GeoSEA的監測網記錄到埃特納火山的東南翼正在滑落
火山山坡在水下滑動可能導致海嘯
埃特納火山的東南翼正緩緩滑落到海裡。德國亥姆霍茲基爾海洋研究中心以及基爾大學的卓越團隊「海洋未來」的科學家，利用以聲波為原理的大地監測網首度測出埃特納火山的山坡在水下移動的跡象：八天之內滑動了大約四公分。未來整個坡體如果突然之間快速滑落，可能會造成海嘯而對整個地區造成大災難。結果發表於國際期刊《科學進展》(Science Advances)。
2016年春季，GEOMAR和基爾大學的團隊在研究船「海神號(POSEIDON)」上，把GeoSEA的收發器安裝到埃特納火山的東翼。圖片來源：Felix Gross (CC by 4.0)
身為歐洲最活躍的火山，埃特納火山受到科學家和義大利政府的嚴密監控。衛星觀測顯示大部分的山坡目前大致都處於穩定狀態，除了東南翼正緩緩滑落到海裡。由於衛星觀測無法看到海面之下的活動，因此山坡在水下是否繼續滑動以及滑動程度目前都還一無所知。不過德國亥姆霍茲基爾海洋研究中心、基爾大學的海洋科學重點研究領域、義大利國立地球物理與火山學研究所的科學家，運用GeoSEA新開發的海底大地監測網(seafloor
geodetic monitoring network)首次偵測出火山山坡在水下的水平垂直運動。
發表於國際期刊《科學進展》的研究結果證實整個火山的東南翼都在滑動。最有可能造成坡體運動的作用力是重力，而非先前推測的岩漿上升。研究人員無法排除整座或者大部分山坡滑落的機率，這可能會產生大海嘯而帶給附近區域極為重大的災難。
研究主要作者Morelia Urlaub表示：「在埃特納火山我們設置了以聲波為原理的海底大地監測網，這是首次把海洋大地測量的技術用在火山上。」她領導的這項研究是「MAGOMET―以海洋大地測量法監測埃特納火山近岸活動(Marine geodesy
for offshore monitoring of Mount Etna)」計畫的一部份。2016年四月，GEOMAR團隊在一條斷層的兩側總共設置了5個聲波監測收發站，該斷層是滑動和穩定山坡的邊界。Urlaub博士說：「我們在斷層線滑動的那側設置三個收發站，推測為穩定的另一側設置二個。」
收發站在任務期間每90分鐘就會送出一次聲波訊號。由於研究人員知道聲波在水裡的傳輸速度，因此訊號在各個收發站之間的傳遞時間就能顯示出它們彼此之間在海床上的距離，誤差小於一公分。Urlaub博士說明：「我們注意到2017年5月斷層不同側收發站之間的距離有明顯變化。八天內山坡往海的方向滑動了四公分並下沉了一公分。」這種運動可以比擬成相當緩慢的地震―「慢滑移事件(slow slip
event)」，因此可說是首次在海底記錄到慢滑移事件的水平運動。這套系統傳出數據的時間總計達到約15個月。
研究人員比對衛星在同期間得到的地面變形數據，得出火山西南翼海面以上的部分移動的距離也差不多。Urlaub博士說：「因此整個火山西南翼都移動了。」
她接著表示：「整體上，我們的研究結果顯示坡體滑動的原因是重力而非岩漿上升。」如果山坡的變形是由火山中心的岩漿活動造成，則岸上的移動量應該會比水裡的還多。這項結論對於危害評估來說相當重要。共同作者，負責調校GeoSEA 監測網的Heidrun Kopp教授表示：「整面山坡正因重力作用而移動。因此未來很有可能會發生巨大的崩塌，在整個地中海地區引起海嘯。」不過，研究結果無法預測這類事件是否會發生，以及可能的發生時間。
Urlaub博士總結：「要瞭解埃特納火山以及其他近海火山附近的地質作用，需要更多基礎科學研究才行。我們的調查結果顯示聲波大地監測網在這方面具有莫大幫助。」
GeoSEA array records sliding of Mount Etna’s southeastern flank
Volcano flank moves under water – Tsunami is a
possible consequence
The southeast flank of Mount Etna slowly slides towards the sea. A team
of scientists from GEOMAR Helmholtz Centre for Ocean Research Kiel and the Kiel
cluster of excellence “The Future Ocean” showed for the first time movement of
Etna’s underwater flank using a new, sound-based geodetic monitoring network.
