Seismic waves-energy waves produced by earthquakes-permit scientists to determine the location, thickness, and properties of Earth's internal zones. They are generated when rock masses are suddenly disturbed, such as when they break or rupture. Vibrations spread out in all directions from the source of the disturbance, traveling at different speeds through parts of Earth's crust and interior that differ in chemical composition and physical properties. The principal categories of these waves are primary, secondary, and surface. All three types of waves are recorded on an instrument called a seismograph.

Primary waves, or P-waves, are the speediest of the three kinds of waves and therefore the first to arrive at a seismograph station after there has been an earthquake. They travel through the upper crust of Earth at speeds of 4 to 5 kilometers per second, but near the base of the crust they speed along at 6 or 7 kilometers per second. In these primary waves, pulses of energy are transmitted as a succession of compressions and expansions that parallel the direction of propagation of the wave itself. Thus, a given segment of rock set in motion during an earthquake is driven into its neighbor and bounces back. The neighbor strikes the next particle and rebounds and subsequent particles continue the motion. Vibrational energy is an accordion-like push-pull movement that can be transmitted through solids, liquids and gases. Of course, the speed of Pwave transmission will differ in materials of different density and elastic properties.

Secondary waves, or S-waves, travel 1 to 2 kilometers per second slower than do P-waves. Unlike the movement of P-waves, rock vibration in secondary waves is at right angles to the direction of propagation of the energy. This type of wave is easily demonstrated by tying a length of rope to a hook and then shaking the free end. A series of undulations will develop in the rope and move toward the hook-that is, in the direction of propagation. Any given particle along the rope, however, will move up and down in a direction perpendicular to the direction of propagation. It is because of their more complex motion that S-waves travel more slowly than P-waves. They are the second group of oscillations to arrive at a seismograph station. Unlike P-waves, secondary waves will not pass through liquids or gases.

Both P- and S-waves are sometimes also termed body waves because they are able to penetrate deep into the interior or body of our planet. Body waves travel faster in rocks of greater elasticity, and their speeds therefore increase steadily as they move downward into more elastic zones of Earth's interior and then decrease as they begin to make their ascent toward Earth's surface. The change in velocity that occurs as body waves invade rocks of different elasticity results in a bending or refraction of the wave. The many small refractions cause the body waves to assume a curved travel path through Earth.

Not only are body waves subjected to refraction, but they may also be partially reflected off the surface of a dense rock layer in much the same way as light is reflected off a polished surface. Many factors influence the behavior of body waves. An increase in the temperature of rocks through which body waves are traveling will cause a decrease in velocity, whereas an increase in confining pressure will cause a corresponding increase in wave velocity. In a fluid where no rigidity exists, S-waves cannot propagate and P-waves are markedly slowed.

Surface waves are large-motion waves that travel through the outer crust of Earth. Their pattern of movement resembles that of waves caused when a pebble is tossed into the center of a pond. They develop whenever P- or S-waves disturb the surface of Earth as they emerge from the interior. Surface waves are the last to arrive at a seismograph station. They are usually the primary cause of the destruction that can result from earthquakes affecting densely populated areas. This destruction results because surface waves are channeled through the thin outer region of Earth, and their energy is less rapidly scattered into the large volumes of rock traversed by body waves.

Questions:
question_stem: Paragraph 2 indicates that compared to P-waves traveling through Earth's upper crust, Pwaves traveling near the base of Earth's crust
choices: tag: A
text: move at higher speeds

tag: B
text: are more likely to reach a seismograph station

tag: C
text: are less likely to cause an earthquake

tag: D
text: are less likely to undergo compression
answer_key: A
explanation: 正确答案 A。题干 indicate 暗示，相比于穿过 upper crust 的 P 波，在 base of crust 的 p 波 如何？A 正确，原文有线索 They travel through the upper crust of Earth at speeds of4 to 5 kilometers per second,but near the base of the crust they speed along at6 or 7kilometers persecond.故 A 正确。B 错更容易到 station；C 错不容易引起地震；D 错不可能经历压缩

question_stem: According to paragraph 3, the main reason that S-waves travel more slowly than P-waves do is that S-waves
choices: tag: A
text: lack a consistent direction

tag: B
text: do not spread through gases or liquids

tag: C
text: are produced by the movement of P-waves

tag: D
text: move in a more complex way than P-waves
answer_key: D
explanation: 正确答案 D。题干“S 比 P 慢的主要原因是，S 波……”；A 错，缺少一致的方向，没提；B 错，不能穿过气体和液体，没提到；C 错，由 P 波的移动产生，没提到；D 正确，可定位原文 It  is because of their more complex motion that S-waves travel more slowly than Pwaves

