地震有徵兆嗎？

原文網址：http://www.mccormick.northwestern.edu/news/articles/2017/10/do-earthquakes-have-a-tell.html

地震有徵兆嗎？

數據科學家和地震學家利用「深層微地動」來預報大地震

研究人員長久以來有很好的理由相信地震本質上就是不可預測的，但西北大學研究人員做出的新發現卻可能大幅撼動此舊思維。

「慢地震」釋放能量的時間長達數小時至數個月。一組跨領域研究團隊發現慢地震或許會引發鄰近地區發生「一般地震」。這項發現可以幫助地震學家在一定時間以前預測出某些大地震即將發生，讓他們能發出預警並做好防備來減少生命財產的損害。

「雖然地殼中應力累積的多寡大部分來說是可以預測的，但透過一般地震來釋放應力的過程性質卻較為渾沌，造成預測它們可能發生的時間相當困難。」西北大學複雜系統研究所的數據科學學者Kevin Chao表示。「但近幾年越來越多的研究發現隱沒帶的大地震發生之前常常有前震或是慢地震出現。」

這項由國家科學基金會補助的研究刊登於《地球物理研究期刊：固體地球》。論文第一作者Chao同時也是西北大學優化與統計學習中心的成員。此外，西北大學文理學院的地球與行星科學教授Suzan van der Lee也參與了這項研究。

Chao和他的同事於數年前將目光轉往台灣其中一處而開始這項研究。台灣擁有大約100座經年累月持續記錄地面移動的地震站。研究團隊在台灣注意到深層微地動的訊號，這是一種以數天至數週為週期重複發生的慢地震。

Chao表示：「深層微地動對微小的應力變化十分敏感。因此我們決定把它當作應力儀，來監測大地震發生之前與之後當地應力的累積與釋放有何變化。」

為了感應並監測深層微地動的活動，Chao的團隊發展出一套精密的數學方法來處理從台灣10座地震站得到的數據。2010年3月台灣南部發生了一場規模6.4的地震，他們發現大約二個月之前深層微地動的行為開始出現變化。舉例來說，在地震發生之前微地動的持續時間增加了二倍，且在地震之後仍持續增加。

雖然科學家首度於2002年發表深層微地動的存在，卻還沒有發現太多在大地震之前行為有所改變的案例。「在規模6.4的地震發生之後，我們注意到事件前後的深層微地動有值得研究的潛力。」Chao表示，「我們辨識出在地震前三週微地動的持續時間有所增加，但起初我們還無法下定結論，因為微地動的發生頻率隨時都有可能受到不同因素而增長。」

但在規模6.4的地震發生三年之後，Chao和同事注意到他們對微地動的觀測結果跟附近GPS站的紀錄有吻合之處，其指出微地動來源附近的地表移動方向有所轉變。

結合大地觀測(像是GPS和地震站)得到的數據以及一系列的演算法，團隊顯示深層微地動的模式變化可以做為附近即將發生地震的徵兆。為了進一步驗證這項發現，Chao檢驗了另外四場地震並發現此種預警模式確實存在。他和Van der Lee期許這項成果可以啟發地震學領域中更多以資料為導向的研究。

他說：「在發展出可靠的中長期地震預警方法之前，對這些微小但有趣的微地動訊號進行更加大量的資料分析是必須的。」

Do Earthquakes Have a ‘Tell’?

Data scientists and seismologists use “deep tremor” to forecast strong earthquakes

Researchers have long had good reason to believe that earthquakes are inherently unpredictable. But a new finding from Northwestern University might be a seismic shift for that old way of thinking.

An interdisciplinary team recently discovered that “slow earthquakes,” which release energy over a period of hours to months, could potentially lead to nearby “regular earthquakes.” The finding could help seismologists better forecast some strong earthquakes set to occur within a certain window of time, enabling warnings and other preparations that may save lives.

“While the build-up of stress in the Earth’s crust is largely predictable, stress release via regular earthquakes is more chaotic in nature, which makes it challenging to predict when they might occur,” said Kevin Chao, a data science scholar in the Northwestern Institute on Complex Systems (NICO). “But in recent years, more and more research has found that large earthquakes in subduction zones are often preceded by foreshocks and slow earthquakes.”

Supported by the National Science Foundation, the research was published in the Journal of Geophysical Research: Solid Earth. Chao, who is also a member of Northwestern’s Center for Optimization and Statistical Learning, served as the paper’s first author. Suzan van der Lee, a professor of earth and planetary sciences in Northwestern’s Weinberg College of Arts and Sciences, also contributed to the work.

Chao and his colleagues began their work several years ago by turning to a region within Taiwan, home to approximately 100 seismic stations that have continuously recorded ground motion for years. It was there the team noticed deep tremors, a type of slow earthquake that typically recurs in days- or weeks-long cycles.

“Deep tremor is very sensitive to small stress changes,” Chao said. “So, we decided to use them as stress meters to monitor local variations in stress build-up and release before and after large earthquakes.”

To detect and monitor this deep tremor activity, Chao’s team developed a sophisticated set of algorithms and applied it to data from 10 seismic stations in Taiwan. They discovered that deep tremor started to change its behavior about two months before the occurrence of a 6.4-magnitude earthquake in March 2010 in southern Taiwan. The tremor’s duration, for example, increased by two-fold before this event and continued to increase afterwards.

Although deep tremor was first reported in 2002, scientists have not found many cases in which behavior changed before large earthquakes. “After the 6.4-magnitude earthquake occurred, we noticed a potential to study deep tremor near the event,” Chao said. “We identified the increase in tremor duration three weeks before the earthquake, but we initially could not draw conclusions because tremor rates increase all the time and for different reasons.

But three years after the 6.4-magnitude, Chao and his colleagues noticed that their observations of tremor activity coincided with nearby a GPS recording, which indicated a flip in the direction of ground motion near tremor sources. 

By combining data from earth observatories, such as GPS and seismic stations, with statistics and a series of algorithms, the team showed that changes in deep tremor patterns could signal an impending earthquake nearby. To further test the finding, Chao examined four additional earthquakes and discovered that similar precursory patterns did exist. He and Van der Lee hope that this work will inspire more data-driven research in the seismology field.

“Much more data analysis of these tiny but fascinating tremor signals is necessary,” he said, “before mid- to short-term earthquake forecasting become reliable.”

原始論文：Kevin Chao, Zhigang Peng, Ya-Ju Hsu, Kazushige Obara, Chunquan Wu, Kuo-En Ching, Suzan van der Lee, Hsin-Chieh Pu, Peih-Lin Leu, Aaron Wech. Temporal variation of tectonic tremor activity in southern Taiwan around the 2010 M L 6.4 Jiashian earthquake. Journal of Geophysical Research: Solid Earth, 2017; 122 (7): 5417 DOI: 10.1002/2016JB013925

引用自：Northwestern University. "Do earthquakes have a 'tell'? Data scientists and seismologists use 'deep tremor' to forecast strong earthquakes."