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 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 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 (45) common shallow events, in which the focus lay just under the epicenter, and deep events, with 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.
24-33 The passage suggests that which of the following must take place in order for any earthquake to occur?
I. Stress must build up
II. Cool rock must descend into the mantle.
III. A fracture must occur.
A) I only
B) II only
C) III only
D) I and III only
E) I, II and III
MINE: A
CO ANS: D
WHY?HELP!作者: poorleafes 时间: 2010-5-10 06:49
24-30 The author’s explanation of how deep events occur would be most weakened if which of the following were discovered to be true?
A) Deep events are far less common than shallow events.
B) Deep events occur in places other than where crustal plates meet.
C) Mantle rock is more ductile at a depth of several hundred kilometers than it is at 50 kilometers.
D) The speeds of both P and S waves are slightly greater than previously thought.
Quiz 30。文中对于deep event的解释是 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.
只有选项B，当 Deep events occur in places other than where crustal plates meet 的时候，上面的解释说不通。作者: IMARCUS 时间: 2010-5-12 06:57
30题的A为什么不对呢？文章有说它是不common的阿！
谢谢了。作者: missouyangs 时间: 2010-5-12 22:22
30题的A为什么不对呢？文章有说它是不common的阿！
谢谢了。
IMARCUS 发表于 2010-5-12 06:57
24-30 The author’s explanation of how deep events occur would be most weakened if which of the following were discovered to be true?
24-31 Information presented in the passage suggests that, compared with seismic activity at the epicenter of a shallow event, seismic activity at the epicenter of a deep event is characterized by
A) shorter P-S intervals and higher peak intensity
B) shorter P-S intervals and lower peak intensity
C) longer P-S intervals and similar peak intensity
D) longer P-S intervals and higher peak intensity
E) longer P-S intervals and lower peak intensity
应该如何定位呢？
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