問題詳情

In most earthquakes the Earth’s crust cracks like porcelain. Stress builds up until afracture forms at 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’s mantle, wherehigh pressure makes rock so ductile that it flows instead of cracking, even under stress severeenough 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 seismologistKiyooWadati convincingly demonstrated their existence. Instead of comparing the arrivaltimes of seismic waves 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 S waves travel at different but fairly constant speeds, the interval between theirarrivals 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 waslong even at the epicenter. Wadati saw a similar pattern when he analyzed data on the intensityof shaking. Most earthquakes had a small area of intense shaking, which weakened rapidlywith increasing distance from the epicenter, but others were characterized by lower peakintensity, felt over a broader area. Both the P-S intervals and the intensity patterns suggestedtwo kinds of earthquakes: the more common shallow events, in which the focus lay just underthe 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 ofmore than 50 kilometers is too ductile to store enough stress to fracture? Wadati’s worksuggested that deep events occur in areas (now called Wadati-Benioff zones) where one crustalplate is forced under another and descends into the mantle. The descending rock issubstantially cooler than the surrounding mantle and hence is less ductile and much more liableto fracture.
73. 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 ofpredicting them
(D) discussing evidence for the existence of deep events and the conditions that allowthem to occur.

參考答案

答案：D
難度：適中0.46875
統計：A(6),B(6),C(22),D(30),E(0)

內容推薦

內容推薦