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一天两场大地震,这预示着什么?

更新时间:2016-4-18 18:33:53 来源:纽约时报中文网 作者:佚名

Do Earthquakes in Japan and Ecuador Signal an Epidemic? Scientists Say No
一天两场大地震,这预示着什么?

Earthquakes of magnitudes exceeding 7.0 struck Japan and Ecuador just hours apart on Saturday. Are the two somehow related?

星期六,超过七级的大地震先后袭击日本与厄瓜多尔,相隔仅几个小时。这两场地震之间有关联吗?

No. The two quakes occurred about 9,000 miles apart. That’s far too distant for there to be any connection between them.

没有。两场地震的发生地相隔约9000英里(约合1.5万公里)。这个距离太遥远了,之间不可能产生任何关联。

Large earthquakes can, and usually do, lead to more quakes — but only in the same region, along or near the same fault. These are called aftershocks. Sometimes a large quake can be linked to a smaller quake that occurred earlier, called a foreshock. In the case of the Japanese quake, seismologists believe that several magnitude-6 quakes in the same region on the previous day were foreshocks to the Saturday event.

大型地震有可能也通常会引发更多地震,但只是在同一个区域,沿着同一断层,或是在同一断层附近。这叫做“余震”。有时候大地震和较早的小型地震有关,这些小地震被称为“前震”。在这次日本地震中,地震学家认为,同一地区在周六地震之前的几天,发生了几次震级为六级的前震。

But the two earthquakes are similar in some ways, aren’t they?

但是两场地震在某些方面非常相像,对不对?

Not really. The magnitude-7.8 quake in Ecuador was what would be considered a classic megathrust event, a type that was first identified through the work of George Plafker, a United States Geological Survey geologist, on the great Alaskan earthquake of 1964. A megathrust quake occurs in the boundary zone where one of the planet’s tectonic plates is sliding under another, a process called subduction.

两者并不像。厄瓜多尔的地震是7.8级,可被视为典型的“大型逆冲区”(megathrust)地震,这种地震最早是美国地质调查局(United States Geological Survey)的地质学者乔治·普拉夫克(George Plafker)在研究1964年的阿拉斯加大地震期间发现的。大型逆冲区地震发生在地壳构造的边缘地带,在这里,地球的一个地壳板块下降到另一个板块之下,这种过程在地质上称为“潜没”(subduction)。

In the case of the Ecuadorean quake, the Nazca, a heavy oceanic plate, is sliding under the South American, a lighter continental plate, at a rate of about two inches a year. Strain builds up at the boundary, which is then released suddenly in the form of an earthquake. Because the boundary area is usually large, megathrust quakes are the most powerful and include the two strongest quakes ever measured by instruments: the magnitude-9.2 1964 Alaskan quake and one in coastal Chile in 1960 of magnitude 9.5.

厄瓜多尔地震的情况是这样的,沉重的大洋板块纳斯卡(Nazca)正在以每年两英寸(约为5厘米)的速度,滑向较轻的大陆板块南美板块之下。因此板块边界便逐渐积聚起张力,并突然以地震的形式释放出来。因为板块边缘地带一般范围很大,大型逆冲区地震通常都很强烈,迄今仪器测到的两场最大地震都是这种类型的地震,一场是1964年9.2级的阿拉斯加地震,另一场是1960年9.5级的智利海岸大地震。

Although there have been plenty of megathrust earthquakes in Japan — including the 2011 Tohoku quake, which led to the Fukushima nuclear disaster — the earthquake on Saturday on the island of Kyushu in southwest Japan was not the megathrust type. Rather, according to the geological survey, the earthquake occurred at shallow depth along a different kind of fault — called a strike-slip — in the top of the Eurasia plate, above any subduction zone.

尽管日本也发生过多次大型逆冲区地震——其中就包括2011年导致了福岛核事故的日本东北大地震——但周六在日本西南部九州岛发生的地震却不是这种类型。根据地质勘测,这场地震不在任何潜没区内,而是发生在欧亚板块之上另一种较浅的断层,称为“走向滑动断层”(strike-slip)。

O.K., but two 7.0-plus quakes in the same day — does that mean earthquake activity is increasing?

好吧,不过同一天发生两场七级以上的地震,这是不是意味着地震在增加呢?

No. The geological survey, which monitors earthquakes around the world, says the average number of quakes per year is remarkably consistent. For earthquakes between magnitude 7.0 and 7.9, there have been some years with more than 20 and others with fewer than 10, but the average, according to the survey, is about 15. That means that there is more than one per month, on average, and by chance, sometimes two quakes occur on the same day. (Also by chance, the world sometimes goes a month or longer without a 7.0-plus quake, as it did between July 27 and Sept. 16 last year.)

不是。监控全球所有地震的地理勘测表明,每年地震的平均数字是相当稳定的。在某些年份,全年会发生20多起震级在7.0至7.9之间的地震,在某些年份,这样规模的地震不到10起,但根据这项勘测,平均数是每年15起。这意味着,平均每个月都会有一场以上这样的地震,所以两场地震是有机会在同一天发生的(偶尔,也会有一个月乃至更长的时间里,地球上完全没有7.0级以上的地震,比如去年的7月27日至9月16日)。

Sometimes it seems that earthquakes are increasing in frequency because, as instrumentation improves and more people occupy more parts of the world, more quakes make the news. The two earthquakes on Saturday both occurred in heavily populated areas with media and communication networks, so word got out quickly and easily. If one had occurred in the middle of the ocean, few people would have noticed.

有时候,地震的频率似乎在增加,这是因为随着探测工具的进步,以及人们在世界的各个角落定居,地震日益成为新闻。周六的两场地震都发生在人口密集、拥有媒体和通讯网络的地区,所以消息传播起来就格外容易和快捷。如果地震发生在大洋之中,就不会有什么人注意到。

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