In most earthquakes the Earth’s crust ..-阿摩線上測驗

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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 KiyooWadati 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 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.
【題組】75. 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.

教甄◆英文科- 104 年 - 104 臺北市市立國民中學教師聯合甄選：英語科#22006

答案：D
難度： 適中

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Joy :) 考上國小英語高三下 (2015/06/22)
D->深層和淺層地震都是由震央所開始的。

地震波主要分為兩種，一種是表面波，一種是實體波。表面波只在地表傳遞，實體波能穿越地球內部。

實體波（Body Wave）：在地球內部傳遞，又分成P波和S波兩種。

P波：P代表主要（Primary）或壓縮（Pressure），為一種縱波，粒子振動方向和波前進方平行，在所有地震波中，前進速度最快，也最早抵達。P波能在固體、液體或氣體中傳遞。

S波：S意指次要（Secondary）或剪力（Shear），前進速度僅次於P波，粒子振動方向垂直於波的前進方向，是一種橫波。S波只能在固體中傳遞，無法穿過液態外地核。

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Mac Liang 高一上 (2017/04/29)
...the more common shallow events, in which the focus lay just under the epicenter, and deep events, with a focus several hundred kilometers down.

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In most earthquakes the Earth’s crust ..-阿摩線上測驗

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