本篇文章共分 6 段，針對地震波中的三種不同性質波動分別說明各自的特性與同、異之處，並詳細分析這三種波動對於地表物體的影響。

本篇考題英文原文與對應之中文翻譯整理如下。練習作答解題時若有對語意不清楚之處，請仔細查閱對照，以提升閱讀理解能力。

Seismic Waves 地震波

1. 地震波的分類

   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.

   地震波—由地震產生的能量波—使科學家能夠確定地球內部區域的位置、厚度和屬性。地震波是在巖體突然受到干擾時產生的，例如當它們破裂或斷裂時。振動從擾動的源頭向各個方向擴散，以不同的速度穿過化學成分和物理性質不同的地殼和內部。這些波的主要類別是一級、二級和表面。所有這三種類型的波都記錄在一個叫做地震儀的儀器上。

2. 一級波的特性

   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 P-wave transmission will differ in materials of different density and elastic properties.

   一級波，或稱 P 波，是三種波中速度最快的，因此在發生地震後最先到達地震儀站。它們以每秒 4 至 5 公里的速度穿過地殼上部，但在地殼底部附近，它們以每秒 6 或 7 公里的速度前進。在這些一級波中，能量的脈衝是以連續的壓縮和膨脹來傳輸的，與波本身的傳播方向平行。因此，在地震期間，一個特定的岩石段被推動到它的鄰近岩石段，並反彈回來。鄰近岩石撞擊下一個粒子並反彈，隨後的粒子繼續運動。震動能量是一種手風琴式的推拉運動，可以通過固體、液體和氣體傳播。當然，P 波的傳輸速度在不同密度和彈性特性的材料中會有所不同。

3. 二次波的特性

   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.

   二次波，或稱 S 波，比 P 波的傳播速度慢 1 至 2 公里每秒。與 P 波的運動不同，二次波的岩石振動與能量的傳播方向成直角。這種類型的波很容易通過將一段繩子綁在鉤子上，然後搖動自由端來證明。繩子上會出現一系列的起伏，並向鉤子方向移動，也就是沿著傳播的方向。然而，沿著繩子的任何特定粒子將在垂直於傳播方向的方向上上下移動。正是因為它們的運動更複雜，所以 S 波比 P 波傳播得更慢。它們是到達地震儀站的第二組震盪。與 P 波不同，次級波不會穿過液體或氣體。

4. 體波的共通性

   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.

   P 波和 S 波有時也被稱為體波，因為它們能夠深入到我們星球的內部或主體內。體波在彈性較大的岩石中傳播速度較快，因此，當它們向下進入地球內部彈性較大的區域時，其速度會穩步增加，然後當它們開始向地球表面上升時，其速度會降低。當體波侵入不同彈性的岩石時，速度的變化導致了波的彎曲或折射。許多小的折射導致體波在地球上呈現出彎曲的旅行路徑。

5. 體波與表面波

   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.

   體波不僅受到折射，而且還可能從密集的岩石層表面部分反射出來，其方式與光從拋光的表面反射出來的方式相同。許多因素影響體波的行為。體波通過的岩石的溫度升高會導致速度下降，而約束壓力的增加會導致波速相應增加。在一個不存在剛性的流體中，S 波無法傳播，P 波則明顯減慢。表面波是穿過地球外殼的大運動波。它們的運動模式類似於將小石子扔進池塘中心時引起的波浪。每當 P 波或 S 波從內部出現時干擾地球表面，它們就會發生。

6. 表面波的影響

   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.

   表面波是最後到達地震儀站的。它們通常是影響人口稠密地區的地震所造成的破壞的主要原因。造成這種破壞的原因是，表面波通過地球外部的薄層區域，因此其能量無法迅速散佈到體波所穿越的大量岩石中。