Within eight days, the flank slipped about four centimeters. A sudden and rapid
descent of the entire slope could lead to a tsunami with disastrous effects for
the entire region. The results have been published today in the international
journal Science Advances.
As Europe’s most
active volcano, Mount Etna is intensively monitored by scientists and Italian
authorities. Satellite-based measurements have shown that the southeastern
flank of the volcano is slowly sliding towards the sea, while the other slopes
are largely stable. To date, it has been entirely unknown if and how movement
continues under water, as satellite-based measurements are impossible below the
ocean surface. With the new GeoSEA seafloor geodetic monitoring network,
scientists from the GEOMAR Helmholtz Centre for Ocean Research Kiel, the Kiel
University, priority research area Kiel Marine Science, and the Istituto
Nazionale di Geofisica e Vulcanologia (INGV) have now been able to detect for
the first time the horizontal and vertical movement of a submerged volcanic
flank.
The results
confirm that the entire southeastern flank is in motion. The driving force of
flank movement is most likely gravity, and not the ascent of magma, as
previously assumed. Catastrophic collapse involving the entire flank or large
parts of it cannot be excluded and would trigger a major tsunami with extreme
effects in the region. The results of the study have been published today in
the international journal Science
Advances.
"At Mount
Etna we used a sound based underwater geodetic monitoring network, the
so-called marine geodesy, on a volcano for the first time ", says Dr.
Morelia Urlaub, lead author of the study. She led the investigations as part of
the "MAGOMET - Marine geodesy for offshore monitoring of Mount Etna"
project. In April 2016, the GEOMAR team placed a total of five acoustic
monitoring transponder stations across the fault line that represents the
boundary between the sliding flank and the stable slope. "We placed three
on the sliding sector and two on the presumably stable side of the fault
line," says Dr. Urlaub.
During their
mission each transponder was sending an acoustic signal every 90 minutes. Since
the speed of sound in water is known, the travel time of the signals between
transponders gave information on the distances between transponders on the
seafloor with a precision of less than one centimeter. "We noticed that in
May 2017 the distances between transponders on different sides of the fault
clearly changed. The flank slipped by four centimeters seawards and subsided by
one centimeter within a period of eight days," explains Dr. Urlaub. This
movement can be compared to a very slow earthquake, a so-called "slow slip
event". It was the first time that the horizontal movement of such a slow
slip event was recorded under water. In total, the system delivered data for
about 15 months.
A comparison
with ground deformation data obtained by satellite showed that the southeastern
flank above sea level moved by a similar distance during the same observation
period. "So the entire southeast flank changed its position," says
Dr. Urlaub.
"Overall,
our results indicate that the slope is sliding due to gravity and not due to
the rise of magma," she continues. If magma dynamics in the centre of the
volcano triggered flank deformation, displacement of the flank would be
expected to be larger onshore than below water. This is crucial for hazard
assessments. "The entire slope is in motion due to gravity. It is
therefore quite possible that it could collapse catastrophically, which could trigger
a tsunami in the entire Mediterranean," explains Professor Heidrun Kopp,
coordinator of the GeoSEA array and co-author of the study. However, the
results of the study do not allow a prediction whether and when such an event
might occur.
"Further
basic research is needed to understand the geological processes at and around
Etna and other coastal volcanoes. Our investigation shows that the sound-based
geodetic monitoring network can be a tremendous help in this respect,"
summarises Dr. Urlaub.
原始論文：Morelia Urlaub, Florian Petersen, Felix Gross, Alessandro
Bonforte, Giuseppe Puglisi, Francesco Guglielmino, Sebastian Krastel, Dietrich
Lange, Heidrun Kopp. Gravitational
collapse of Mount Etna’s southeastern flank. Science Advances,
2018; 4 (10): eaat9700 DOI: 10.1126/sciadv.aat9700
引用自：Helmholtz Centre for Ocean Research Kiel
(GEOMAR). "GeoSEA array records sliding of Mount Etna's southeastern
flank."
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