研究日本的慢地震或許能闡釋海嘯的產生機制

原文網址：www.sciencedaily.com/releases/2017/06/170615142730.htm

研究日本的慢地震或許能闡釋海嘯的產生機制

一組國際研究團隊利用架設在日本東部外海海床及鑽井中的儀器來蒐集數據，據他們所言，瞭解隱沒帶由緩慢滑移所產生的地震，或許有助於研究人員瞭解大地震和海嘯的形成機制。

賓州大學地質科學系的教授Demian Saffer表示：「此區域位於海溝附近，是這道板塊邊界系統中最淺的部分。如果在此發生的滑移產生了地震，就有機會形成大型海嘯。」

太平洋板塊和歐亞板塊在此隱沒帶聚合，其中太平洋板塊往歐亞板塊下方滑動。這類型的地震帶構成了環繞太平洋的「火環」，會有此名稱是因為下潛的隱沒板塊一旦到達適當深度，就會融化並在地表形成火山。喀斯喀特山脈的聖海倫火山是這些火山中的其中一座，同樣的還有位於東京西南方約100公里的富士山。另外，隱沒帶也常常跟大地震的形成有所關連。

研究人員將研究重點放在慢地震上，這類滑移事件的發生期間長達數天或者數週不等。最近由其他團隊進行的研究指出，對於板塊邊界發生的地震和斷層整體的滑移模式來說，慢地震都是相當重要的一部份，另外它們也可以解釋斷層或是隱沒帶中累積的能量，一部份的去向為何。

James Allan表示：「這些珍貴的結果對於瞭解海嘯產生的風險來說相當重要。」他是國家科學基金會海洋科學部門的計畫主任，此單位提供資金給國際海洋發現計劃(IODP)。「就像2004年發生在東南亞的一樣，此等巨浪足以影響數萬人的生命安全並可能造成數十億美元的經濟損失。這項研究強調出以科學鑽探為基礎的調查成果，以及長期蒐集海洋和地質資料是相當重要的工作。」

2009年和2010年，IODP(綜合海洋鑽探計畫，現為國際海洋發現計畫)的NanTroSEIZE子計畫在日本本州西南外海的南海海槽鑽了兩孔鑽井；2010年，研究人員在佈署觀測網時，除了在海床上安裝感應器，也在鑽井內部設置了監測儀器。Saffer和共同作者，日本海洋科技機構的主任技術研究員荒木英一郎，將他們的成果刊登於6月16日的《科學》(Science)。兩孔鑽井相距10.6公里，橫跨此區域最近一次大地震時，滑動發生位置的淺層邊界。這場規模為8.1的大地震發生於1944年，隨之產生的海嘯高達8公尺並襲擊了東京。

Saffer表示：「斷層帶淺處的能量究竟有多少百分比是由慢地震宣洩出來？百分之百或者根本為零？在我們取得這些數據之前，沒有人知道答案。我們發現大致上有百分之50的能量是在慢地震時釋放出來。另外百分之50則可能造成上方板塊的永久壓縮變形，或者是儲存下來而成為100年或者150年之後下一個地震的能量來源。我們不知道哪種情形為真，但對於海嘯災害來說，這已經造成了巨大的差別。藉由週期性的釋放應力，慢地震可以減輕產生海嘯的風險，但它的實際作用可能更加複雜，不只是作為一個避震器而已。」

在日本東部地區會重複發生規模8地震的板塊交界，研究人員發現靠海的那一側有發生一系列的緩慢滑移事件。它們的持續時間長達數天到數個禮拜，有些是受到區域內其他無關的地震誘發，有些則是同時發生。根據研究人員所述，慢地震群集每12到18個月就會發生一次。

Saffer表示：「這些慢地震發生的區域並未被壓縮，所以我們過去才會認為這些位處海溝附近的淺層區域有類似避震器的作用，可以阻止深處發生的地震到達地表。但是我們現在發現這些區域會以數天到數週的時間發生規模5到6的慢地震。」

這類地震一般來說很少被注意到，因為它們進行的速度相當緩慢且發生在遠洋地區。

研究人員也注意到跟隱沒帶沒有直接關聯，發生在一段距離以外的地震也可以誘發慢地震，因此研究人員認為研究區域比過往認為得還要敏感。誘發慢地震的原因是震動，而非此區域受到的直接應變。

Saffer表示：「現在的問題是當慢地震發生時它們是否會釋放出應力。要留心的是不能認為結論僅有緩慢發生的地震事件會減輕災害，因為我們的結果同樣也顯示隱沒帶的外圍區域會累積應變。此外，慢地震的作用只有減輕真正會發生大地震的深部所承受的應力嗎？我們也不知道。」

###

參與此計畫的還有德國布萊梅大學海洋環境科學中心的Achim J Kopf、紐西蘭地質與核子科學研究所暨美國德克薩斯大學地球物理研究所的Laura M. Wallace、日本神奈川縣海洋科技機構的木村俊則和町田祐彌、日本東京大學地球與行星科學系的井出哲、加拿大地質調查所太平洋地質科學中心的Earl Davis，以及所有IODP 365航次上登船的科學家。

本研究由美國國家科學基金會和IODP資助。

Japanese slow earthquakes could shed light on tsunami generation

Understanding slow-slip earthquakes in subduction zone areas may help researchers understand large earthquakes and the creation of tsunamis, according to an international team of researchers that used data from instruments placed on the seafloor and in boreholes east of the Japanese coast.

