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地震之謎:“人造地震”是如何發生的?

更新時間:2019/9/11 21:24:06 來源:紐約時報中文網 作者:佚名

The mystery of unexplained earthquakes
地震之謎:“人造地震”是如何發生的?

Fear seizes the TV presenter at the very moment her TV studio starts to shake. She stops, mid-monologue, and falls silent. The shaking worsens. Other presenters seated around the awkwardly large plastic table sit stock still, save for a few worried glances left and right. Then the shaking gets stronger. The rattling of equipment above them can be heard. One broadcaster turns his gaze upward to see. The main presenter gasps. It’s time to go.

電視演播室開始搖晃,女主播頓時面露恐懼,話說了一半就停下來,陷入沉默。搖晃加劇。圍坐在笨重的大塑料桌旁的其他主持人,呆坐不動,擔心地左右瞥了幾眼。震動更強烈了。他們頭頂上的設備發出咔嗒聲。一位主播向上望,倒抽了一口冷氣。該撤了。

As the South Korean live TV team hastily discarded body microphones and abandoned their set, the seismic ripples of a 5.5 magnitude earthquake continued to shudder across Pohang. It was a powerful jolting. Other footage shows people running from buildings as walls collapse behind them. An entire city of half a million residents was left in shock. But this quake wasn’t a freak natural event. It was started by people.

這個韓國直播電視團隊隨即匆忙扔掉身上的麥克風,丟下了他們的設備,此時一場5.5級地震的震波正在震撼著浦項(Pohang)全城。這是一次劇烈的地震。其他視頻片段顯示,人們從大樓逃出來,他們身后的建筑物墻身隨即倒塌。這個擁有50萬居民的城市一片驚慌。但這次地震并不是一次反常的自然事件。而是人為造成的。

That’s the conclusion of a report published in March by a team of experts who tried to find out what caused the event in Pohang on 15 November 2017. It left 135 people injured and 1,700 had to be temporarily relocated to emergency housing. Thousands of buildings were damaged, costing $75m ($60m). Because a geothermal drilling project had been operational nearby at the time, a big question needed to be answered: Whodunit? Humans or nature? To find out if industrial activity had set off the quake, the South Koreans called on a new breed of seismologist: the earthquake detectives.

一組專家今年3月份發表的一份報告得出了上述結論。他們試圖找出2017年11月15日浦項地震的原因。這次地震造成135人受傷,1700人被迫暫時轉移到緊急住所。數千座建筑被毀,經濟損失達7500萬美元。地震時附近有一個正在開工的地熱鉆探工程,因此一個有待回答的大問題是:是誰干的?人類還是自然?為了查明是否這項工程引發了地震,韓國請來了新一代地震學家:地震偵探。

They are the ones tasked with combing through seismic records and industry data to see if the shaking was natural or not. It is not an easy thing to prove either way. But these scientists are now coming up with surer methods of identifying the culprit. They are forensics for the Earth.

他們的任務是梳理地震記錄和行業數據,以判斷地震是天災還是人禍。二者都不容易證明。但這些科學家現在提出了更可靠的方法來找出罪魁禍首。他們是地球的法醫。

With more drilling and fracking occurring around the world, human-induced or anthropogenic earthquakes have become an increasingly common concern. About 100,000 oil wells are now drilled every year and the use of geothermal energy, which sometimes involves injecting fluid into hot rock in order to create steam, could increase six-fold by 2050. By removing large quantities of fossil fuel or by flooding fractured rock with liquid, it’s possible to upset the balance of stresses below and set an earthquake in motion.

隨著地表鉆探和水力壓裂開采能源技術在世界各地越來越多的運用,人類誘導或人為產生的地震已成為一個日益普遍關注的問題。現在,每年大約有10萬口油井開鉆,到2050年,地熱能的使用可能會增加6倍。地熱能開采有時需要向熱巖中注水以產生蒸汽。但開采大量的化石燃料或需要向斷裂的巖石中注水,有可能破壞地下巖層的應力平衡,引發地震。

While we like to use metaphors like “on solid ground” in English, on a geological scale the stuff below our feet is anything but. It’s full of shifting planes of material with varying densities. There are faults and fractures, often with ribbons of fluid running through them. There are sediments, clays and bedrock. Not to mention, on an even bigger scale, gigantic tectonic plates rubbing against or pulling apart from one another. In some places, the ground is like a tower of toy bricks just waiting to topple.

