Understanding seismic waves: Teachers' notes

Students use a model based on wire helical coils ('slinkies') to look at how an earthquake generates P-waves and S-waves.

Learning objectives

Students will:

  • observe the way a single push generates both longitudinal (P) waves and transverse (S) waves
  • observe the difference in arrival times of waves travelling different distances
  • observe the fact that these waves transmit energy, but that the medium itself does not move along much overall
  • recognise that different types of earthquake produce distinctive patterns of P-wave and S-wave arrivals at seismic stations.

Notes

If you want to demonstrate P-waves and S-waves with a wire helix (a 'slinky') first, then this method should give you good results:

  • Get two students to each hold one end of a 'slinky' and stretch it out across the room so it is hanging freely between them. Compress 4-5 coils at one end and then wait for all vibrations to die down before releasing the compressed coils (the two students should keep hold of their ends of the slinky without moving). This generates a nice compressional P-wave which will bounce back and forth along the whole length.
  • For S-waves, the students remain in the same place. Pull one end of the 'slinky' (about 5 coils in from the end) at right angles to the length until it is deflected about 1-2 diameters. Wait until all the vibrations have died down before releasing these coils and creating a transverse S-wave that bounces up and down the whole length.

The main activity using the box and 'slinkies' is best done as a demonstration, working with a small group of student participants. One group member (or the teacher) provides the push for an earthquake, while five other group members each hold onto the free end of a coil, to act as 'seismic stations'.

Try to ensure that students keep the coil tension the same when they stand at different distances, to make this a 'fair' test through comparable media: if a 'slinky' is made half length this means that the student holding it must now hold it in the middle rather than the end (keeping the unused half compressed in their hands).

Resources needed

Each group will need: