颱風可以改變地震發生的模式
By Josef Zens
地球的地殼一直受到應力作用。偶爾，這些應力會在大地震時釋放出來。地震發生的原因通常來自於地球表面板塊的緩慢移動。然而，還有另外一項迄今少有關注的影響因素：強烈的侵蝕作用可以暫時讓一個區域的地震活動出現明顯變化。最近德國地質科學研究中心(GFZ)的研究人員和國際同仁合作之下，證實了台灣曾經受此因素的影響。這項結果發表於期刊《科學報告》(Scientific Reports)。
颱風造成的土地侵蝕移動了許多塊體並堆積至地勢較低的地方，使得地震活動增加。圖片來源：Philippe Steer, University of Rennes
由於位在菲律賓海板塊撞上亞洲大陸邊緣的地方，這座西太平洋的島嶼可說是全世界構造運動最活躍的地區之一。十一年前莫拉克颱風掃過了台灣沿岸——這場熱帶氣旋被認為是台灣有歷史紀錄以來最嚴重的其中一次。
在2009年8月的短短三天之內，莫拉克颱風降下了每平方公尺三千公升的雨量。相較而言，柏林和布蘭登堡平均一年接收到的雨量大約為每平方公尺550公升。大量的降水造成了災情慘重的水災以及遍布各地的山崩。莫拉克颱風造成了超過600人死亡，直接經濟損失則相當於大約30億歐元*。
*《莫拉克颱風之災情勘查與分析》(2011)估計經濟損失達到904.7億台幣。
由法國雷恩大學的Philippe
Steer領導的國際團隊，統計了此次侵蝕事件之後的地震數量。他們得出莫拉克颱風過後的二年半之間，相較於事件之前小規模地震與淺層地震的數目有顯著提升，而且這些變化只發生在遭到嚴重侵蝕的地區。論文資深作者，GFZ的研究人員Niels
Hovius表示：「我們對地震活動的變化解釋是地表經歷嚴重侵蝕之後，深度小於15公里的淺層地殼受到的應力也跟著增加了。」為數眾多的山崩移除了大量土石，接著河流把這些物質帶離了受到颱風破壞的地區。Hovius解釋：「土石被逐漸移除之後改變了地殼淺部的應力狀態，程度大到足以讓逆衝斷層發生更多地震。」
在台灣可以看見的這類活躍山脈的特色為地下分佈著「逆衝斷層」，這種斷層一側的岩石會往上滑至另外一側的岩石之上，而當應力過大時岩石就會破裂。板塊移動與互相卡住時持續累積的壓力通常是造成斷層滑動的原因，隨之產生的地震經常會造成山崩並大幅加劇侵蝕作用。不過GFZ的研究人員與合作同仁首度證明了反過來也是可行的：劇烈的侵蝕作用能影響地震活動——即使從地質角度來看只是一瞬間。Niels
Hovius表示：「地表作用和構造運動只在轉瞬之間有所關聯。」這位研究人員繼續說道：「地震是最危險且破壞力最強的天災。若要更加實際地評估地震產生的災害，深入了解構造運動和外部作用如何引發地震是相當重要的，對於人口密集的地區來說更是如此。」
Typhoon changed earthquake patterns
The Earth’s crust is under constant
stress. Every now and then this stress is discharged in heavy earthquakes,
mostly caused by the slow movement of Earth's crustal plates. There is,
however, another influencing factor that has received little attention so far:
intensive erosion can temporarily change the earthquake activity (seismicity)
of a region significantly. This has now been shown for Taiwan by researchers
from the GFZ German Research Centre for Geosciences in cooperation with
international colleagues. They report on this in the journal Scientific Reports.
The island in the western Pacific Ocean is anyway one
of the most tectonically active regions in the world, as the Philippine Sea
Plate collides with the edge of the Asian continent. Eleven years ago, Typhoon
Morakot reached the coast of Taiwan. This tropical cyclone is considered the
one of the worst in Taiwan's recorded history.
Within only three days in August 2009, three thousand
litres of rain fell per square metre. As a comparison, Berlin and Brandenburg
receive an average of around 550 liters per square meter in one year. The water
masses caused catastrophic flooding and widespread landsliding. More than 600
people died and the immediate economic damage amounted to the equivalent of
around three billion euros.
The international team led by Philippe Steer of the
University of Rennes, France, evaluated the earthquakes following this erosion
event statistically. They showed that there were significantly more
small-magnitude and shallow earthquakes during the two and a half years after
typhoon Morakot than before, and that this change occurred only in the area
showing extensive erosion. GFZ researcher and senior author Niels Hovius says:
"We explain this change in seismicity by an increase in crustal stresses
at shallow depth, less than 15 kilometres, in conjunction with surface
erosion". The numerous landslides have moved enormous loads, rivers
transported the material from the devastated regions. "The progressive
removal of these loads changes the state of the stress in the upper part of the
Earth's crust to such an extent that there are more earthquakes on thrust
faults," explains Hovius.
So-called active mountain ranges, such as those found
in Taiwan, are characterized by "thrust faults" in the underground,
where one unit of rocks moves up and over another unit. The rock breaks when
the stress becomes too great. Usually it is the continuous pressure of the
moving and interlocking crustal plates that causes faults to move. The
resulting earthquakes in turn often cause landslides and massively increased
erosion. The work of the GFZ researchers and their colleagues now shows for the
first time that the reverse is also possible: massive erosion influences
seismicity - and does so in a geological instant. Niels Hovius: "Surface
processes and tectonics are connected in the blink of an eye.” The researcher
continues: "Earthquakes are among the most dangerous and destructive
natural hazards. Better understanding earthquake triggering by tectonics and by
external processes is crucial for a more realistic assessment of earthquake hazards,
especially in densely populated regions.”
原始論文：Philippe
Steer, Louise Jeandet, Nadaya Cubas, Odin Marc, Patrick Meunier, Martine
Simoes, Rodolphe Cattin, J. Bruce H. Shyu, Maxime Mouyen, Wen-Tzong Liang,
Thomas Theunissen, Shou-Hao Chiang, Niels Hovius. Earthquake statistics
changed by typhoon-driven erosion. Scientific Reports, 2020; 10
(1) DOI: 10.1038/s41598-020-67865-y
引用自：GFZ GeoForschungsZentrum Potsdam, Helmholtz
Centre. "Typhoon changed earthquake patterns”
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