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请Jeffxf加入地震那一篇的额外四题 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 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.
1. The passage is primarily concerned with
(A) demonstrating why the methods of early seismologists were flawed
(B) arguing that deep events are poorly understood and deserve further study
(C) defending a revolutionary theory about the causes of earthquakes and methods of predicting them
(D) discussing evidence for the existence of deep events and the conditions that allow them to occur
(E) comparing the effects of shallow events with those of deep events
2. 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
3. It can be inferred from the passage that if the S waves from an earthquake arrive at a given location long after the P waves, which of the following must be true?
(A) The earthquake was a deep event.
(B) The earthquake was a shallow event.
(C) The earthquake focus was distant.
(D) The earthquake focus was nearby.
(E) The earthquake had a low peak intensity.
4. The method used by Wadati to determine the depths of earthquakes is most like which of the following?
(A) Determining the depth of a well by dropping stones into the well and timing how long they take to reach the bottom
(B) Determining the height of a mountain by measuring the shadow it casts at different times of the day
(C) Determining the distance from a thunderstorm by timing the interval between the flash of a lightning bolt and the thunder it produces
(D) Determining the distance between two points by counting the number of paces it takes to cover the distance and measuring a single pace
(E) Determining the speed at which a car is traveling by timing how long it takes to travel a known distance
answer: CEAC
第一题比较有争议性，有人说D,但是答案是给C,希望大家一起讨论 | 
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发表于 2007-4-23 08:12:00
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