Understanding seismic waves

You can use a box and several wire helixes ('slinkies') attached to it with bulldog clips to show how an earthquake generates P- and S- waves.

Five students, the 'observers', each hold on to the unattached end of a 'slinky', so it is stretched out loosely across the room (not touching the bench top).

Modelling seismic waves: pushes at X.

1) Push box at X

Another person — 'the earthquake' — should stand at X and give the box a quick push.

What kind of wave does each of the observers detect, an S-wave (transverse) or a P-wave (longitudinal)?

Modelling seismic waves: pushes at Y.

2) Push box at Y

What happens if the earthquake push is at Y instead?

Now repeat the single push at X, with the three observers on the opposite side of the file all at the same distance.

Pull the 'slinkies' to the same length and coil spacing.

What do you notice about the arrival time of the pulse at each of these observers?

Observers at different distances but keeping the tension the same.

3) Observers at different distances

Now have the three 'observers' standing at different distances, but keeping the coils with the same spacing, as shown on the right.

You will need to pull them out to much bigger lengths than shown, though. One observer could hold the helix at full length, but the two observers who move closer to the box need to hold onto 'spare' coils so that the space between coils looks the same for all three.

What happens if you send the pulse now?

Teachers' notes

Understanding seismic wavesUnderstanding seismic waves

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