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足以形成地震规模的“炸弹气旋”

更新时间:2018-1-3 21:36:28 来源:纽约时报中文网 作者:佚名

How 'bomb cyclones' can let us peer deep inside the Earth
足以形成地震规模的“炸弹气旋”

Given its location on the Atlantic seaboard of North America, Maine is no stranger to seasonal storms. I’ve lived through multiple hurricanes and a historic ice storm that paralysed my home state for weeks.

由于位置关系,位于北美大西洋沿岸的缅因州经常有季节性风暴光顾。我在这里就曾经遇上过好多次导致整个州陷入瘫痪数周的飓风和冰风暴。

In November, however, around 484,000 residents in Maine – nearly two-thirds of the state – were left in the dark when a sudden and ferocious storm knocked out their power for more than a week in some areas. Most of us were completely unprepared as there had been few warnings of what was coming.  This was no ordinary storm.

然而,2017年11月,一场突如其来的狂烈风暴横扫了缅因州,导致大约有48.4万州内民众——大概占全州总人口的三分之二——失去了电力供应而陷入黑暗长达一周之久。尽管有关部门在风暴来袭前曾经发出了几次警告,但是大多数灾民仍然对此猝不及防。很显然,这场风暴与众不同。

Maine was hit by a “bomb cyclone”, a portentous term for a weather system that meteorologists call explosive cyclogenesis or, in less hyperbolic moments, rapid and intense low-pressure development.

这次袭击缅因州的是"炸弹气旋"(bomb cyclone)——一种气象学家称之为"爆发性气旋",或者更学术化地称作"迅速而猛烈的低压发育"的天气系统。

The hurricane-speed gusts these systems generate can uproot trees and damage power lines. They can also bring flash flooding and erosion. They’re so powerful that a research team in Japan recently discovered that the ocean waves whipped up by bomb cyclones can light up the Earth’s interior with seismic energy. Monitoring these seismic signals could now provide geologists with new tools to help them explore the Earth’s internal structure.

这种速度和飓风(hurricane)不相上下的暴风能够把大树连根拔起,破坏输电线路,还会引发洪水和水土流失。日本的一个研究小组最近发现,炸弹气旋激起的海浪足以形成地震规模的能量,从而引发地壳深处的反应。对这种地震信号进行监控可以帮助地质学家探索地球内部构造。

Chris Legro, lead meteorologist with the National Weather Service in Gray, Maine, says the strong winds generated by bomb cyclones create large ocean waves, often in the order of 25 to 35ft (7.6m-10.7m). It is when these huge waves interact with the sea floor that they generate subtle shaking that can be measured around the world on seismographs.

位于缅因州格雷的美国国家气象服务局(National Weather Service)首席气象学家克里斯·雷格罗(Chris Legro)称,炸弹气旋引发的强劲风力会诱发浪高达7.6-10.7米的巨大海浪。海浪与海床相互作用会引发轻微的晃动,在世界各地用地震仪即可监控这种晃动。

“They [the seismic waves] travel through the Earth's interior to reach the other side of the globe, and can provide insight into the type of material they traveled through,” Legro explains.

"[地震波]会穿过地球内部抵达地球的另一端,我们由此就可以分析地震波经过物质的特性,"雷格罗说。

Deep-earth tremors

深层震动

Combining data from a network of more than 200 seismic sensors operated by Japan’s National Research Institute for Earth Science and Disaster Prevention, researchers Kiwamu Nishida and Ryota Takagi observed a rare, faint type of deep-earth tremor, known as an S wave microseism, which they attributed to a North Atlantic bomb cyclone off the coast of Greenland in December 2014.

