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Modelling Waves

In this activity, students learn about the differences between S-waves and P-waves, collectively known as body waves, which move through the inner rock part of the earth and are created during earthquakes.

Seismic waves are the energy that radiates out from the centre of an earthquake (the epicentre). They consist of body waves (P- and S-waves) and surface waves (L-waves).

P-waves (primary or compressional waves) are longitudinal or compression waves, able to move through solids, liquids, and gases at speeds ranging between 300-5,000 metres per second. As they travel through rock, they move particles back and forth in the same direction that the wave is moving. When P-waves strike an object they push and pull the object. It's like a train engine bumping into a railroad car, which then bumps into another. This movement continues through the whole length of the train. In solids, these waves travel twice as fast as S-waves. For example, P-waves move at 330 m/s in air, 1,450 m/s in water, and 5,000 m/s in granite.

P-waves are initiated and transmitted when a sudden release of energy compresses rocks near the earthquake focus. As the waves continue on to compress adjacent rocks the initially compressed rocks expand elastically past their original volume. Then they return to their original shape.

S-waves (secondary waves or shear waves) occur just after P-waves and travel more slowly. They move the ground in an up-and-down motion, similar to the way waves move in the ocean As these waves move, they displace rock particles perpendicular (i.e. up and down) from the wave direction. They can only move through solids.

Objectives

  • Explain the difference between s-waves and p-waves.

  • Use a slinky to model s-waves and p-waves.

Materials

  • Per Demo or Group:
    slinky

Key Questions

  • Which demonstration is like a P-wave?
  • Which demonstration is like a S-wave?

What To Do

  1. Choose 2 volunteers.
  2. One person holds one end of a slinky and a second person holds the other end.
  3. Have the volunteers move apart from each other, stretching out the slinky.
  4. One of the students holding the slinky end pushes the slinky straight forward so the slinky shortens between the 2 students. Alternatively, 1 person should count about 20 coils at his/her end and compress the coils together. Release these coils, remembering to hold onto the very end. The resulting compression wave is like a P-Wave.
  5. Ask students what they think is happening.
  6. One person wiggles his or her end of the slinky up and down. The resulting transverse wave is like an S-Wave.
  7. Ask students what they think is happening now.
  8. Explain the different kinds of waves and ask them which one was like P and which one was like S.
  9. Demonstrate it again if you want to re-enforce the learning.

Extensions

  • Which do you think would do more damage? P- or S- waves? Why?

Other Resources

IRIS Earthquake Science | Capturing the Motion of an Earthquake

Kids Know It | Video | How a Seismograph Works

IRIS Earthquake Science | How will 3 buildings, engineered equally, on different bedrock, react to an earthquake?

Prepared BC | Earthquake and Tsunami Guide PDF

Government of British Columbia | Earthquake Preparedness and Response

Government of Canada | Natural Resources Canada |  Earthquakes Canada

U.S. Geological Survey | Earthquake Hazards Program | Earthquakes for Kids

Cybersleuth Kids | Earthquake Resources