雖然在英語中我們喜歡用“在穩固的基礎上”這樣的比喻,但從地質的角度來看,我們腳下的大地絕不穩固,全是由不同密度物質組成的移動巖層,有斷層和裂縫,并有地下水流穿過。這里還有沉積物、粘土和基巖。更不用說從更宏觀的角度,有巨大的地殼構造板塊相互擠壓或撕離。在一些地帶,地表就像一座玩具磚塔,只消一推即倒。

Bill Ellsworth remembers the first time he saw images of people fleeing buildings as the Pohang earthquake rattled the city.

埃爾斯沃思(Bill Ellsworth)猶記得他第一次看到浦項地震時人們逃出大樓時的情景。

“They were very fortunate that no-one was killed, having seen some of the security cam footage,” he says. Ellsworth, from Stanford University’s Center for Induced and Triggered Seismicity, was part of the international team that investigated what happened.

他說:“我后來看到了一些監控攝像頭的視頻,他們非常幸運,沒有人死亡。”埃爾斯沃思為斯坦福大學誘發和觸發地震活動中心(Stanford University's Center for Induced and Triggered Seismicity)的學者,是調查浦項地震的國際團隊的一員。

The stakes for these investigators were high. They knew at the outset that to label Pohang a human-induced earthquake would be a big deal. Earthquakes are measured on the Richter scale – which is “logarithmic” – meaning an increase in one point signifies a 10-fold increase in strength. A quake of about 3 on the Richter scale would be felt by inhabitants, and 4 would be enough to knock objects off shelves. A 5.5 or higher magnitude event caused by human activity is very rare, and although it is still considered moderate, it would be enough to damage buildings.

這些調查人員冒著很大風險。他們從一開始就知道,將浦項地震定性為人為地震是非同小可之事。地震采用里克特級數(Richter scale)計量,這是“對數級”,意味著每增加一級,強度就增加10倍。居民可以感覺到里氏3級左右的地震,4級地震足以將貨架上的物品震落。由人類活動引起的5.5級或更高級別的地震非常罕見,盡管仍被認為是中等強度,但已經足以摧毀建筑物。

The day after Pohang was rocked by tremors, NexGeo – the company operating the experimental geothermal power plant – denied it had any responsibility for what happened. But as Ellsworth and his team began scouring the evidence, a different story began to emerge. He and his fellow experts considered seismic data from the area as well as information from NexGeo – which cooperated with the investigation – about the drilling activity.

浦項地震發生的第二天,經營這家實驗性地熱發電廠的NexGeo公司否認對此事負有任何責任。但當埃爾斯沃思和他的團隊開始搜集證據時,一個不同的故事開始浮出水面。他和其他專家研究了該地區的地震數據,以及配合調查的NexGeo公司提供的有關鉆井活動的信息。

Geothermal plants work by using heat from the ground to generate electricity. There are various ways to do this, some for example make use of steam released directly from geothermal reservoirs. In other cases, the rock may be hot but there is not enough fluid to bring heat to the surface in the form of steam. To fracture the rock and release that heat, NexGeo planned to inject fluid into the ground.

地熱發電廠利用地熱能發電。有多種方法可以做到這一點,例如利用直接從地熱儲層釋放的蒸汽。但也有這樣的情況,巖石可能是滾燙的,但沒有足夠的水以蒸汽的形式把熱量帶到地表。為了打破巖石并釋放這些熱量,NexGeo公司計劃向地下注水。

Before the firm’s team could get to that stage, they had to drill deep into the earth. It was during this process that things went wrong.

在該公司的團隊執行注水之前,他們必須深入地下鉆孔。正是在這個過程中出了問題。

When subterranean rock is drilled, it gets smashed into tiny particles by the drill bit and has to be removed. This is done by flushing a relatively dense fluid nicknamed “mud” down through the centre of the drill, which then exits at the bottom and flushes pulverised rock around the drill bit up to the surface. But the South Korean drillers unexpectedly hit an area of cracked and fractured rock about 3.8km (2.4 miles) down. A large quantity of the mud escaped into those cracks instead of flowing upwards. This increased the pressure in that area.

