GWD-24-35 earthquake形成的那题

cappuccino202

提问！！！GWD-24-35

GWD-24-Q30 ~ 37

In most earthquakes the Earth’s crust cracks like porcelain, Stress builds up until a fracture forms at a depth of a few kilometers and thecrust (5) slips to relieve the stress. Some earthquakes, however, take place hundreds of kilometers down in the Earth’s mantle, where high pressure makes rock so ductile that it flows instead of (10) cracking, even under stress severe enough to deform it like putty. How can there be earthquakes at such depths? That such deep events do occur has been accepted only since 1927 when the seismologist Kiyoo Wadati convincingly demonstrated their existence. Instead of comparing the arrival times of seismic waves at different locations, as earlier researchers had done, Wadati relied on a time difference between the arrival of primary(P) waves and the slower secondary(S)waves. Because P and S waves travel at different but fairly constant speeds, the interval between their arrivals increases in proportion to the distance from the earthquake focus, or initial rupture point.

For most earthquakes, Wadati discovered, the interval was quite short near the epicenter; the point on the surface where shaking is strongest. For a few events, however, the delay was long even at the epicenter. Wadati saw a similar pattern when he analyzed data on the intensity of shaking. Most earthquakes had a small area of intense shaking, which weakened rapidly with increasing distance from the epicenter, but others were characterized by a lower peak intensity, felt over a broader area. Both the P-S intervals and the intensity patterns suggested two kinds of earthquakes: the more common shallow events, in which the focus lay just under the epicenter, and deep events, with a focus several hundred kilometers down.

The question remained: how can such quakes occur, given that mantle rock at a depth of more than 50 kilometers is too ductile to store enough stress to fracture? Wadati’s work suggested that deep events occur in areas (now called Wadati-Benioff zones) where one crustal plate is forced under another and descends into the mantle. The descending rock is substantially cooler than the surrounding mantle and hence is less ductile and much more liable to fracture.


Q35：The author uses the comparisons to porcelain and putty in order to

A.explain whythe Earth’s mantle is under great pressure

B.distinguish the earthquake’s epicenter from its focus
C.demonstrate the conditions under which a Wadati-Benioff zone forms
D.explain why S waves are slower than P waves
E.illustrate whythecrustwill fracture but the mantle will not

为什么不选C啊为什么？

Warrengxt

选E不选C的两个原因：
1.从对文章分析来看：How can there be earthquakes at such depths? 第一段中的这个问句是为了解释为什么会有一些地震发生在几百公里以下。“take place hundreds of kilometers down in the Earth’s mantle”  但是这一问句是为了提出Wadati-Benioff这个人的科学发现，并且把之前的porcelain and putty的比较很好的隔开了。也就是说WB这个人的发现和这个比较无关，他只是在证明确实有一些地震发生在几百公里以下。所以根据之前porcelain and putty的比较，以及类似instead of cracking的词语，不难发现作者比较的目的。
2.从做题技巧来看，两个东西对比必然有区别，答案E中的but。。not。。很明显突出了他们的区别。如果没时间了或者不会的话可以直接选出这个答案。当然不是保证对的。

cappuccino202

选E不选C的两个原因：
1.从对文章分析来看：How can there be earthquakes at such depths? 第一段中的这个问句是为了解释为什么会有一些地震发生在几百公里以下。“take place hundreds of kilometers down in the Earth’s mantle”  但是这一问句是为了提出Wadati-Benioff这个人的科学发现，并且把之前的porcelain and putty的比较很好的隔开了。也就是说WB这个人的发现和这个比较无关，他只是在证明确实有一些地震发生在几百公里以下。所以根据之前porcelain and putty的比较，以及类似instead of cracking的词语，不难发现作者比较的目的。
2.从做题技巧来看，两个东西对比必然有区别，答案E中的but。。not。。很明显突出了他们的区别。如果没时间了或者不会的话可以直接选出这个答案。当然不是保证对的。

-- by 会员 Warrengxt (2011/8/2 10:28:03)

第二点想到了，但是第一点还是不太能说服自己。。。