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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( ).

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

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 ( ).

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

3.The method used by Wadati to determine the depths of earthquakes is most like which of the following?

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.

4.The passage supports which of the following statements about the relationship between the epicenter and the focus of an earthquake?

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

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