往地下巖石鉆孔時,巖石會被鉆頭粉碎成微小的顆粒,這些巖石顆粒必須被清除出鉆井。要做到清除,需要從鉆頭中心射出一種密度相對較大的鉆井液,鉆井液從鉆頭中流出到達鉆孔底部,然后將鉆頭周圍的碎石沖到地面上。但是,韓國鉆探人員意外地在地下約3.8千米深的地方擊中了一塊破裂的巖石。大量的泥漿沒有向上涌出,而是流進了裂縫,因此增加了那個巖區的壓強。

“For whatever reason there was a pathway that allowed the fluid to escape the borehole,” explains Ellsworth. By flushing even more fluid down, the drillers secured their borehole. But the huge pressure now present caused what no-one wanted: seismicity.

埃爾斯沃思解釋說:“不管是什么原因,有一條裂縫讓鉆井液從鉆孔中流出。”沖下更多的鉆井液后,鉆井工們保住了他們的鉆孔。但是現在巨大的水壓造成了人們不想看到的結果:地震活動。

“It triggered some very tiny events, events that were so small they were not noted at the time,” says Ellsworth.

埃爾斯沃思說:“水壓引發了一些非常輕微的地震,因為強度非常小,當時沒有人注意到。”

What the drilling team don’t appear to have realised then, but which spatial analysis of these mini earthquakes later showed, was that the drilling had actually crossed a fault line – a boundary underground where two planes of earth meet. Movement of earth can happen along these faults. That’s what causes earthquakes.

后來對這些輕微地震的空間分析發現,鉆探實際上已經碰到斷層線,即地下兩個巖層面相交的邊界。而鉆探小組當時似乎沒有意識到這一點。沿著這些斷層可能發生地層移動,這就是導致地震的原因。

Ideally, fault lines in areas subjected to drilling or fluid injection are known about and are usually avoided. In this case, partly because there had been no indication of a fault line at the surface, the South Korean team had no idea what they had drilled into. As Ellsworth puts it: “That was very unlucky.”

理想情況是,若果事前已知鉆井或灌注井液的巖區有斷層線,通常是可以避開斷層線鉆井。但在這次事故中,部分原因是地表沒有跡象表明地下存在斷層線,韓國的鉆探隊根本不知道自己鉆到了什么地方。正如埃爾斯沃思所說:“這是非常不幸的。”

“This fault was what we call critically stressed – only a small change in conditions could cause that fault to move, which is ultimately what happened.”

“這個斷層就是我們所說的臨界應力,只要有地層有微小變化就能導致斷層移動,而最終發生的正是這種情況。”

Those first, smaller earthquakes, overlooked at the time, were a sign that something wasn’t right. It was only a few weeks later that the 5.5 magnitude earthquake hit.

起初那些輕微的地震被忽視了,那其實是一個信號,說明出了問題。僅僅幾周后,5.5級地震就發生了。

Data collected by Ellsworth and his colleagues convinced them that the event was human-induced. While there has been some debate over the results, the findings have already been accepted by the South Korean government, which says it will now dismantle the geothermal plant.

埃爾斯沃思和他的同事們收集的數據使他們相信,這次地震是人為造成的。盡管對調查結果存在一些爭議,但韓國政府已經接受了調查結果,并表示將拆除地熱發電廠。

Could the drilling team have noticed the early seismicity and stopped drilling just in time? It’s possible, says Ellsworth, but they were relying on a relatively simple traffic light system to help them judge whether drilling was safe. This involves monitoring seismicity and only ceasing to drill should a certain magnitude of quake be reached. Ellsworth points out that in this case the magnitude of those mini quakes was very small, but plotting where they occurred reveals the presence of a fault. That kind of more comprehensive analysis could, in theory, have alerted drilling operators to the gravity of the situation earlier.

