Earthquakes are vibrations of the ground caused by movement of the subsurface. The grinding of tectonic plates sends shock waves traveling through the ground with enough strength to rupture everything in their path. An earthquake will often shift the two sides of a fault line, so that the ground on one side may move in an opposite direction from the earth on the other side.
The pressure between two plates will build until something snaps, jarring the earth in an instant. The site of this violent movement is known as the focus, or hypocenter, of the earthquake. The epicenter is the point on the earth’s surface above the focus, from which damage can spread in radial waves.
Forces of Nature: ThinkQuest 2000 (Team #C003603)
http://library.thinkquest.org/C003603/english/earthquakes/whatarethey.shtml
Eight years ago, I was sleeping soundly in my bed when a sudden jarring motion startled me awake. The posts of my bed were rattling noisily against the wall, and the whole house was shaking back and forth like a bulldozer was lifting it off its foundation. Objects began falling off tables and surfaces. A loud, rough booming noise filled the air. Seconds after it began, everything was quiet again. The earth had stopped shaking.