The mystery of unexplained earthquakes

地震之谜：“人造地震”是如何发生的？

 

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.

 

电视演播室开始摇晃，女主播顿时面露恐惧，话说了一半就停下来，陷入沉默。摇晃加剧。围坐在笨重的大塑料桌旁的其他主持人，呆坐不动，担心地左右瞥了几眼。震动更强烈了。他们头顶上的设备发出咔嗒声。一位主播向上望，倒抽了一口冷气。该撤了。

 

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.

 

这个韩国直播电视团队随即匆忙扔掉身上的麦克风，丢下了他们的设备，此时一场5.5级地震的震波正在震撼着浦项（Pohang）全城。这是一次剧烈的地震。其他视频片段显示，人们从大楼逃出来，他们身后的建筑物墙身随即倒塌。这个拥有50万居民的城市一片惊慌。但这次地震并不是一次反常的自然事件。而是人为造成的。

 

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.

 

一组专家今年3月份发表的一份报告得出了上述结论。他们试图找出2017年11月15日浦项地震的原因。这次地震造成135人受伤，1700人被迫暂时转移到紧急住所。数千座建筑被毁，经济损失达7500万美元。地震时附近有一个正在开工的地热钻探工程，因此一个有待回答的大问题是：是谁干的？人类还是自然？为了查明是否这项工程引发了地震，韩国请来了新一代地震学家：地震侦探。

 

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.

 

他们的任务是梳理地震记录和行业数据，以判断地震是天灾还是人祸。二者都不容易证明。但这些科学家现在提出了更可靠的方法来找出罪魁祸首。他们是地球的法医。

 

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.

 

随着地表钻探和水力压裂开采能源技术在世界各地越来越多的运用，人类诱导或人为产生的地震已成为一个日益普遍关注的问题。现在，每年大约有10万口油井开钻，到2050年，地热能的使用可能会增加6倍。地热能开采有时需要向热岩中注水以产生蒸汽。但开采大量的化石燃料或需要向断裂的岩石中注水，有可能破坏地下岩层的应力平衡，引发地震。

 

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.

 

虽然在英语中我们喜欢用“在稳固的基础上”这样的比喻，但从地质的角度来看，我们脚下的大地绝不稳固，全是由不同密度物质组成的移动岩层，有断层和裂缝，并有地下水流穿过。这里还有沉积物、粘土和基岩。更不用说从更宏观的角度，有巨大的地壳构造板块相互挤压或撕离。在一些地带，地表就像一座玩具砖塔，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.

 

埃尔斯沃思（Bill Ellsworth）犹记得他第一次看到浦项地震时人们逃出大楼时的情景。

 

“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.

 

他说：“我后来看到了一些监控摄像头的视频，他们非常幸运，没有人死亡。”埃尔斯沃思为斯坦福大学诱发和触发地震活动中心（Stanford University's Center for Induced and Triggered Seismicity）的学者，是调查浦项地震的国际团队的一员。

 

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.

 

这些调查人员冒着很大风险。他们从一开始就知道，将浦项地震定性为人为地震是非同小可之事。地震采用里克特级数（Richter scale）计量，这是“对数级”，意味着每增加一级，强度就增加10倍。居民可以感觉到里氏3级左右的地震，4级地震足以将货架上的物品震落。由人类活动引起的5.5级或更高级别的地震非常罕见，尽管仍被认为是中等强度，但已经足以摧毁建筑物。

 

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.

 

浦项地震发生的第二天，经营这家实验性地热发电厂的NexGeo公司否认对此事负有任何责任。但当埃尔斯沃思和他的团队开始搜集证据时，一个不同的故事开始浮出水面。他和其他专家研究了该地区的地震数据，以及配合调查的NexGeo公司提供的有关钻井活动的信息。

 

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.

 

地热发电厂利用地热能发电。有多种方法可以做到这一点，例如利用直接从地热储层释放的蒸汽。但也有这样的情况，岩石可能是滚烫的，但没有足够的水以蒸汽的形式把热量带到地表。为了打破岩石并释放这些热量，NexGeo公司计划向地下注水。

 

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.

 

在该公司的团队执行注水之前，他们必须深入地下钻孔。正是在这个过程中出了问题。

 

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.

