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.
Q36：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. 我觉得c肯定对，但是为什么a错呢？
文章中说" interval between their arrivals increases in proportion to the distance from the earthquake focus, or initial rupture point" 这句话说明,选取的测试点,离震中越远,"interval"就越大.
看选项,根据题干中给的信息:"the S waves from an earthquake arrive at a given location long after the P waves"无法判断出,这样的地震到底是哪一种地震.因为,不管是deep还是shadow,只要你选取的测试点(location)远离震中,"interval"肯定是大的. 直接排除ABE.
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发表于 2010-6-27 22:01:47
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