鉆探小組是否注意到了早期的地震活動并及時停止鉆探?埃爾斯沃思說,這是有可能的,但是鉆探小組當時依靠的是一個相對簡單的交通燈式的系統來判斷鉆井是否安全。該系統只有地震達到一定的震級時才能監測到有地震發生,才能停止鉆探。埃爾斯沃思指出,在這次事故中,這些輕微地震的震級非常小,因此這個監測系統未能察覺,而調查小組測繪發生這些微地震的區域,發現此處有斷層存在。理論上,這種更全面的分析可以讓鉆井操作人員更早意識到形勢的嚴重性。

But listening to the ground and making sense of the various rumblings going on below is no easy task. How do we do it? Five and a half thousand miles away in the south of England, one scientist has found himself embroiled in another seismic detective case – this time involving an oil drilling operation nestled in the pleasant countryside of Surrey.

但是,監聽大地的聲音并弄清地下發出的各種轟鳴聲意味什么問題并非易事。我們該怎么做呢?在5500英里之外的英格蘭南部,一位科學家發現自己卷入了另一起地震偵探案件。這一次和位于薩里郡(Surrey)宜人鄉村的石油鉆探作業有關。

It’s a bright afternoon in early spring when Stephen Hicks cracks open the big black box by a huge solar panel at the edge of a field. Through the hedge, a few horses look at us bemusedly before trotting off. “We’ve got five of these in the area,” he says, enthusiastically digging through the cables and components in the box to check everything is in place as it should be.

一個早春時節陽光明媚的下午,希克斯(Stephen Hicks)在田邊打開了一塊巨大的太陽能電池板旁邊的一個大黑盒子。隔著樹籬,幾匹馬困惑地看著我們,然后小跑著離開了。“我們這里有5個這樣的盒子,”他一邊說,一邊滿懷熱情地在盒子里的電線和零件中翻來翻去,檢查是否一切就緒。

Hicks is a seismologist at Imperial College London. He has found himself leading a local investigation that aims to find the cause of a series of small earthquakes in the area. It’s not something that Surrey, with its gentle rolling hills and babbling streams, is used to. But back on 27 February the location was hit by a 3.1 magnitude quake in the early hours of the morning. That was the strongest so far and while not hugely damaging, it was an unusual event. The UK only gets two or three such quakes a year.

希克斯是倫敦帝國理工學院(Imperial College London)的地震學家。他正在領導當地的一項調查,旨在找出該地區一系列輕微地震的原因。在山丘平緩起伏,溪流潺潺不息的薩里郡,這看來不會是應該發生的事。但今年2月27日凌晨,該地區發生3.1級地震。這是迄今為止最強烈的一次地震,雖然沒有造成巨大的破壞,但卻是一起不同尋常的事件。英國每年只有兩到三次這樣的地震。

Because a firm called UK Oil and Gas (UKOG) has been extracting oil nearby, many locals are worried that the activity is disturbing ancient fault lines and causing the quakes. There has been a series of heated protests at the drilling site. Many have turned to scientists to see if they can prove what is really going on, which is where Hicks comes in.

由于英國石油天然氣投資公司(UKOG)一直在附近開采石油,許多當地人擔心這種活動會破壞古老的斷層線,引發地震。鉆井現場曾進行過一系列激烈的抗議活動。許多人求助于科學家,看看他們能否證明到底發生了什么,這正是希克斯要研究的。

“That’s what we call the digitiser,” he says, enthusiastically pointing to a small box inside the black case. “That’s just turning analogue signal into some sort of digital form and then we can later convert it into velocity, metres per second or acceleration.”

希克斯興致勃勃地指著黑色盒子里的一個小盒子說:“這就是我們所說的數字轉換器,就是把模擬信號轉換成某種數字形式,然后我們可以把它轉換成速度、每秒多少米或加速度。”

After stamping on the ground he shows me the huge spikes that pop up a few minutes later on the near-real-time chart of tremors he can check online. Having multiple instruments in the field means that incidental noise, say from passing vehicles (or stamping scientists), can be discounted. Only when tremors appear uniformly on a number of the seismic monitors does that indicate an earthquake.

