GWD 24 Earthquake问题

rabbityvonne

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 initial 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 (45) common shallow events, in which the focus lay just under the epicenter, and deep events, with 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.

TN-10-Q36:

Q36：It can be inferred from the passage that if the S waves from an earthquake arrive at a given location long after the P waves, which of the following must be true?

A.      The earthquake was a deep event.

B.       The earthquake was a shallow event.

C.      The earthquake focus was distant.

D.      The earthquake focus was nearby.

E.    The earthquake had a low peak intensity

答案是C,我选的是对的，但是不知道为什么A不对，觉得逻辑上还没理顺，也直接导致后面Q37做错

Q37：The method used by Wadati to determine the depths of earthquakes is most like which of the following?

A.      Determining the depth of a well by dropping stones into the well and timing how long they take to reach the bottom

B.       Determining the height of a mountain by measuring the shadow it casts at different times of the day

C.      Determining the distance from a thunderstorm by timing the interval between the flash of a lightning bolt and the thunder it produces

D.      Determining the distance between two points by counting the number of paces it takes to cover the distance and measuring a single pace

E.       Determining the speed at which a car is traveling by timing how long it takes to travel a known distance

答案是C

这题真的没明白

谢谢大家了！

gaoliang641

这里面有两个概念是要明确的：一个是focus，一个是epiccenter，文章怕我们不懂，还分别对两个作了解释：
focus: 是地震的发源地。 在地下。
epiccenter：是地震波传到地面时候的人能够感受到的最强震点的地方。

P和S波同时从Focus点发出，一个是快的，一个是慢的。 在epiccenter, 我们先感受到P波，然后再感受到S波。
如果是深层的地震，感受到P和S的间隔就长。 如果是浅层地震，感受到P和S的时间就短。

同理，闪电就是P波，声音就是S波。

rabbityvonne

解释的很清楚，谢谢！