"This area is the shallowest part of the plate boundary system," said Demian Saffer, professor of geosciences, Penn State. "If this region near the ocean trench slips in an earthquake, it has the potential to generate a large tsunami."

Two tectonic plates meet here, the Pacific Plate and the Eurasian Plate, in a subduction zone where the Pacific plate slides beneath the Eurasian plate. This type of earthquake zone forms the "ring of fire" that surrounds the Pacific Ocean, because once the end of the plate that is subducting -- sliding underneath -- reaches the proper depth, it triggers melting and forms volcanoes. Mt. St. Helens in the American Cascade Mountains is one of these volcanoes, as is Mt. Fuji, about 60 miles southwest of Tokyo. Subduction zones are often also associated with large earthquakes.

The researchers focused their study on slow earthquakes, slip events that happen over days or weeks. Recent research by other groups has shown that these slow earthquakes are an important part of the overall patterns of fault slip and earthquake occurrence at the tectonic plate boundaries and can explain where some of the energy built up on a fault or in a subduction zone goes.

"These valuable results are important for understanding the risk of a tsunami," says James Allan, program director in the National Science Foundation's Division of Ocean Sciences, which supports IODP. "Such tidal waves can affect the lives of hundreds of thousands of people and result in billions of dollars in damages, as happened in Southeast Asia in 2004. The research underscores the importance of scientific drillship-based studies, and of collecting oceanographic and geologic data over long periods of time."

In 2009 and 2010, the IODP (Integrated Ocean Drilling Program, now the International Ocean Discovery Program) NanTroSEIZE project drilled two boreholes in the Nankai Trough offshore southwest of Honshu, Japan, and in 2010 researchers installed monitoring instruments in the holes that are part of a network including sensors on the seafloor. Saffer and Eiichiro Araki, senior research scientist, Japan Agency for Marine-Earth Science and Technology, co-lead authors, published their results today (June 16) in Science. The two boreholes were 6.6 miles apart, straddling the shallow boundary of slip in the last major earthquake in this area, which occurred in 1944 and measured magnitude 8.1. The accompanying tsunami that hit Tokyo was 26 feet in height.

"Until we had these data, no one knew if zero percent or a hundred percent of the energy in the shallow subduction zone was dissipated by slow earthquakes," said Saffer. "We have found that somewhere around 50 percent of the energy is released in slow earthquakes. The other 50 percent could be taken up in permanent shortening of the upper plate or be stored for the next 100- or 150-year earthquake. We still don't know which is the case, but it makes a big difference for tsunami hazards. The slow slip could reduce tsunami risk by periodically relieving stress, but it is probably more complicated than just acting as a shock absorber."

The researchers found a series of slow slip events on the plate interface seaward of recurring magnitude 8 earthquake areas east of Japan. These slow earthquakes lasted days to weeks, some triggered by other unconnected earthquakes in the area and some happening spontaneously. According to the researchers, this family of slow earthquakes occurred every 12 to 18 months.

"The area where these slow earthquakes take place is uncompacted, which is why it has been thought that these shallow areas near the trench act like a shock absorber, stopping deeper earthquakes from reaching the surface," said Saffer. "Instead we have discovered slow earthquakes of magnitude 5 or 6 in the region that last from days to weeks."

These earthquakes typically go unnoticed because they are so slow and very far offshore.

The researchers also note that because earthquakes that occur at a distance from this subduction zone, without any direct connection, can trigger the slow earthquakes, the area is much more sensitive than previously thought. The slow earthquakes are triggered by the shaking, not by any direct strain on the area.

"The question now is whether it releases stress when these slow earthquakes occur," said Saffer. "Some caution is required in simply concluding that the slow events reduce hazard, because our results also show the outer part of the subduction area can store strain. Furthermore, are the slow earthquakes doing anything to load deeper parts of the area that do cause big earthquakes? We don't know."

###

Also part of this project were Achim J Kopf, MARUM-Center for Marine Environmental Sciences; Laura M. Wallace, GNS Sciences, New Zealand and University of Texas Institute of Geophysics; Toshinori Kimura and Yuya Machida, Japan Agency for Marine-Earth Science and Technology, Kanagawa, Japan; Satoshi Ide, Department of Earth and Planetary Science, University of Tokyo; Earl Davis, Pacific Geoscience Centre, Geological Survey of Canada; and IODP Expedition 365 shipboard scientists.

The National Science Foundation and the IODP funded this work.

原始論文：Eiichiro Araki et al. Recurring and triggered slow-slip events near the trench at the Nankai Trough subduction megathrust. Science, 2017 DOI: 10.1126/science.aan3120

引用自：Penn State. "Japanese slow earthquakes could shed light on tsunami generation." ScienceDaily. ScienceDaily, 15 June 2017.