他往地上狠狠跺了幾下腳,然后向我展示了可以在線查看的近乎實時的震動圖表,幾分鐘后圖表上出現了巨大峰值。現場有多個儀器,這意味著偶然的噪音,比如過往車輛(或跺腳的科學家)發出的雜音可以忽略不計。只有當許多地震監測儀一致出現震動時,才表明發生了地震。

The kit costs about £10,000 ($12,500) and is owned by the British Geological Survey. Hicks and his colleagues helped to set the five units in place during the summer of 2018. He’s been monitoring the signals ever since. But unlike Pohang, there doesn’t seem to be a smoking gun here.

這套設備為英國地質調查局(British Geological Survey)所有,價值約1萬英鎊。2018年夏天,希克斯和他的同事協助安裝了5套設備。此后他一直在監控信號。但與浦項不同,這里似乎沒有確鑿的證據。

Almost all of the 90 or so quakes Hicks has detected in the last eight months are tiny, less than magnitude 1. And they are occurring at a relatively shallow depth, about 2.5km (1.5 miles) down, but not as shallow as the drilling, which is happening at about 700m or 800m (2,300-2,600ft). Not only that, the quakes have been distributed around the area – known as the Weald basin – not clustered near the drilling site.

在過去的8個月里,希克斯所探測到的90次左右的地震都很微弱,不到1級。它們發生在相對較淺的深度,約地下2.5千米處,但又比鉆探位置深,鉆探深度約為700米或800米。不僅如此,地震分布在威爾德盆地(Weald basin)周圍,而不是集中在鉆井現場附近。

“We think it’s coincidence,” he says. “The swarm – we call them swarms of earthquakes, you get bunches of them in time – it’s kind of random.”

他說:“我們認為這是巧合,這種集群,我們稱之為地震群,你在某個時間會發現很多。這是隨機的。”

In this case, the quake mechanism is “strike-slip”, where planes of earth are moving side-by-side. Kind of like two shoes rubbing together, says Hicks. And while he doesn’t think they are human-induced, they are still interesting, because quakes like this at such a shallow depth aren’t usually recorded with such a high resolution in the UK.

薩里郡的這種地震,是“橫移斷層”,即巖層相互平面移動所造成。希克斯說,這有點像兩只鞋子摩擦在一起。雖然他不認為這些地震是人為造成的,但覺得其生成原因仍然令人感興趣,因為在英國,像這樣在如此淺的地表發生的地震通常不會有如此高分辨率的記錄。

“Regardless of the cause, it’s still an interesting sequence,” he says.

他說:“不管原因是什么,這仍然是一系列令人感興趣的事件。”

As we stroll through the countryside, near to the epicentre of the biggest quake – an amusingly boring field – we bump into a lady named Jackie Wilson, who is walking her dog. When that 3.1 quake hit in February, her cat “leapt off the bed”, she says.

我們在鄉間漫步時,走到薩里郡最大地震的震中,一個有趣的鉆井場所附近,我們遇到一位名叫威爾遜(Jackie Wilson)的女士,她正在遛狗。她說,今年2月3.1級地震發生時,她的貓“從床上跳了下來”。

“Somebody did come round with a petition for the locals to sign against all of this drilling going on,” she adds, “I guess it has kicked off since then, hasn’t it?”

她補充道:“確實有人帶著請愿書,請當地人簽名反對所有正在進行的鉆探活動。我想從那以后地震活動就開始了,不是嗎?”

Hicks has fielded emails, calls and tweets from locals similarly curious about what’s going on. But for now he is sticking to his conclusion that the quakes are natural. A few weeks after we meet, he and colleagues publish preliminary results of their investigation online. “Overall, we find no indicators in the earthquake parameters that would strongly suggest an induced source,” they write.

希克斯已經回復了當地人發來的電子郵件、電話和推特,當地人也同樣好奇這場地震到底是怎么回事。但是現在希克斯仍堅持自己的結論,認為地震是自然的。我們見面幾周后,他和同事在網上公布了初步調查結果。他們寫道:“總的來說,我們在地震參數中沒有發現強烈暗示人為原因的跡象。”

But public concern that tremors might be man-made is becoming a more common story around the world. Especially when drilling or geothermal activity happens in the same place as earthquakes.