日本国立地球科学及灾害预防研究院拥有由200个地震波传感器组成的网络。研究员西田究和高木涼太发现了一种称为"S波微震"的罕见而微弱的地层深层震动,他们认为,S波微震就是由于2014年在格陵兰外海爆发的北大西洋炸弹气旋引起的。

It’s the first time scientists have observed this particular type of tremor on the ocean floor. Previously, experts had only detected P waves (tremors that animals can feel before an earthquake), but were unable to detect the more elusive S waves (tremors that humans feel during earthquakes).

这是科学家首次观察到在海床里发生的此类震动。之前,科学家只探测到过P波(动物能够在地震前感受到的震动),但是这次他们探测到了更少见的S波(人类在地震过程中感受到的震动)。

“I think the significance of this discovery is an incremental improvement in how we can investigate the structure of the Earth's interior,” says Chris Goldfinger, director of the Active Tectonics and Seafloor Mapping Laboratory at Oregon State University’s College of Earth, Ocean, and Atmospheric Sciences. “The use of storm-generated microseisms turns what was ‘noise’ into a potential signal, so it's very clever and something that was previously filtered out or ignored.”

"我认为,这一发现的重要之处在于帮助我们增长了对地球内部构造的认识,"俄勒冈州立大学地球、海洋及大气科学学院主动地层及海床绘图实验室主任克里斯·歌德芬格尔(Chris Goldfinger)表示。"风暴引起的微震动曾经只是噪音,而现在是有用的信号,有效利用过去曾被筛除或忽略的信号是非常聪明的举措。"

A great deal of what we know about the Earth's internal structure has been learned through the use of seismic waves, usually from naturally occurring earthquakes, although sometimes from large explosions and other sources. By examining the speed these waves travel through the Earth at, it can reveal details about the composition of the rocks beneath the Earth’s crust. Reflections caused by changing rock types and between solid and liquid materials also cause seismic waves to bounce around, revealing yet more information.

地震波帮助我们搞清楚了很多地球内部构造的未解之谜。地震波的来源一般是自然发生的地震,有时也利用大规模爆破和其他来源。通过分析地震波穿过地球的速度,可以了解地壳下部岩层的组成。不同种类的岩石,以及固体/液体界面会对地震波形成反射,从而揭露地球内部的更多信息。

“Weather events are not used to do this at all, so that's what is unique about the discovery,” says Goldfinger. While earthquakes will remain the primary tool, weather events provide a new source of seismic waves that can be used to look into the Earth's interior, he adds.

"之前从未考虑过天气事件,这就是这一发现的独特之处,"歌德芬格尔説。地震仍然保留首要工具的地位,天气事件则提供了新的地震波来源,据此可以研究地球内部构造,他接着说。

But according to Peter Bromirski and Peter Gerstoft, researchers at Scripps Institution of Oceanography in California, microseisms work better than earthquakes, which occur sporadically and are clustered on plate boundaries. Microseisms, by contrast, are ever-present and seem to come from all directions.

但是加利福尼亚州Scripps海洋学研究院的研究员彼得·布罗莫斯基(Peter Bromirski)和彼得·哲斯托夫特(Peter Gerstoft)表示,微震动的表现比地震还要好,后者的发生概率很低,并且集中在地质板块边缘。而微震动则更常发生,并来自所有方向。

And while all storm systems generate microseisms of various types, some are more useful to those studying the Earth’s interior than others.

所有风暴系统都会产生不同种类的微震动,然而某些类型的风暴对于研究地球构造的地质学家更有用。

“Bomb cyclones generally have smaller spatial extent than typical extratropical cyclones, so the source area of signals they generate can potentially be better constrained,” says Gerstoft. Imagine a garden hose being shaken up and down with force. S waves cause material to ripple in a similarly explosive, but confined manner. P waves compress and stretch the land, like when a slinky when it is stretched out on the floor and one end is pushed rapidly forward.

