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Plate Tectonics and Earth's Dynamics

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Lesson Focus

Earthquake! Where Was It?

Earth & Space Science

Using the time interval between the arrival of primary waves and secondary waves for three different seismograph stations, students determine the location of the epicenters of two earthquakes.

Now You Know

After completing this tutorial, you will be able to complete the following:

• After completing this Activity Object, students will be able to:
• Explain that earthquakes produce both primary waves and secondary waves, that primary waves travel faster than secondary waves, and that the time interval between the arrivals of the primary and secondary waves can be used to determine the distance to the earthquake epicenter.
• Solve for distance, using an “interval between arrival times vs. distance to the epicenter” graph.
• Explain that a minimum of three seismograms are needed to locate the epicenter of an earthquake.

Everything You'll Have Covered

Earthquakes occur when rock breaks, suddenly releasing stored up energy. The spot where the rock first breaks during an earthquake is called the focus. Because the focus is usually deep underground, scientists often talk about the point on the Earth's surface that is directly above the focus instead. This point is called the epicenter.

The energy that is released travels through the Earth in the form of primary (P) and secondary (S) seismic waves. Every earthquake generates both types of waves. The speed of seismic waves depends on which type of wave (P or S), and on the type of material the waves are traveling through, but not on the size of the earthquake. Because of differences in how P and S waves travel in the Earth, P waves are approximately twice as fast as S waves. Therefore, they travel the distance from the epicenter to a seismograph station in less time than the S waves take.

The amount of time that passes between when the P waves arrive and when the S waves arrive can be used to determine how far away a person is from the earthquake epicenter. To illustrate how this works, imagine a race between two students (this also makes a good student activity, either to introduce the Activity Object or to overcome student confusion). The students leave the starting line at the same time, but one is jogging (or walking fast) while the other just walks at a normal pace. An observer close to the starting line sees the jogger slightly ahead of the walker. Another observer, farther away, sees the jogger farther ahead of the walker. The farther away the observer, the more of a lead the jogger has, and the size of the lead can be used to determine how far the observer is from the starting line. In an earthquake, the epicenter is the starting line, the seismograph stations are the observers, and the P and S waves are the jogger and the walker, respectively.

With data from just one seismograph station, scientists can determine the distance to the epicenter, but they cannot say exactly where the epicenter is, because they don't know the direction. Drawing a circle around the station, with a radius equal to the distance from the station to the epicenter, shows all of the possible locations. Points inside the circle are too close to the station, while points outside the circle are too far away.

Plotting the points that are the correct distance from a second seismograph station gives a second circle, which intersects the first circle in two points. These are the only points that are the correct distance from both stations. To narrow the epicenter's location down to a single point requires data from a third seismograph station.

Tutorial Details

 Approximate Time 25 Minutes Pre-requisite Concepts Earthquake, seismic wave, seismogram, seismograph Course Earth & Space Science Type of Tutorial Concept Development Key Vocabulary plate, tectonics, plate tectonics