 

往地下岩石钻孔时，岩石会被钻头粉碎成微小的颗粒，这些岩石颗粒必须被清除出钻井。要做到清除，需要从钻头中心射出一种密度相对较大的钻井液，钻井液从钻头中流出到达钻孔底部，然后将钻头周围的碎石冲到地面上。但是，韩国钻探人员意外地在地下约3.8千米深的地方击中了一块破裂的岩石。大量的泥浆没有向上涌出，而是流进了裂缝，因此增加了那个岩区的压强。

 

“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.

 

埃尔斯沃思解释说：“不管是什么原因，有一条裂缝让钻井液从钻孔中流出。”冲下更多的钻井液后，钻井工们保住了他们的钻孔。但是现在巨大的水压造成了人们不想看到的结果：地震活动。

 

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

 

埃尔斯沃思说：“水压引发了一些非常轻微的地震，因为强度非常小，当时没有人注意到。”

 

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.

 

后来对这些轻微地震的空间分析发现，钻探实际上已经碰到断层线，即地下两个岩层面相交的边界。而钻探小组当时似乎没有意识到这一点。沿着这些断层可能发生地层移动，这就是导致地震的原因。

 

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.”

 

理想情况是，若果事前已知钻井或灌注井液的岩区有断层线，通常是可以避开断层线钻井。但在这次事故中，部分原因是地表没有迹象表明地下存在断层线，韩国的钻探队根本不知道自己钻到了什么地方。正如埃尔斯沃思所说：“这是非常不幸的。”

 

“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.”

 

“这个断层就是我们所说的临界应力，只要有地层有微小变化就能导致断层移动，而最终发生的正是这种情况。”

 

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.

 

起初那些轻微的地震被忽视了，那其实是一个信号，说明出了问题。仅仅几周后，5.5级地震就发生了。

 

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.

 

埃尔斯沃思和他的同事们收集的数据使他们相信，这次地震是人为造成的。尽管对调查结果存在一些争议，但韩国政府已经接受了调查结果，并表示将拆除地热发电厂。

 

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.

 

钻探小组是否注意到了早期的地震活动并及时停止钻探？埃尔斯沃思说，这是有可能的，但是钻探小组当时依靠的是一个相对简单的交通灯式的系统来判断钻井是否安全。该系统只有地震达到一定的震级时才能监测到有地震发生，才能停止钻探。埃尔斯沃思指出，在这次事故中，这些轻微地震的震级非常小，因此这个监测系统未能察觉，而调查小组测绘发生这些微地震的区域，发现此处有断层存在。理论上，这种更全面的分析可以让钻井操作人员更早意识到形势的严重性。

 

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，监听大地的声音并弄清地下发出的各种轰鸣声意味什么问题并非易事。我们该怎么做呢？在5500英里之外的英格兰南部，一位科学家发现自己卷入了另一起地震侦探案件。这一次和位于萨里郡（Surrey）宜人乡村的石油钻探作业有关。

 

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.

 

一个早春时节阳光明媚的下午，希克斯（Stephen Hicks）在田边打开了一块巨大的太阳能电池板旁边的一个大黑盒子。隔着树篱，几匹马困惑地看着我们，然后小跑着离开了。“我们这里有5个这样的盒子，”他一边说，一边满怀热情地在盒子里的电线和零件中翻来翻去，检查是否一切就绪。

 

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.

 

希克斯是伦敦帝国理工学院（Imperial College London）的地震学家。他正在领导当地的一项调查，旨在找出该地区一系列轻微地震的原因。在山丘平缓起伏，溪流潺潺不息的萨里郡，这看来不会是应该发生的事。但今年2月27日凌晨，该地区发生3.1级地震。这是迄今为止最强烈的一次地震，虽然没有造成巨大的破坏，但却是一起不同寻常的事件。英国每年只有两到三次这样的地震。

 

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.

 

由于英国石油天然气投资公司（UKOG）一直在附近开采石油，许多当地人担心这种活动会破坏古老的断层线，引发地震。钻井现场曾进行过一系列激烈的抗议活动。许多人求助于科学家，看看他们能否证明到底发生了什么，这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.”

 

希克斯兴致勃勃地指着黑色盒子里的一个小盒子说：“这就是我们所说的数字转换器，就是把模拟信号转换成某种数字形式，然后我们可以把它转换成速度、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.

 

他往地上狠狠跺了几下脚，然后向我展示了可以在线查看的近乎实时的震动图表，几分钟后图表上出现了巨大峰值。现场有多个仪器，这意味着偶然的噪音，比如过往车辆（或跺脚的科学家）发出的杂音可以忽略不计。只有当许多地震监测仪一致出现震动时，才表明发生了地震。

 

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.