"与一般的热带外气旋相比,炸弹气旋影响到的范围较小,因此信号来源位置可以较为精确地判定,"哲斯托夫特表示。设想一下用手用力上下摆动一根浇水软管。S波导致的物质波动也是一种与之类似的突然但范围有限的波动。P波会压缩并拉伸大地,就好像在地板上上下迅速掀动一个袍子。

Seismic flashlights

地震手电筒

The Japanese researchers detected two types of S waves were produced by the bomb cyclone they studied – SV waves that have an eliptical motion and can turn into P waves, and SH waves that move material horizontally.

日本研究人员发现,炸弹气旋形成了两种类型的S波——呈椭圆运动,且可转化为P波的SV波;以及沿水平方向剧烈运动的SH波。

Each wave type illuminates a different part of Earth’s interior due to individual travel speeds and characteristics, says Goldfinger. “It's like having a toolbox of many different flashlights, each one of which can illuminate a different part of the interior,” he says. “The new findings add one more flashlight, lighting up a previous dark patch, and maybe illuminating something new about what's there.”

速度和性质不同的各个波形反映了地球内部的不同部分,歌德芬格尔说。"就像是装了各种手电筒的工具箱,每只手电筒都能照亮箱子的一部分,"他说。"新发现增添了一种手电筒,照亮了之前黑暗的一块地方,还可能会照亮跟多地方。"

But what gives a bomb cyclone the power to produce these faint S waves on the bottom of the ocean?

那么炸弹气旋为什么能够在海床上产生微弱的S波?

The main cause is the explosive way in which they develop, says Legro.

主要原因在于其突然性的能量释放过程,雷格罗说。

“The typical meteorological definition of explosive cyclogenesis is the central pressure dropping 24 millibars in 24 hours,” he says. “It is that rapid change in pressure that can cause strong winds at the surface, as air rushes in towards the center to fill the void.”

"爆发性气旋的标准气象学定义是,中心压力在24小时内下降24微巴,"他说。"压力迅速下降后,周围的空气来填补空洞,从而在表面产生强风。"

On average, this happens about 40 times a year between September and April in the Northern Hemisphere. These storms most often develop in the Northwest Atlantic and the Northwest Pacific. Locally, the Northeast United States can expect a handful of these weather bombs every winter. Some may pass relatively harmlessly out to sea, while others, like the one Maine experienced earlier this month, can track through the Northeast.

在北半球,每年9月到下一年4月,这种天气现象会出现大约40次。此类风暴一般在西北大西洋和西南太平洋发育形成。每年冬天,美国东北部都会遭遇几次此类风暴。有些风暴较为温和地通过陆地向海洋运动,而有些——例如缅因州经历过的——则会横扫东北部地区。

While it might not have been the first thing on the minds of residents in Maine clearing felled trees and picking up the debris after November’s bomb cyclone, for seismologists it was a valuable opportunity.

11月的炸弹气旋过后,缅因州居民都在忙着清理倒下的树木和遍地的瓦砾,而地震学家们此时却兴奋异常。

Unfortunately for the Japanese scientists, their monitors sit in a “shadow zone” for any seismic waves passing through the Earth’s interior from Maine. The planet’s liquid core deflects and absorbs the seismic energy, meaning seismographs being used by Nishida and his team in Japan did not pick up the tremors generated by the storm.

很不凑巧,日本的监控装置位于从缅因州发出的地震波的"阴影区"。地球的液态内核折射并吸收了地震能量,因此西田究和他的同事使用的地震仪没有拾取到这场风暴引发的震动。

“The bomb cyclone was devastating, but the seismic excitation was not extraordinary,” says Nishida. He hopes it might be possible to find signals that may have been picked up in other parts of the world, but they have still to get their hands on this data.

"炸弹气旋摧毁力巨大,但是它所激发的地震却没有探测到,"西田究说。他希望世界其他地区的地震仪拾取到了信号,但他们仍然需要自己处理这些数据。

So, much like the residents of Maine, the geologists have also been left in the dark, if only temporarily.

和缅因州居民一样,地质学家们也陷入了漆黑一片——至少暂时如此。

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