尼泊爾下一場地震的破裂會更加全面

原文網址：https://www.ethz.ch/en/news-and-events/eth-news/news/2019/01/waiting-for-the-complete-rupture.html

尼泊爾下一場地震的破裂會更加全面

Peter Rüegg

2015年尼泊爾遭受一場規模7.8的地震襲擊，但這個國家可能還得面對另一場更加強烈的地震威脅。瑞士聯邦理工學院(ETH)的研究人員運用新的電腦模型，模擬了歐亞板塊和印度板塊之間的地震破裂過程背後的物理機制，從而得到上述結論。

在地震強力威脅之下的日常生活――加德滿都菜市場中的老百姓。圖片來源：Colourbox

2015年4月，一場強震襲擊了尼泊爾，尤以首都加德滿都附近最為嚴重。這場規模7.8的地震摧毀了許多村莊、交通設施以及古蹟，總計罹難人數約有9000人。

更糟的是，蘇黎世聯邦理工學院的地球科學家團隊，運用新的電腦模型來模擬喜馬拉雅山脈附近，歐亞板塊跟印度板塊間的碰撞帶，結果指出尼泊爾可能還得面對另一場強烈許多、規模8以上的地震威脅。ETH地球物理研究所Taras Gerya教授的博士生Luca Dal Zilio，跟ETH的研究團隊合作，透過新的模型首度把該破裂帶周期性發生的地震以高解析度模擬出來，並呈現在一條剖面上。

這篇研究最近發表於期刊《自然通訊》(Nature Communications)，主要作者Dal Zilio解釋：「2015年的地震發生時，喜馬拉雅山分隔兩個大陸板塊的主要斷層只有一部份破裂而已。在印度板塊隱沒至歐亞板塊下方的破裂帶中，前緣近地表的部分並沒有滑動，仍在累積應力。」

一般來說，大型地震時兩個板塊之間的錯動，幾乎可以完全釋放掉震源附近累積的應力。不過Dal Zilio說：「我們的模型顯示廓爾喀地震雖然降低了部份破裂帶的應力，但喜馬拉雅山腳附近的破裂帶前緣，所受的張力實際上是增加的。這種看似矛盾的現象，使廓爾喀地震這類的『中型』地震能夠創造出有利於大地震發生的條件。」

引發廓爾喀地震的斷層系統有100多公里，但其震動幅度只能把斷層較深部的應力釋放出來。破裂帶近地表的部分在這之後會繼續累積新的應力、甚至比之前更多。

根據Dal Zilio和同事進行的模擬結果，還需要兩到三個廓爾喀地震才能累積夠多應力，使規模8.1以上的地震發生。如此劇烈的地震會讓破裂帶從深到淺都發生破壞並直達地表，沿著喜馬拉雅山脈延伸數百公里長。在長度總計約2000公里的斷層系統上，發生地震的這個區段所累積的應力，最後會完全釋放出來。

歷史紀錄顯示過去曾發生這類超大型事件。比方說，1950年的阿薩姆地震的規模有8.6，破裂帶的長度有數百公里並涵蓋各個深度範圍。1505年發生的大地震強度足以在喜馬拉雅斷層上造成約800公里長的破裂。

ETH地震學和地球動力學的教授Edi Kissling表示：「新的模型顯示喜馬拉雅山脈發生的強烈地震不只一種，至少有兩種；而且它們的週期有部分重疊。」超級地震的發生週期可能為400至600年，而廓爾喀地震這類的「中型」地震則數百年發生一次。由於它們的週期彼此重疊，因此研究人員認為強烈而危及人類的地震會以不規律的時間間隔發生。

不過研究人員還是無法預測下一場超級大地震會在什麼時候來襲。Kissling說：「沒有人可以預測地震，就算有新的模型也一樣。不過，我們可以更加了解在特定地區地震造成的損害情形，進而做出預防措施。」

研究人員用的高解析度二維模型納入了前人在廓爾喀地震後發表的研究結果，並利用蘇黎世聯邦理工學院的尤拉主機電腦來進行模擬。Dal Zilio說：「三維模型的結果會更加精確，也能讓我們解析喜馬拉雅山東側跟西側的差異。但是，模擬全長2000多公里的破裂帶需要的龐大運算能力，就連CSCS(瑞士國家超級電腦中心)的超級電腦都無法達成。」

尼泊爾位在兩個大陸板塊――印度板塊和歐亞板塊交會之處。在此印度板塊隱沒到歐亞板塊下方的地函當中。由於印度板塊隱沒到地函時會產生吸力，使得印度次大陸每年往北移動約4公分。

隱沒作用使得兩個板塊在長達2000公里的斷層系統中互相摩擦，累積十分可觀的應力。當這些應力瞬間釋放出來，使兩個板塊迅速錯動時就造成了地震。此為尼泊爾和喜馬拉雅南側的山麓地帶，一再受到極為強烈的地震襲擊的原因。

Waiting for the complete rupture

Nepal was struck by an earthquake with a magnitude of 7.8 in 2015, but the country may still face the threat of much stronger temblor. This is the conclusion reached by ETH researchers based on a new model that simulates physical processes of earthquake rupture between the Eurasian and Indian Plates.

