Earthquake Body Waves

There are two types of body waves produced by an earthquake, P waves, think of these as primary waves, and S waves, think of these as secondary. The two waves travel through the body of the Earth and are very different from one another. In order for students to really understand the difference between the two, I asked them to step into the hall and feel the difference.


P waves, in a solid material

Students were asked to stand in a straight line facing the same direction, to stand with arms out, elbows locked, and touching one another. They were aligning themselves like atoms would in a solid material, the Earth's crust for example.

Watch and see how the wave moves through these "atoms."

The wave moved quickly and easily though the "atoms". This type of movement is an example of a compressional wave. It's because of the wave's ability to move quickly that the P wave is the first to arrive at seismic stations.

S waves, in a solid material

Students were then asked to face the wall and lock arms. Again, they are atoms locked together in a solid material, but this time a S wave moves through them. S waves are transverse waves and move in an S shape. similar to a snake. 

Watch this video to see these "atoms" as a S wave travels through them.

Again, the wave moved through the "atoms", but this time much slower and certainly with more force. It's this S wave that travels much more slowly to a seismic station, but it's carrying much more power. 

How are these two types of waves different?

Which type of wave would you suspect causes more damage?

P waves, in a liquid material

Again, students were asked to form a straight line and face the same direction, to stand with arms out, elbows locked, and this time, NOT touching one another. Close, but NOT touching. In this case,
they were aligning themselves like atoms would in a LIQUID material. The atoms in a liquid are not "locked" into place and it's this physical property that allows a liquid to move about and take the shape of it's container.

How do you think a P wave will travel through the "atoms?"

You can see that P waves are able to travel in a liquid and still move quickly. 

The demonstration was run once more, but this time a S wave was sent through a liquid. Students stood side by side, arms on hips, but since they were liquid "atoms", they were not touching. 

What do you think happened? 

Can a S wave travel though a liquid?