In most earthquakes theEarth’s crust cracks like porcelain, Stress builds up until a fracture forms ata depth of a few kilometers and the crust (5) slips to relieve the stress. Someearthquakes, however, take place hundreds of kilometers down in the Earth’smantle, 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 canthere be earthquakes at such depths? That such deep events do occur has beenaccepted only since 1927 when the seismologist Kiyoo Wadati convincinglydemonstrated their existence. Instead of comparing the arrival times of seismicwaves at different locations, as earlier researchers had done, Wadati relied ona time difference between the arrival of primary(P) waves and the slower secondary(S) waves. Because P and Swaves travel at different but fairly constant speeds, the interval between theirarrivals increases in proportion to the distance from the earthquake focus, orinitial rupture point.
For most earthquakes,Wadati discovered, the interval was quite short near the epicenter; the pointon 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: howcan such quakes occur, given that mantle rock at a depth of more than 50kilometers is too ductile to store enough stress to fracture? Wadati’s work suggestedthat deep events occur in areas (now called Wadati-Benioff zones) where onecrustal plate is forced under another and descends into the mantle. Thedescending rock is substantially cooler than the surrounding mantle and henceis less ductile and much more liable to fracture.
Q14 According to the passage, the majority of compensation commites put the greatest emphasis on which of the following when determining compensation for their executives? A.Long-term corporate performance B.The threat of government regulation C.Salaries paid to executives of comparable corporations D.The probable effect the determination will have on competitors E.The probable effect the economic climate will have on the company
-------------------------------------------------------------------- Q15 The passage suggests which of the following about government legislation requiring that corporate boards undergo reform? A.Such legislation is likely to discourage candidates from joining corporate boards. B.Such legislation is likely to lead to reduced competition among companies. C.The performance of individual companies would be affected by such legislation to a greater extent than would the economy as a whole. D.Such legislation would duplicate initiatives already being made by corporate boards to improve their own performance. E.Corporate boards themselves could act to make such legislation unnecessary. -------------------------------------------------------------------- Q16 Which of the following best describes the organization of the passage? A.A problem is acknowledged, the causes are explored, and a solution is offered. B.A question is raised, opposing points of view are evaluated, and several alternative answers are discussed. C.A means of dealing with a problem is proposed, and the manner in which a solution was reached is explained. D.A plan of action is advanced, and the probable outcomes of that plan are discussed. E.Two competing theories are described and then reconciled.
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发表于 2012-4-10 18:23:40
