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# Earthquake on the Playground

Adapted with permission from L.W. Braile and S.J. Braile and the Incorporated Research Institutions for Seismology (IRIS).

While seismologists conduct much of their research indoors at a computer, going outside to collect data is a vital aspect of their work, as in the activity below. Seismologists travel to some of the most remote places on Earth to install seismographs far away from the vibrations of human civilization. Here they can record the high-quality data necessary for them to conduct their research, including locating earthquakes.

Safety
Drawing compasses have sharp points, so make sure students use them properly. P-wave students should jog at a steady pace and should not sprint. Wear appropriate footwear and clothing — no flip-flops!

Materials

• 3 stopwatches
• Graph paper and pencils
• Meter stick/tape measure
• Drawing compasses
• Object, such as a cone, to mark the epicenter

Procedure

1. Outside with your classmates, simulate P and S waves by jogging (to model the faster “primary waves”) and walking (to model the slower “secondary waves”). Practice jogging and walking at a constant velocity to ensure consistency.

2. Decide which of you will serve as station timers. To determine the velocity of student P and S waves, station timers (students using stopwatches) measure the time it takes to jog and walk to a point at a distance of 10, 20, and 30 meters from the source. In other words, time how long it takes students, traveling a straight path, to arrive at the timers. To improve the accuracy of the P and S wave velocity measurements, complete several trials and take the average. Write down your findings.

3. Compute average travel times for the student P and S waves at various distances, and graph the data as a travel-time curve. By plotting how long it takes seismic waves to travel various distances, you’re modeling the way scientists create travel-time curves. You will need this curve for later in the activity.

4. Next, mark the corners of a 30-meter square space as well as the “epicenter,” the place within that square that will be the source of student P and S waves. Create “seismic stations,” where students with stopwatches stand at three corners of the square. See a diagram of this set-up.

5. Work together to assemble six student P and S waves at the epicenter marker, with S waves standing back-to-back and their associated P waves standing directly in front of each of them, each facing one of three seismic stations. Until the earthquake occurs, they represent stored potential energy in rocks.

6. Representing an earthquake, P-wave students jog outward from the epicenter toward the seismic stations. Start the stopwatches when the students start jogging, and stop them when they reach the seismic stations. Record the times. S-wave students walk outward from the epicenter toward the seismic stations. Start the stopwatches when the students start walking, and stop them when they reach the seismic stations. Record these times. Next, subtract the S-wave time from the P-wave time (S-P times) and record the time differences between P and S wave arrivals.

7. Use this measurement along with the travel time curves they created earlier to calculate the distance from each station to the epicenter, then combine all three distances to locate the epicenter by triangulation. Using the example travel-time curve online, if the S-P travel time is 0.5 seconds from a particular seismic station, then the distance from the station to the epicenter should be 10 meters. Find the distance to the other two seismic stations and record your findings. To triangulate the epicenter, use the graph found here. Put the point of the drawing compass on the first station and, using the scale, measure out the distance to the epicenter and draw a circle. Repeat this for the other two stations. Where the circles meet is the epicenter of the earthquake. See an example.

8. Once you’ve calculated the location of the playground earthquake, compare your result with the actual epicenter in the 30-meter-square space. Discuss possible reasons for any inaccuracies in determining the actual earthquake location.

For a longer version of the activity including pictures of the setup, see http://web.ics.purdue.edu/~braile/edumod/walkrun/walkrun.htm.

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