An earthquake is the sudden movement of Earth's crust caused by plate tectonics. Earthquakes occur along fault lines, cracks in Earth's crust where tectonic plates meet. They occur where plates are subducting, spreading, slipping, or colliding. As the plates grind together, they get stuck and pressure builds up. Finally, the pressure between the plates is so great that they break loose. Depending on how much pressure has built up, the ground may tremble slightly or shake forcefully. Scientists describe the intensity of an earthquake using the Richter Scale. It measures earthquakes on a scale of 1 to 10. People barely feel a magnitude 3 earthquake, and windows might rattle at magnitude 4. A magnitude 6 earthquake is considered major, causing houses to move and chimneys to fall. The largest earthquake on record had a magnitude of 9.5.

Source: American Museum of Natural History