地震在海底邊坡造成的矛盾效應
活動大陸邊緣是海洋板塊滑入大陸板塊下方的場所。此處可能會發生地震以及海嘯，並把陸緣斜坡上的沉積物運送到深海海溝。最近地質學家在2011年日本東北大地震附近的一座海底邊坡上，發現地震造成坡面的沉積物受到侵蝕的證據。
2016年，研究船太陽號(RV
Sonne)上的科學家和船員正在等待新的沉積物岩芯送上來。圖片來源：T.
Schwestermann
過往多數研究推測地震只會透過海底山崩之類的作用來搬運沉積物，也就是讓厚達數公尺的大量沉積物一起滑入深海。但最近發現了另外一種邊坡表面再活動的機制，其中牽涉到大範圍地區僅有表面一層薄薄的沉積物剝落下來。乍看之下消失幾公分的沉積物或許並不重要，然而，因為它的影響範圍十分廣大，對某些研究來說具有相當深遠的意涵，像是有歷史紀錄之前發生的地震、有機碳沉入深海的過程，甚至是海底山崩可能造成的海嘯，這些研究都需要用到地震讓海底沉積物再活動的機制。奧地利因斯布魯克大學地質學研究所的助理教授Jasper
Moernaut解釋：「邊坡表面再活動的機制，其理論基礎來自於研究海盆裡的沉積物。但想要真正了解這個重要的過程，就需要調查實際發生的地點：海底邊坡。」
留心間隙
研究人員從日本外海的一座斜坡上採集了沉積物，結合物理化學分析方法，他們發現其中有幾層數公分厚的沉積物不見了。隨後進行的定年結果，顯示這些沉積物缺失的原因可能是地震造成的晃動。「我們在僅有15公分的沉積物岩芯中發現了不只一道缺失，而是三道時，真的感到十分訝異。」因斯布魯克大學地質學研究所的博士生Ariana
Molenaar表示，「我們定年這三道缺失之後，發現它們跟附近區域三起最強、規模超過八的地震有關，代表這種作用是有系統地重複發生。」
從來沒有人用此方法來探討深海斜坡――侵蝕作用進行的斜坡是人們絕對不會想到要去採集沉積物岩芯的地點。因斯布魯克大學地質學研究所的教授Michael
Strasser表示：「我們研究的先驅之處在於，首度以海底邊坡為對象來研究此作用，結果顯示我們的研究方法確實具有潛力。」研究人員最近開始把他們的策略運用在不同的環境――甚至是湖泊當中，希望可以更加瞭解此種新發現的作用。
在海底邊坡造成的矛盾效應
地震除了可以移除海底邊坡最上面幾公分的沉積物，造成的晃動還會引發另外一種截然不同的效應：留下來的沉積物實際上會變得更加穩固。這種稱為「地震強化」(seismic strengthening)的作用是因為強烈的晃動會壓縮沉積物。Jasper
Moernaut表示：「此作用使得海底邊坡變得十分穩定，海底山崩的數量因而大為減少。」這帶來了一個好消息：雖然活動大量邊緣時常發生強烈地震，但是由海底山崩造成的海嘯卻較為稀少。
Contradictory effect of
earthquakes on submarine slopes
Active margins, where an oceanic plate
slides under a continental plate, may cause earthquakes and tsunamis. Further,
they are known for shifting sediments from margin slopes into deep ocean
trenches. Geologists now found evidence of earthquake-triggered surface
sediment erosion on a submarine slope close to the area of the 2011 Tohoku-oki
earthquake.
Whereas most previous research assumed that sediment
transport by earthquakes only happened by sliding of sediment packages (i.e.
submarine landslides), that are several meters thick, the recently-discovered
process of surficial remobilization involves the stripping of only a thin
veneer of sediment over an extensive area. At first view a few missing
centimeters of sediment do not look very spectacular. However, the fact that it
affects a vast area has tremendous implications for all studies based on the
remobilization of marine sediment by earthquakes, such as research on
pre-historical earthquakes, deposition of organic carbon into the deep ocean
and even the potential tsunami hazard by submarine landslides. “Surficial
remobilization was hypothesized based on studies of basin deposits. However, to
really understand this important process it is crucial to investigate the place
where it takes place: the submarine slopes,” explains Jasper Moernaut,
Assistant Professor at the Institute of Geology, University of Innsbruck.
Mind the gap
The researchers combined chemical and physical
analyses to detect small centimeter scale gaps in the sediment taken from a
slope offshore Japan. Subsequent dating then revealed the potential of the gaps
being caused by seismic shaking. “We were quite amazed when we found that not
only one, but three gaps were present in this small 15 cm section of sediment
core,” says Ariana Molenaar, PhD student at the Institute of Geology. “When we
dated these three gaps we found that they link to the three strongest regional
earthquakes with a magnitude larger than eight, indicating that this is a
systematically repeating process.”
No one before has examined deep sea slopes with this
method. A slope site where erosion takes place is surely the last place one
would take a sediment core. “Our pilot study is the first to target a submarine
slope to investigate this process, showing the potential of this method,” says
Michael Strasser, Professor at the Institute of Geology. The research team is
now applying their strategy in different settings – even in lakes − to further
advance their understanding of this newly-discovered process.
Contrasting
effect on submarine slopes
Besides the shedding of the uppermost few
centimeters, earthquake shaking has another very contrasting effect on the
submarine slope: the sediments that remain actually get stronger. This process,
called “seismic strengthening”, occurs due to the compaction of sediments by
violent shaking. “In the ocean, this leads to very stable slope sequences and
thus a remarkable absence of submarine landslides,” says Jasper Moernaut. So
the good news is that − despite the frequent occurrence of strong earthquakes
at active ocean margins − tsunamis triggered by submarine landslides are
relatively uncommon in these regions.
原始論文：Ariana
Molenaar, Jasper Moernaut, Gauvain Wiemer, Nathalie Dubois, Michael Strasser. Earthquake Impact on Active Margins:
Tracing Surficial Remobilization and Seismic Strengthening in a Slope
Sedimentary Sequence. Geophysical
Research Letters, 2019; 46 DOI: 10.1029/2019GL082350
引用自：University
of Innsbruck. “Contradictory effect of earthquakes on submarine slopes.”
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