但公眾對地震可能是人為造成的擔憂,在世界各地正變得越來越普遍。尤其是當鉆探或地熱活動與地震發生在同一地點時。

People are clearly getting used to the concept of anthropogenic seismicity, which can be either human-induced or triggered. The latter is a slightly different condition in which earthquakes are mostly caused by tectonic activity but human activity plays a role in how they occur.

很明顯,人們已經能接受人為活動可能造成地震的概念,地震可以由人類行為誘發或直接觸發。后者與前者略微不同,在這種情況下,地震主要是由地質構造活動引起的,但人類活動對地震的發生起著一定作用。

It’s natural for the public to have concerns, says Francesco Grigoli at ETH Zurich. Grigoli has studied what tools are available to earthquake detectives when they try to work out the cause of seismic disturbances.

蘇黎世聯邦理工學院(ETH Zurich)的格里戈利(Francesco Grigoli)說,公眾有這樣的擔憂是很自然的。格里戈利研究了地震偵探在試圖找出地震原因時可用的工具。

“There is not a standard recipe for identifying any event, discriminating any event,” he explains. He and colleagues studied the Pohang quake less than a year after it occurred, but they were limited to public seismicity data from a Japanese station, many miles from the epicentre, and did not at the time have access to NexGeo’s information on drilling.

他解釋說:“沒有一個標準的方法來識別任何地震事件,并將地震辨析分類。”浦項地震發生不到一年后,他和同事們研究了這次地震,但他們的研究僅限于一個日本監測站公開的地震活動數據,該監測站距離震中很遠,而且他當時無法獲得NexGeo公司的鉆探信息。

He makes the point that more open data can make a huge difference when it comes to deciding whether a quake was anthropogenic or not.

他的觀點是,在判斷地震是否人為造成時,更多公開的數據可以帶來不同結果。

Plus, the resolution of seismic monitoring has improved greatly. If motivated to do so, drilling companies can today use highly sensitive listening arrays that hear “every last pop and crack”, says James Verdon at the University of Bristol.

此外,地震監測技術的分辨率也有了很大的提高。布里斯托大學(University of Bristol)的韋爾登(James Verdon)表示,如果鉆井公司受到鼓勵,就可以使用高度敏感的監聽陣列,能夠聽到“每一次巖石最新爆裂的聲音”。

“This gives us thousands, or even hundreds of thousands, of data points with which to make a much more detailed assessment of seismic hazard,” he explains.

他解釋說:“這為我們提供了成千甚至上萬的數據點,我們可以用這些數據點更詳細地評估地震災害。”

One paper published earlier this year described how microseismic monitoring apparently helped to keep fluid-induced seismicity to a minimum at a geothermal project in Finland. In that case, the team listened carefully to small-scale seismicity, which in turn motivated them to occasionally lower the rate of fluid injection or wait for longer periods between pumping. The authors of the paper think this kept any more serious shaking at bay.

今年早些時候發表的一篇論文描述了在芬蘭的一個地熱工程中,微震監測明顯有助于將流體誘發的地震活動降到最低。在該案例中,鉆井團隊仔細監聽小規模的地震活動,因而促使他們不時降低注液速度,或者拉長兩次注水之間的等待時間。這篇論文的作者們認為,這阻止了較嚴重的地震發生。

Some will never be comfortable with the idea of drilling near populated places. The potential consequences, they argue, are just too great, even if the probability of setting off a big earthquake remains small. Earthquake detectives, though, can in theory help to assess the situation while drilling is going on, not just after the fact, and raise the alarm should that drilling become dangerous. By listening carefully to what the ground is telling us, companies and governments may be better equipped to react – before it is too late.

有些人對人口密集地區附近鉆井永遠心懷不安。他們認為,即使引發大地震的可能性很小,但潛在的后果太嚴重。盡管如此,在理論上,地震偵探們可以幫助評估正在進行的鉆探情況,并在鉆探變得危險時發出警報,而不僅僅是在事故發生后才采取行動。通過仔細傾聽大地發出的聲音,企業和政府或許能更好地做出反應,以免為時過晚。

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