In April 2015, Nepal – and especially the region around the capital city, Kathmandu – was struck by a powerful tremor. An earthquake with a magnitude of 7.8 destroyed entire villages, traffic routes and cultural monuments, with a death toll of some 9,000.

However, the country may still face the threat of much stronger earthquakes with a magnitude of 8 or more. This is the conclusion reached by a group of earth scientists from ETH Zurich based on a new model of the collision zone between the Indian and Eurasian Plates in the vicinity of the Himalayas. Using this model, the team of ETH researchers working with doctoral student Luca Dal Zilio, from the group led by Professor Taras Gerya at the Institute of Geophysics, has now performed the first high-resolution simulations of earthquake cycles in a cross-section of the rupture zone.

“In the 2015 quake, there was only a partial rupture of the major Himalayan fault separating the two continental plates. The frontal, near-surface section of the rupture zone, where the Indian Plate subducts beneath the Eurasian Plate, did not slip and remains under stress,” explains Dal Zilio, lead author of the study, which was recently published in the journal Nature Communications.

Normally, a major earthquake releases almost all the stress that has built up in the vicinity of the focus as a result of displacement of the plates. “Our model shows that, although the Gorkha earthquake reduced the stress level in part of the rupture zone, tension actually increased in the frontal section close to the foot of the Himalayas. The apparent paradox is that ‘medium-sized’ earthquakes such as Gorkha can create the conditions for an even larger earthquake,” says Dal Zilio.

Tremors of the magnitude of the Gorkha earthquake release stress only in the deeper subsections of the fault system over lengths of 100 kilometres. In turn, new and even greater stress builds up in the near-surface sections of the rupture zone.

According to the simulations performed by Dal Zilio and his colleagues, two or three further Gorkha quakes would be needed to build up sufficient stress for an earthquake with a magnitude of 8.1 or more. In a quake of this kind, the rupture zone breaks over the entire depth range, extending up to the Earth’s surface and laterally — along the Himalayan arc — for hundreds of kilometres. This ultimately leads to a complete stress release in this segment of the fault system, which extends to some 2,000 kilometres in total.

Historical data shows that mega events of this kind have also occurred in the past. For example, the Assam earthquake in 1950 had a magnitude of 8.6, with the rupture zone breaking over a length of several hundred kilometres and across the entire depth range. In 1505, a giant earthquake struck with sufficient power to produce an approximately 800-kilometre rupture on the major Himalayan fault.

“The new model reveals that powerful earthquakes in the Himalayas have not just one form but at least two, and that their cycles partially overlap,” says Edi Kissling, Professor of Seismology and Geodynamics. Super earthquakes might occur with a periodicity of 400 to 600 years, whereas “medium-sized” quakes such as Gorkha have a recurrence time of up to a few hundred years. As the cycles overlap, the researchers expect powerful and dangerous earthquakes to occur at irregular intervals.

However, they cannot predict when another extremely large quake will next take place. “No one can predict earthquakes, not even with the new model. However, we can improve our understanding of the seismic hazard in a specific area and take appropriate precautions,” says Kissling.

The two-dimensional and high-resolution model also includes some research findings that were published after the Gorkha earthquake. To generate the simulations, the researchers used the Euler mainframe computer at ETH Zurich. “A three-dimensional model would be more accurate and would also allow us to make statements about the western and eastern fringes of the Himalayas. However, modelling the entire 2,000 kilometres of the rupture zone would require enormous computational power that not even the supercomputers at the CSCS can provide,” says Dal Zilio.

Nepal lies at the point where two continents meet: India and Eurasia. It is here that the Indian Plate subducts into the mantle beneath the Eurasian plate. Due to the suction effect exerted by the Indian Plate as it sinks into the mantle, the Indian subcontinent moves north by up to 4 centimetres a year.

As a result, the plates rub against each other over the length of this 2,000-kilometre fault system, allowing considerable amounts of stress to build up. During an earthquake, the sudden release of this stress causes an abrupt displacement of the plates next to each other. This is why Nepal and the southern foothills of the Himalayas repeatedly experience very powerful earthquakes.

原始論文：Luca Dal Zilio, Ylona van Dinther, Taras Gerya, Jean-Philippe Avouac. Bimodal seismicity in the Himalaya controlled by fault friction and geometry. Nature Communications, 2019; 10 (1) DOI: 10.1038/s41467-018-07874-8

引用自：ETH Zurich. "Waiting for the complete rupture."