The earth is a strange place. This big green-and-blue marble of ours can do some pretty crazy things just by interacting with itself. An earthquake is a prime example of this: the surface of our planet kind of just smashes into itself, causing all kinds of havoc above and below the surface. The greatly varying makeup of the earth only serves to make earthquakes even more peculiar than they already are.
To show students (and/or anyone else who wants to pay attention) how Earth’s internal composition affects the effects of an earthquake, you can sit around and wait for an actual earthquake to occur, or you can use this quick and easy science demo from Demo Science.
For this one, you’ll need a fairly large bowl (think of a “popcorn bowl”), a glass bottle, and a pen, pencil, or other thin, cylindrical item. Fill the bowl about halfway with water, then place the bottle in the middle of the bowl. If the bowl overflows, either you’ve put in too much water or your bottle is too big. Or possibly both. You had one job, come on!
Wait for the water to settle, so the surface is nice and smooth. Have your smelly little students gather around and observe as you use your pen to tap the surface of the water near the side of the bowl. This, of course, causes ripples, but watch closely, kids—what else is happening?
P Waves & S Waves
As the water ripples away from where your pen struck the surface, the ripples that hit the bottle will mostly reflect back toward the pen. This is because they cannot move through the bottle as the could through the water.
This reaction is roughly analogous to that of secondary waves (or, S waves) in an earthquake. Primary waves (or, P waves) move faster and have more energy than S waves during an earthquake; additionally, S waves can move through solid objects, but not through liquids, while P waves can move through both.
When the earth begins to quake, P waves travel completely through the center of the earth. S waves move rapidly through the solid parts of Earth’s interior, much like the ripples in the water, but, like the tiny waves hitting the bottle and bouncing back, S waves are reflected back to their origin when they reach Earth’s liquid core.