PBS高端访谈:为什么墨西哥城易受地震的影响?

 　　Yesterday's earthquake in Mexico was the second in less than two weeks and brought back memories of the terrible tremor¹ that hit 30 years to the day yesterday.

　　So many are asking about why the country and region seem more prone² to these earthquakes.

　　We look at those questions with a seismologist. She is Lucy Jones. And this is for our Leading Edge science segment this week.

　　Lucy Jones, welcome to the program. We have been watching these heartbreaking scenes of trying to get people out of collapsed³ buildings, including a school, young children.

　　Is this part of Mexico particularly vulnerable to this kind of quake?

　　Well, there's one issue with Mexico City, which is that it's built in an old lake bed. The ground underneath⁴ it is extremely soft and wet.

　　When the seismic⁵ waves come from wherever they come from and into the soft soil, they get larger.

　　They have to slow down. They have to carry the same amount of energy. They get bigger.

　　That happens in any soil. Here in Los Angeles, we might have a factor of two or three amplification⁶.

　　In Mexico City, it's a factor of 100. So, every earthquake is much worse when it's in Mexico City.

　　So, are buildings typically built that much stronger to withstand earthquakes in these situations?

　　When you say how is a building built, there are buildings built currently right now to the best code which are trying to be handling that, but older structures haven't had that advantage.

　　And, you know, there was that horrible earthquake in 1985 that damaged and got rid of a lot of buildings and helped understand what's going on with this lake bed.

　　But it was so far away -- it was actually 250 miles away from the city -- that by the time the waves got there, they were only very slow waves.

　　And that's what got amplified⁷. So, very big buildings got damaged in that event, but the smaller ones just didn't really respond to those very distant waves.

　　This time around, now the earthquake is much closer, even though it's somewhat smaller. It's closer, so the waves are coming in big.

　　They still have their high energy. And we're seeing a lot of damage to those smaller buildings.

　　As far as I can tell, a lot of them are older buildings. Everywhere in the world, your building code is not retroactive.

　　Your building is as good as the code that was in place when it was built. If that's 1940, it's not much of a code.

　　Am I right that there have been not been aftershocks, Lucy Jones? And, if not, why? Isn't that unusual?

　　There haven't been large aftershocks. There are still plenty of aftershocks. So far, they have all been below magnitude 5.

　　Main shock of magnitude 7, that's on the lower edge of the distribution, but it's still easily within the normal range.

　　Same thing applied⁸ to the Loma Prieta earthquake in San Francisco in 1989.

　　What we see, though, is, when earthquakes are deeper, they're likely to have these smaller aftershock sequences.

　　And this earthquake started 30 miles below the earth's surface.

　　And just finally, this is the second earthquake to hit Mexico in a couple of weeks. Everybody down there has to be on edge.

　　Well, I think now seismologists would look at the magnitude 7 as a sort of extended aftershock or a distant triggered earthquake.

　　Usually, we use the word aftershocks for the large majority of triggered events that are very near the first one.

　　But we see this ability to trigger out to greater distances, maybe 300 or 500 miles, to a magnitude 8.

　　And so this earthquake could have been at least hastened by the occurrence of the first magnitude 8.

　　And it doesn't mean that you have got to have a whole lot of other earthquakes coming. But it does -- emotionally, it definitely feels scary.

　　Seismologist Lucy Jones, we thank you very much.

　　Thank you for having me.

1 tremor   

n.震动，颤动，战栗，兴奋，地震

参考例句：

• There was a slight tremor in his voice.他的声音有点颤抖。
• A slight earth tremor was felt in California.加利福尼亚发生了轻微的地震。

2 prone   

adj.(to)易于…的，很可能…的；俯卧的

参考例句：

• Some people are prone to jump to hasty conclusions.有些人往往作出轻率的结论。
• He is prone to lose his temper when people disagree with him.人家一不同意他的意见,他就发脾气。

3 collapsed   

adj.倒塌的

参考例句：

• Jack collapsed in agony on the floor. 杰克十分痛苦地瘫倒在地板上。
• The roof collapsed under the weight of snow. 房顶在雪的重压下突然坍塌下来。

4 underneath   

adj.在...下面，在...底下；adv.在下面

参考例句：

• Working underneath the car is always a messy job.在汽车底下工作是件脏活。
• She wore a coat with a dress underneath.她穿着一件大衣，里面套着一条连衣裙。

5 seismic   

a.地震的，地震强度的

参考例句：

• Earthquakes produce two types of seismic waves.地震产生两种地震波。
• The latest seismic activity was also felt in northern Kenya.肯尼亚北部也感觉到了最近的地震活动。

6 amplification   

n.扩大，发挥

参考例句：

• The voice of despair may be weak and need amplification.绝望的呼声可能很微弱，需要扩大。
• Some of them require further amplification.其中有些内容需进一步详细阐明。

7 amplified   

放大，扩大( amplify的过去式和过去分词 )； 增强； 详述

参考例句：

• He amplified on his remarks with drawings and figures. 他用图表详细地解释了他的话。
• He amplified the whole course of the incident. 他详述了事件的全过程。

8 applied   

adj.应用的；v.应用，适用

参考例句：

• She plans to take a course in applied linguistics.她打算学习应用语言学课程。
• This cream is best applied to the face at night.这种乳霜最好晚上擦脸用。