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 kilo¬meters 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 con¬vincingly demonstrated their existence. Instead of comparing the arrival times of seismic waves at different locations, as earlier researchers had done, Wadati relied on a lime 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 be¬tween 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 sur¬face 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 low¬er 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.
1.The passage is primarily concerned with（ ）.
2.It can be inferred from the passage that if the S waves from an earthquake arrive at a given location long af¬ter the P waves, which of the following must be true?
3.The passage suggests that which of the following must take place in order for any earthquake to occur?
4.The author’s explanation of how deep events occur would be most weakened if which of the following were discovered to be true?
