An earthquake is a sudden movement of the earth's crust caused by the release of stress accumulated along geologic faults or by volcanic activity.

Some 80 percent of all the planet's earthquakes occur along the rim of the Pacific Ocean, called the "Ring of Fire" because of the amount of volcanic activity there. Most earthquakes occur at fault zones, where tectonic plates; giant rock slabs that make up the Earth's upper layer; collide or slide against each other. These impacts are usually gradual and unnoticeable on the surface. However, immense stress can build up between plates. When this stress is released quickly, it sends massive vibrations, called seismic waves, often hundreds of miles through the rock and up to the surface. Other quakes can occur far from faults zones when plates are stretched or squeezed.

Scientists assign a magnitude rating to earthquakes based on the strength and duration of their seismic waves. A quake measuring 3 to 5 would be considered minor or light; 5 to 7 is moderate to strong; 7 to 8 is major, and 8 or more is great.

On average, a magnitude 8 quake strikes somewhere every year and some 10,000 people die worldwide annually in earthquakes. Collapsing buildings claim by far the majority of lives, but the destruction is often compounded by mud slides, fires, floods, or tsunamis. Smaller temblors that usually occur in the days following a large earthquake can complicate rescue efforts and cause further death and destruction.