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Hi, we're from Mr. Darrell Ransom's 3rd grade class at Roberts Park Elementary School in North of Virginia.

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Hi, we're the Treehouse Detectives and we see you're doing an activity to find out where an earthquake occurred.

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Could you tell us something about it?

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Sure. Our mentors from the Society of Women Engineers helped us with the activity called triangulation.

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It must have something to do with threes.

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Yes, we learned that in order to find the origin or epicenter of an earthquake, we needed data from three different seismic stations.

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Did you need any special materials?

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No, just a world map, a seismogram, a graph, and a drawing compass.

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What did you do next?

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First, we looked at the seismic recordings and determined when the S and P waves each arrived at the seismic stations.

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How could you tell which one was which?

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We know the P waves travel faster and arrive first.

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We then found the difference between the two times in seconds.

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For example, at the Denver, Colorado station, the P waves arrived at 10 hours and 16 minutes and the S waves arrived at 10 hours, 18 minutes, and 30 seconds.

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I know, the difference was 2 minutes and 30 seconds.

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That's correct. We then used the graph to mark off 2 minutes and 30 seconds along the edge of the piece of paper.

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We slid the paper along the graph between the S waves and the P waves to find the distance in kilometers that the waves had traveled.

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How far did the waves travel?

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About 1600 kilometers. After finding the distance the waves traveled, we located the city on the map.

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We used the map scale and our drawing compass to measure that distance.

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We placed the point of our compass on the city and drew a circle.

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Why a circle?

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We know that the earthquake occurred 1600 kilometers from Denver, but that could be 1600 kilometers in any direction.

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So we used 1600 kilometers as our radius, and the earthquake occurred somewhere along the circle.

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How do you pinpoint the location?

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We repeated the steps with the next seismic recording for Terre Haute, Indiana.

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But the two circles intersect at two different points. How can you tell which one is the epicenter?

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That's why you need a third seismic station.

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Once all three stations are plotted on the map, you find where the three circles intersect.

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The intersection is the approximate epicenter of the earthquake.