1
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This must be a seismograph. I recognize it from TV. I wonder if Mr. Lyle is here.

2
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Oh, hi, Mr. Lyle.

3
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Hi, girls.

4
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We're the Treehouse Detectives. We called you about an earlier tremor.

5
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Now we need to know if you've recorded anything on your seismograph today.

6
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As a matter of fact, we did, and here it is right here.

7
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Wow! Was that an earthquake? Can you really tell from that reading?

8
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We're not sure at this time. We're analyzing the data because not all detected tremors are related to earthquakes.

9
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Analyzing your data is very important in the scientific process.

10
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In this area, most of the tremors we detect are relatively small, travel a short distance, and generally occur between 8 a.m. and 5 p.m.

11
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This suggests that they're caused by human conditions.

12
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We learned that there are earthquakes because there are breaks in the Earth's crust that are moving along fault lines.

13
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Are there faults in Virginia?

14
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Yes, there are numerous faults in Virginia, but not all of them are active faults.

15
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Wow! Does Virginia have a lot of earthquakes?

16
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Not really. In the last 30 years, there have been more than 150 earthquakes in Virginia.

17
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But these are low-intensity events seldom felt by people.

18
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Where do most Virginia earthquakes occur?

19
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Most seem to occur in the central and western part of the state.

20
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We don't live near that part of the state.

21
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Hmm. What other things could cause a reading on your seismograph?

22
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Anything that shakes the ground can be recorded.

23
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Here's a seismogram, or paper recording, of that construction blast from the other day.

24
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It looks so small, but it created such a big boom.

25
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Our seismometer measuring instrument is very sensitive.

26
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This is what automobile traffic and even storm waves from the beach look like.

27
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That's pretty neat.

28
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The seismometer is buried 70 meters below us.

29
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It is so sensitive that we had to put it in a very quiet place.

30
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That's way down there.

31
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Would you like to see a seismogram of a confirmed earthquake?

32
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Yes.

33
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What are the little square marks?

34
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How can you figure anything out?

35
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You guys have lots of questions.

36
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Well, every square ticks off a minute.

37
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Oh, so you can tell when an earthquake happens and how long it lasts.

38
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That's correct.

39
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To most people, these look like squiggly lines, but when I look at them, I see S and P waves.

40
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Like sound waves.

41
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They are somewhat like sound waves.

42
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These are vibrations in the earth.

43
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That doesn't look at all like the tremor we felt.

44
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Does that mean it wasn't an earthquake?

45
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Not necessarily.

46
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A local earthquake could look very different from this.

47
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Let's put one of these in our treehouse.

48
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Then we can measure the next tremor.

49
00:02:22,000 --> 00:02:24,000
I think Dr. D is down the hall building a seismograph.

50
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Dr. D, a seismograph?

51
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We're there.