Why Do Earthquakes Happen?

The tectonic plates of the earth are in constant motion, sliding against each other. But they don't just slide smoothly; rocks catch against each other halting movment even though the plates are still in motion.

Earthquakes occur when, eventually, the rocks break free and release the builtup the pressure along the fault line.

This sudden release of energy causes the seismic waves that make the ground shake.

The spot underground where the rock breaks is called the focus of the earthquake. The place right above the focus (on top of the ground) is called the epicenter.