10 cracking even under stress severe enough to deform

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(10) 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. (15) when the seismologist Kiyoo Wadati convincingly demonstrated their exis- tence. Instead of comparing the arrival times of seismic waves at different locations, as earlier researchers had (20) done, Wadati relied on a time differ- ence between the arrival of primary(P) waves and the slower secondary(S) waves. Because P and S waves travel at different but fairly constant (25) speeds, the interval between their arrivals increases in proportion to the distance from the earthquake focus, or initial rupture point. For most earthquakes, wadati dis- (30) covered, the interval was quite short near the epicenter; the point on the sur- face where shaking is strongest. For a few events, however, the delay was long even at the epicenter. Wadati saw (35) a similar pattern when he analyzed data on the intensity of shaking. Most earth- quakes had a small area of intense shaking, which weakened rapidly with increasing distance from the epicenter. (40) 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 (45) 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 (50) such quakes occur, given that mantle rock at a depth of more than 50 kilo- meters is too ductile to store enough stress to fracture? Wadati’s work sug-
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gested that deep events occur in areas (55) (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 (60) hence is less ductile and much more liable to fracture. ------------------------------------------------------------------------------------------------------- 30. The author’s explanation of how deep events occur would be most weakened if which of the following were discovered to be true? A. Deep events are far less common than shallow events. B. Deep events occur in places other than where crustal plates meet. C. Mantle rock is more ductile at a depth of several hundred kilometers than it is at 50 kilometers. D. The speeds of both P and S waves are slightly greater than previously thought. E. Below 650 kilometers earthquakes cease to occur. Answer: -------------------------------------------------------------------------------------------------------
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