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Passage 79 (16/22)In most earthquakes the Earth’s crust crackslike porcelain. Stress builds up (build up: v.树立, 增进, 增大) until a fracture formsat a depth of a few kilometers and the crust 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 ofcracking, even under stress severe enough to deform it like putty. How can there be earthquakes at such depths?
That such deep events dooccur has been accepted only since 1927, when theseismologist Kiyoo Wadati convincingly demonstrated theirexistence. Instead of comparing the arrival times of seismic waves atdifferent locations, as earlier researchers had done. Wadati relied on a timedifference 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 inproportion to (in proportion to: adv.与...成比例) the distance from the earthquake focus (earthquakefocus: 震源), or rupture point.
For most earthquakes, Wadati discovered, theinterval was quite short near the epicenter (epicenter: n.震中, 中心), the point on the surfacewhere shaking is strongest. For a few events, however, the delay was long evenat the epicenter. Wadati saw a similar pattern when he analyzed data on theintensity of shaking. Most earthquakes had a small area of intense shaking,which weakened rapidly with increasing distance from the epicenter, but otherswere characterized by a lower peak intensity, felt over a broader area. Boththe P-S intervals and the intensity patterns suggested two kinds ofearthquakes: the more common shallow events, in which the focus lay just underthe epicenter, and deep events, with a focusseveral hundred kilometers down.
The question remained: how can such quakes occur, given (in view of (in view of: adv.考虑到, 由于): CONSIDERING “given what she knew about others'lives, how could she complain about her own? Marilyn F.”) that mantle rock at a depth of more than 50 kilometers is tooductile to store enough stress to fracture? Wadati’s work suggested that deepevents occur in areas (now called Wadati-Benioff zones) where one crustal plateis forced under another and descends into the mantle. The descending rock issubstantially cooler than the surrounding mantle and hence is less ductile andmuch more liable to fracture.
1. Thepassage is primarily concerned with
(A) demonstrating why the methods of earlyseismologists were flawed
(B) arguing that deep events are poorlyunderstood and deserve further study
(C) defending a revolutionary theory aboutthe causes of earthquakes and methods of predicting them
(D) discussing evidence for the existenceof deep events and the conditions that allow them to occur（C）
(E) comparing the effects of shallow eventswith those of deep events
这道主旨题是绝对不可能选C的
1. 文章通篇都没有提predict的问题，这是个典型的无关信息
2. 我上面highlight的部分都充分证明了文章的重点是在讲deep event
文章根本不是在讲earthquake的成因，只是在讲KW用了一种方法来探求deep event的存在
期待反驳 |
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发表于 2010-5-4 22:54:17
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