Energy
Your book does a great job of describing seismic waves in Section 6.7 (pp 184). Let’s just redefine them here to refresh your memory before we move on.
The energy released by an earthquakes travels through the Earth as three types of seismic waves: P-waves, S-waves, and R(L)-waves. However, each travels through the Earth at a different speed.
- P-waves (primary waves) move the fastest; go through rock and liquid
- S-waves (secondary waves) move second fastest; only go through rock
- R(L)-waves (rolling, long, or last waves) move slowest ; only go through rock
These waves travel through the Earth. What do we mean the waves travel through the Earth? Think of sound waves. We can’t see sound, but we know that the energy of sound exists. We can hear sound being transferred from one type of material to another. If you were to break a pencil, you would hear a “snap.” As stress you applied to bend the pencil causes it to snap, the pencil would vibrate very quickly, causing the molecules of air near it to vibrate with it. These vibrations would be passed along the air molecules until it reached your eardrum, causing your eardrum to vibrate and for you to hear the “snap.”
This same movement of waves happens in the Earth. But instead of the breaking rocks causing air molecules to vibrate, the rocks and Earth materials near the break point (focus) would vibrate. These vibrations move through the rock materials as waves. (Make a note about the rocks nearest the focus vibrating…)