(单词翻译:单击)

Good morning, class. Today Professor Liu will give us a lecture about earthquakes. Welcome.

Thank you. As you know, some earthquakes are not powerful enough to be felt on the Earth's surface while in others, people can, indeed, feel the earth shaking. And of course, there might be violent movement, often with catastrophic results. What we feel during an earthquake or huge explosion are a series of waves called seismic waves.

In an earthquake, you might initially feel some minor trembling, then perhaps violent shaking, and then some more tremors. The reason why you feel these different types of movement is because there are different kinds of seismic waves. There are two major types of seismic waves: body waves and surface waves.

The waves that you feel first are body waves. These mainly travel deep through the Earth's interior rather than at the surface. Some of them travel very fast, and that's why you feel them first. But because they travel deep inside the Earth, they're felt only as tremors. There are two types of body waves. The first type of body wave is the P wave or primary wave. It's thefastest kind of seismic wave. These waves can move through any type of material, including rock and fluids. Because of this and because of their speed, these waves are the first to reach the Earth's surface. You likely won't even hear or feel these waves, but animals may hear them. They're not strong enough to cause significant damage. The other type of body wave is the S wave or secondary wave. Because S waves travel at half the speed of P waves, you won't feel them in the beginning.

Unlike body waves, surface waves move at or near the Earth's surface. These waves are similar to ripples on water. They are usually the most destructive type of seismic wave because of their lower frequency, larger amplitude, and longer duration. There are two types of surface waves. The first kind of surface wave is called a love wave. It's the fastest surface wave. The other kind of surface wave is the Rayleigh wave. It rolls along the ground just like a wave rolls across a lake or an ocean. As a result, it moves the ground up and down and side to side in the same direction that the wave is moving. Most of the shaking felt from an earthquake is due to the Rayleigh wave, which can be much larger than the other waves. They are slower than body waves, and sometimes cars might appear to be moving up and down with these waves.

Question 16. Why do we feel different types of movement in an earthquake?

Question 17. Why do we feel body waves only as tremors?

Question 18. What is the most destructive type of seismic wave?

Question 19. Which seismic wave brings us shaking felt from an earthquake?