Cutaway of Earth's layers

Earthquakes are the phenomena experienced during sudden movements of the Earth's crust. Under the Earth's crust lies the asthenosphere, the upper part of the mantle composed of liquid rock. The plates of the Earth's crust essentially "float" on top of this layer, and can be forced to shift as the upwelling molten material below moves. As the plates shift (and thus interact with each other), an enormous amount of energy is released in the form of waves. Although they can occur anywhere on the planet with little or no warning, the most extreme earthquakes occur near plate boundaries, as the plates converge (collide), diverge (move away from another), or shear (grind past one another). Moving rock and magma within volcanoes can also trigger earthquakes. In all of these cases, large sections of the crust can fracture and move to-and-fro to dissipate the released energy. This "shaking" is the sensation felt during an earthquake. The energy released is often described in terms of "magnitude," a logarithmic scale used to describe how energetic an earthquake was; a quake of magnitude 2 is hardly noticeable without special monitoring equipment, while quakes over magnitude 8 may actually cause the ground to visibly heave and roll. Since the scale is logarithmic, a magnitude 8 quake is not four times more energetic than a magnitude 2 quake, but one billion times more energetic!


Credit: Photo by United States Geological Survey

For more info: http://www.geog.ucsb.edu/~dylan/mtpe/