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An earthquake is a trembling or shaking movement of a planet's surface. Earthquakes typically result from the movement of continental and oceanic plates, as described by plate tectonics. Stress builds up along the boundaries of these plates and is released as strain (i.e. slippage) along geological faults. The resulting release of energy creates seismic waves.

Earthquakes occur every day on Earth, however, the vast majority of them are minor and cause no damage. Large earthquakes can cause serious destruction and massive loss of life. Large earthquakes usually have a substantial initial tremor, followed by numerous aftershocks (smaller tremors). The underground point of origin of an earthquake is called its "focus" and the location on the surface directly above the focus is the "epicenter." When the epicenter of an earthquake is located underneath an ocean, it is called a seaquake. Seaquakes can give rise to tsunamis.

The magnitude and intensity of an earthquake used to be quantified on the Richter scale, but now multiple scales are used. The scale reported to the public is so close to the Richter scale that seismologists do not bother to explain the difference.

Some earthquakes are caused by the movement of magma in volcanoes, and such quakes can be an early warning of volcanic eruptions. A rare few earthquakes are caused by the build-up of large masses of water behind dams, such as the Kariba Dam in Zambia, Africa. Finally, earthquakes (in a broad sense) can also result from the discharge of explosives. Thus Western scientists have been able to monitor, using the tools of seismology, nuclear weapons tests performed by governments that were not disclosing information on these tests along normal channels.

See also: List of earthquakes, seismology, geophysics, New Madrid Fault Zone, San Andreas Fault.

External links

• EQNET: Earthquake Information Network
• Geowall- An interesting 3d presentation system for looking at and understanding earthquake data.