question_stem: In paragraph 3, the author presents the characteristics of S-waves by
choices: tag: A
text: comparing and contrasting S-waves and P-waves

tag: B
text: arguing that S-waves mostly behave like P-waves

tag: C
text: introducing new research findings about how S-waves move

tag: D
text: contrasting past and present understandings of how S-waves behave
answer_key: A
explanation: 正确答案 A。题干“作者通过什么方法展示 S 波的特点”；A 正确，把 S 与 P 进行比较，原文中，反复出现 unlike 来表现两者的不同，故 A 正确；B 错误，argue 通过驳斥/辩驳证明了 S 与 P 很相似，文章中没有出现 argue，因此 B 错；C 错，new research 没提到；D 错，对比过去和现在完全没有；

question_stem: The word “invade” in the passage is closest in meaning to
choices: tag: A
text: break

tag: B
text: enter

tag: C
text: escape from

tag: D
text: pass over
answer_key: B
explanation: 题干“invade”意为“入侵”；选项 A 破坏；选项 B 进入；选项 C 逃离；D 路过

question_stem: Which of the following can be inferred about body waves from paragraph 4?
choices: tag: A
text: P-and S-waves are restricted to Earth's deep interior.

tag: B
text: The refraction of a body wave causes the wave to change its speed.

tag: C
text: A body wave changes its velocity and direction many times as it passes through Earth.

tag: D
text: Body waves become more elastic as they move deeper into Earth and less elastic as they move toward Earth's surface.
answer_key: C
explanation: 正确答案 C。题干“关于体波能推断出什么”；A 错，P 和 S 受限于地球深处，be restricted to 错误；B 错，体波的反射引起速度的改变；C 正确，体波改变速度和方向，是因为它在地球不同的层次，原文线索 Body waves travel faster in rocks of
greater  elasticity,and  their  speeds  therefore  increase  steadily  as  theymove  downward  into
more elastic zones of Earth's interior and then decrease as they begin to make their ascent
toward Earth's surface.D 错，深便可塑，表层则不可塑，文章没提

question_stem: According to paragraph 5, body waves travel more slowly through a mass of rock if the rock is
choices: tag: A
text: at a high temperature

tag: B
text: under high pressure

tag: C
text: covered with fluid

tag: D
text: not particularly dense
answer_key: A
explanation: 正确答案 A。题干“体波慢如果岩石……”A 正确，原文 An  increase  in the temperature
ofrocks  through  which  bodywaves  are  traveling  will  cause  a  decrease  invelocity,意为“穿过高温岩石则速度慢”，故 A 正确，其他选项皆未提及

question_stem: According to paragraph 6. why do surface waves create much of the destruction caused by an earthquake?
choices: tag: A
text: The energy of surface waves diminishes relatively slowly as they travel through the outer crust of Earth.

tag: B
text: The regions in which surface waves tend to be strongest are areas with unusually large human populations.

tag: C
text: Surface waves travel at a more rapid rate than do P- or S-waves.

tag: D
text: Surface waves reach locations on the surface of Earth sooner than do the body waves coming from that earthquake.
answer_key: A
explanation: 正确答案 A。题干“为什么表面波创造毁灭”；A 正确，能量减少相对更慢因为穿过地壳外层，可定位原文 This destruction results because surface waves are channeled through the thin outer region of Earth,and their energy is less rapidly scattered into the large volumes ofrock traversed by
body waves.B 错，strongest 极端表达；C 错，虚假比较，表面波比 P 和 S 快；D 错虚假比较，到达速度 sooner 没提

question_stem: According to paragraph 6, surface waves differ from body waves in which of the following ways?
choices: tag: A
text: Surface waves are more regular in their motion than are body waves.

tag: B
text: Surface waves are generated later than are body waves.

tag: C
text: Surface waves travel through larger volumes of rock than do body waves.

tag: D
text: Surface waves lose speed much faster than do body waves.
answer_key: B
explanation: 正确答案 B。题干“表面波不同于体波，在哪方面”；A 错，虚假比较，more regular 没提及；B 正确，可定位至 Surface waves are the last to arrive at a seismograph station.故 B 正确；C 错，体积大小没提及；D 错丢失速度更快 ，也没提及