4 In most earthquakes the Earth’s crust cracks like porcelain. Stress builds up until a fracture forms at the 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 arri- val 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 a rupture point. For most earthquakes, Wadati discovered, the interval was quite short near the epicenter, the point on the surface where shaking is the strongest. For a few events, however, the delay was long enough 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 the 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 flexible 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. The passage is primarily concerned with______.

A. demonstrating why the methods of early seismologists were flawed
B. defending a revolutionary theory about the causes of earthquakes and methods of predicting them
C. discussing the evidence for the existence of deep events and the conditions that al- low them to occur
D. comparing the effects of shallow events with those of deep events