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（ ）.
2.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 （ ）.
3.The method used by Wadati to determine the depths of earthquakes is most like which of the following?
4.The passage supports which of the following statements about the relationship between the epicenter and the focus of an earthquake?
问题1选项
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 the evidence for the existence of deep events and the conditions that allow them to occur
问题2选项
A.shorter P-S intervals and higher peak intensity
B.shorter P-S intervals and lower peak intensity
C.longer P-S intervals and lower peak intensity
D.longer P-S intervals and higher peak intensity
问题3选项
A.Determining the distance from a thunderstorm by thing the interval between the flash of a lightning bolt and thunder it produces.
B.Determining the height of a mountain by measuring the shadow it casts at different times of the day.
C.Determining the depth of a well by dropping stones into well and timing how long they take to reach the bottom.
D.Determining the distance between two points by counting the number of paces if takes to cover the distance and measuring a single pace.
问题4选项
A.P waves originate at the focus and S waves originate at the epicenter.
B.In deep events the epicenter and the focus are reversed.
C.In shallow events the epicenter and the focus coincide.
D.In both deep and shallow events the focus lies beneath the epicenter
第1题:
主旨大意题。题目是问文章主要是关于什么的。通过文章内容可知，文章主要是向读者介绍deep events的知识。Wadat通过对P waves与S waves的研究发现deep events的存在。不同类型的地震其P-S intervals是不同的，因此有了shallow events与deep events的区别。文章也告诉了读者deep events是如何发生的。所以D选项“讨论深层事件存在的证据以及允许它们发生的条件”正确。
第2题:
细节事实题。根据文章第二段最后一句“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. 由于P波和S波以不同但相当恒定的速度传播，它们到达的间隔与震源或破裂点的距离成正比。”以及第三段最后一句 “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. P-S间隔和强度模式都表明有两种地震：一种是更常见的浅层地震，震源就在震中下方；另一种是震源在几百公里以下的深层地震”可知与地震活动在一个浅的中心事件，地震活动的中心事件的特点是“通过更接近的纵横间隔波和更高的峰值强度表现的”，D选项正确。
第3题:
推理判断题。题目提问Wadati用来确定地震深度的方法最像下列哪一种？distant用词不准。根据原文“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 the earthquake focus-or rupture point. 因为P波和S波以不同但相当恒定的速度传播，它们到达的距离与震源（震源或破裂点）的距离成正比。所以，只要地震的S波在P波之后很长时间才到达一个给定的位置肯定是真的，所以A选项“用闪电与雷暴之间的间隔来确定雷暴与雷暴之间的距离”符合题意。
第4题:
细节事实题。本题是问文章支持下列哪一个陈述关于中心和地震的集点之间的关系。根据文章内容以及最后一段“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. 问题依然存在：考虑到50公里以上深处的地幔岩石韧性太强，无法储存足够的应力而导致断裂，这样的地震怎么会发生呢？Wadati的工作表明，深部事件发生在一个地壳板块被压在另一个地壳板块下并进入地幔的区域（现在称为Wadati-Benioff带）。下降的岩石比周围的地幔要冷得多，因此韧性较差，更容易断裂。”可知D“无论是深地震还是浅地震，震源都位于震中下方”选项。
