered stable and provide early warning of these
massive landslide and tsunami events.
Co-author Thomas Forbriger, from Karlsruhe Institute
of Technology, said: “We wouldn’t have discovered or been able to analyse this
amazing event without networks of high-fidelity broadband seismic stations
around the world, which are the only sensors that can truly capture such a
unique signal.”
原始論文：Kristian
Svennevig, Stephen P. Hicks, Thomas Forbriger, Thomas Lecocq, Rudolf
Widmer-Schnidrig, Anne Mangeney, Clément Hibert, Niels J. Korsgaard, Antoine
Lucas, Claudio Satriano, Robert E. Anthony, Aurélien Mordret, Sven Schippkus,
Søren Rysgaard, Wieter Boone, Steven J. Gibbons, Kristen L. Cook, Sylfest
Glimsdal, Finn Løvholt, Koen Van Noten, Jelle D. Assink, Alexis Marboeuf,
Anthony Lomax, Kris Vanneste, Taka’aki Taira, Matteo Spagnolo, Raphael De
Plaen, Paula Koelemeijer, Carl Ebeling, Andrea Cannata, William D. Harcourt,
David G. Cornwell, Corentin Caudron, Piero Poli, Pascal Bernard, Eric Larose, Eleonore
Stutzmann, Peter H. Voss, Bjorn Lund, Flavio Cannavo, Manuel J. Castro-Díaz,
Esteban Chaves, Trine Dahl-Jensen, Nicolas De Pinho Dias, Aline Déprez, Roeland
Develter, Douglas Dreger, Läslo G. Evers, Enrique D. Fernández-Nieto, Ana M. G.
Ferreira, Gareth Funning, Alice-Agnes Gabriel, Marc Hendrickx, Alan L. Kafka,
Marie Keiding, Jeffrey Kerby, Shfaqat A. Khan, Andreas Kjær Dideriksen, Oliver
D. Lamb, Tine B. Larsen, Bradley Lipovsky, Ikha Magdalena, Jean-Philippe Malet,
Mikkel Myrup, Luis Rivera, Eugenio Ruiz-Castillo, Selina Wetter, Bastien
Wirtz. A rockslide-generated tsunami in a Greenland fjord rang Earth
for 9 days. Science, 2024; 385 (6714): 1196 DOI: 10.1126/science.adm9247
引用自：University College London.
"Climate-change-triggered 2023 mega-landslide caused Earth to vibrate for
nine days."
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