提问！！！GWD-24-35【从没有人问过！！】请NN解答！

cmy2

处女帖。。。

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 the crust (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 (lines 2 and 8) in order to

A.explain why the 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 why the crust will fracture but the mantle will not

答案选E，我看完不知道选什么。。。这篇阅读不知道为啥有7道题，觉得看得挺明白，只错了这一题，不明白啊。。。

Q35：The author uses the comparisons to porcelain and putty (lines 2 and 8) in order to

A.explain why the 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 why the crust will fracture but the mantle will not

答案选E，我看完不知道选什么。。。这篇阅读不知道为啥有7道题，觉得看得挺明白，只错了这一题，不明白啊。。。

cmy2

ding!

cappuccino202

同问，为什么不选C？

EmilyYoung90

同问！ 为什么不选C

2013rainbow

从putty结束后的那个问句开始，作者已经转移了话题。所以C是错位信息。说瓷器和油灰是来形象比喻地壳和地幔的质地，也一次告诉你它们在地震时发生不同变化的原因：那就是他们的质地。