Almost anything to do with earthquake seismology can be demonstrated with a slinky or lots of them!

This type of demonstration is best done before a crowd where you can bring up lots of volunteers. There is quite a controversy among ‘slinky-ologists’ as to the best type of slinky to use. Some prefer the antique all-steel versions (and are quite proud of being ‘retro’). I prefer the genuine “Slinky-brand” florescent plastic slinkys. They are much longer, and have better response than the generic types.

Seismic Waves

As in the figure, you start with a stretched slinky. This is where you call for somebody in the audience to be an endpoint. Give it a good stretch, so it only has a slight sag. The first seismic wave to demonstrate is a compression wave, or P-wave. These are best generated by gathering a few coils of the slinky and quickly releasing them. You will see the wave moving very rapidly and reflecting. At this point, I like to tell the person that they are a wave detector, or seismometer, and they must say ‘Ow!’ when the wave hits them.

Shear waves (S-waves) can be generated by lifting a coil (slightly away from the hand), and releasing it. The audience will see that it travels more slowly to the seismometer (who must say ‘Ow’ again).

At this point I like to demonstrate the ability of seismometers to detect the amplitude of the wave hitting them, by giving a loud ‘Ow’ for a big wave, and a softer exclamation for a small wave. (Keep hitting them with different-sized waves until they are properly calibrated!).

Locating Earthquakes

For this you will need three slinkys and some more demonstrators. Set up two more human seismometers around a central human earthquake. Have one slinky go far away at full stretch, have another hold extra coils (for the medium distance). Finally, have them hold a lot of coils for the short distance.

Have your ‘epicenter’ hold all three of the other ends. Now it becomes somewhat tricky. You have to calibrate the new seismometers with individual waves. Make sure they are giving a good yell at wave impact. The earthquake itself can gather coils and release them (major talent!), but I find you usually have to help them. Gather and release the coils. If you are lucky, you will get the proper rapid succession of yells, and wait for the applause!

You can then go on to explain how we don’t have real-live teenagers (or whatever the audience) standing in fields as seismometers (because they sleep and eat too much!). I then go into slides on real seismometers, and how computers locate the earthquakes.

Buildings

If you have lots of time, a vertical slinky can demonstrate the fundamental modes of building response. I even use it to demonstrate the effect of soft foundation soils.