Waves Tell Stories

When an earthquake occurs, it releases energy as seismic waves that travel through and around the Earth. These waves are not all the same. P-waves (primary) are compressional waves that move fastest, pushing and pulling rock like a spring. They travel through solids, liquids, and gases. S-waves (secondary) are slower and move rock in a side-to-side shearing motion.

Critically, S-waves cannot travel through liquids. This single fact is how we know Earth's outer core is liquid — S-waves from earthquakes disappear when they hit it, creating a shadow zone on the opposite side of the planet.