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Hey Dr. D. Hi guys. Good to see you here in San Francisco, California. I understand from

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the email that you want to know more about earthquake waves. Yes. I put together some

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demonstrations for you. Great. Let's talk about three different kinds of waves. The

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first two are S and P waves. They're called body waves because they travel through the

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body of the earth. Mr. Lyle showed the other triage detectives those on the seismogram.

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P stands for primary or compressional waves and S is for secondary or shear waves. P waves

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make the earth vibrate back and forth along the direction of motion. Let me show you with

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this slinky over here. Those don't look like the waves I'm used to seeing, like ocean waves.

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The wave you're used to seeing has up and down or side to side motion. This is like

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the shear wave or S wave. It looks like this. Another important difference between S and

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P waves is that P waves travel faster than S waves. Almost twice as fast. I guess that

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means that P waves will always arrive first. That's right and the difference in time from

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the arrival of the S and P waves help us find the epicenter of the earthquake. What's an

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epicenter? It's the point on the earth directly above the focus or the point where the earthquake

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originates. Have you ever heard about counting the seconds between seeing the lightning

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and hearing the thunder? I have but I've never done it. Well it works because light and sound

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travel at different speeds. For example, when you see the lightning start counting. 1001,

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1002, 1003. The sound arrived about three seconds after the lightning. That means the

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lightning is about one kilometer away. For earthquakes, first you feel a P wave and then

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seconds later you feel a more powerful S wave. For local earthquakes, every second that you

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count means you're about eight kilometers further away from the epicenter. I don't think

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I'd be counting the seconds if I was in an earthquake. I think I would either. The third

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and slowest of these waves, the surface wave, is the most destructive of them all. Here

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in California they have to build structures to withstand these dangerous waves. How do

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they know how to do that? One way is for engineers to use shaky tables like these, only much

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bigger, to test their designs. Let's try it out. I'll bet my structure can't withstand

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the greatest earthquake. We'll see who's the best engineer.

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Yes! I guess we have a little bit more to learn about the power of earthquakes.

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Dr. D, didn't you say we could experience an earthquake here at the museum?

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You need to go see my friend, Dr. Tang.