How does an earthquake create both P-waves and S-waves?

After Investigation 11.1, I searched for a way to clear up the question over how an earthquake can create both P-waves and S-waves.  I was coming up short, until my friend and fellow student teacher, Ms. Yip, offered me a suggestion that worked out perfectly.  Most of you saw me demo this at the beginning of class last Friday.

For those who were not able to see the demonstration, here is a link to the video I recorded:

Investigation 11.1 Extra: Creating Both P-Waves & S-Waves

In a nutshell, I created a P-wave using an "earthquake wave simulation spring", as Ms. Katherine calls them.  But I also had a spring set up 90º perpendicular to the first spring.  I then held the ends of both springs in my hand, and moved them together while I created the P-wave.  This created an S-wave on the second spring – it's really best if you take a few minutes to watch the video, because I think it does a great job of demonstrating this.

It's most important to remember that earthquake energy goes out from the focus in every direction, much like an explosion.  As a result, for every P-wave, there is an S-wave generated 90º in both directions.  Since the waves go out in every direction, there's going to be an S-wave generated that happens to line up with every P-wave.  I can't think of a better way to describe this, other than to remind everyone that the waves aren't going out in lines, but in spheres.  Think about Investigation 15.1: the waves we saw were not straight lines.

Please let me know if you have any questions!