You no doubt recognize an image like this. It’s a model of our own blue planet with a hunk cut out of it so we can see the inside. The arrows represent earthquake waves. The more purplish ones are S waves, or sheer waves, which are a type of transverse wave and travel only through solids. Other waves, the P waves (pressure waves) are able to travel through the liquid outer core. When the waves once again reach the crust, they cause smaller earthquakes. While the rippling effect caused by S waves can only be felt where the waves can reach without crossing the outer core, the pressure of the P waves can be felt clear on the other side of the Earth. By monitoring where the little earthquakes occur along the Earth’s crust, we can examine the nature of P and S waves, and we can learn more about…