 

这套设备为英国地质调查局（British Geological Survey）所有，价值约1万英镑。2018年夏天，希克斯和他的同事协助安装了5套设备。此后他一直在监控信号。但与浦项不同，这里似乎没有确凿的证据。

 

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.

 

在过去的8个月里，希克斯所探测到的90次左右的地震都很微弱，不到1级。它们发生在相对较浅的深度，约地下2.5千米处，但又比钻探位置深，钻探深度约为700米或800米。不仅如此，地震分布在威尔德盆地（Weald basin）周围，而不是集中在钻井现场附近。

 

“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.”

 

他说：“我们认为这是巧合，这种集群，我们称之为地震群，你在某个时间会发现很多。这是随机的。”

 

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.

 

萨里郡的这种地震，是“横移断层”，即岩层相互平面移动所造成。希克斯说，这有点像两只鞋子摩擦在一起。虽然他不认为这些地震是人为造成的，但觉得其生成原因仍然令人感兴趣，因为在英国，像这样在如此浅的地表发生的地震通常不会有如此高分辨率的记录。

 

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

 

他说：“不管原因是什么，这仍然是一系列令人感兴趣的事件。”

 

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.

 

我们在乡间漫步时，走到萨里郡最大地震的震中，一个有趣的钻井场所附近，我们遇到一位名叫威尔逊（Jackie Wilson）的女士，她正在遛狗。她说，今年2月3.1级地震发生时，她的猫“从床上跳了下来”。

 

“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?”

 

她补充道：“确实有人带着请愿书，请当地人签名反对所有正在进行的钻探活动。我想从那以后地震活动就开始了，不是吗？”

 

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.

 

希克斯已经回复了当地人发来的电子邮件、电话和推特，当地人也同样好奇这场地震到底是怎么回事。但是现在希克斯仍坚持自己的结论，认为地震是自然的。我们见面几周后，他和同事在网上公布了初步调查结果。他们写道：“总的来说，我们在地震参数中没有发现强烈暗示人为原因的迹象。”

 

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.

 

但公众对地震可能是人为造成的担忧，在世界各地正变得越来越普遍。尤其是当钻探或地热活动与地震发生在同一地点时。

 

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.

 

很明显，人们已经能接受人为活动可能造成地震的概念，地震可以由人类行为诱发或直接触发。后者与前者略微不同，在这种情况下，地震主要是由地质构造活动引起的，但人类活动对地震的发生起着一定作用。

 

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.

 

苏黎世联邦理工学院（ETH Zurich）的格里戈利（Francesco Grigoli）说，公众有这样的担忧是很自然的。格里戈利研究了地震侦探在试图找出地震原因时可用的工具。

 

“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.

 

他解释说：“没有一个标准的方法来识别任何地震事件，并将地震辨析分类。”浦项地震发生不到一年后，他和同事们研究了这次地震，但他们的研究仅限于一个日本监测站公开的地震活动数据，该监测站距离震中很远，而且他当时无法获得NexGeo公司的钻探信息。

 

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.

 

他的观点是，在判断地震是否人为造成时，更多公开的数据可以带来不同结果。

 

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，地震监测技术的分辨率也有了很大的提高。布里斯托大学（University of Bristol）的韦尔登（James Verdon）表示，如果钻井公司受到鼓励，就可以使用高度敏感的监听阵列，能够听到“每一次岩石最新爆裂的声音”。

 

“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.

 

他解释说：“这为我们提供了成千甚至上万的数据点，我们可以用这些数据点更详细地评估地震灾害。”

 

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.

 

今年早些时候发表的一篇论文描述了在芬兰的一个地热工程中，微震监测明显有助于将流体诱发的地震活动降到最低。在该案例中，钻井团队仔细监听小规模的地震活动，因而促使他们不时降低注液速度，或者拉长两次注水之间的等待时间。这篇论文的作者们认为，这阻止了较严重的地震发生。

 

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.

 

有些人对人口密集地区附近钻井永远心怀不安。他们认为，即使引发大地震的可能性很小，但潜在的后果太严重。尽管如此，在理论上，地震侦探们可以帮助评估正在进行的钻探情况，并在钻探变得危险时发出警报，而不仅仅是在事故发生后才采取行动。通过仔细倾听大地发出的声音，企业和政府或许能更好地做出反应，以免为时过晚。