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Encyclopedia > Earthquakes
Global earthquake epicenters, 1963–1998.

An earthquake is a trembling or a shaking movement of the Earth's surface. Earthquakes typically result from the movement of faults, quasi-planar zones of deformation within its uppermost layers. The word earthquake is also widely used to indicate the source region itself. The solid earth is in slow but constant motion (see plate tectonics) and earthquakes occur where the resulting stress exceeds the capacity of Earth materials to support it. This condition is most often found at (and the resulting frequent occurrence of earthquakes is used to define) the boundaries of the tectonic plates into which the Earth's lithosphere can be divided. Events that occur at plate boundaries are called interplate earthquakes; the less frequent events that occur in the interior of the lithospheric plates are called intraplate earthquakes.



Earthquakes occur every day on Earth, but the majority of them are minor and cause no damage. Large earthquakes can cause serious destruction and massive loss of life through a variety of agents of damage, including fault rupture, vibratory ground motion (i.e., shaking), inundation (e.g., tsunami, seiche, dam failure), various kinds of permanent ground failure (e.g. liquefaction, landslide), and fire or a release of hazardous materials. In a particular earthquake, any of these agents of damage can dominate, and historically each has caused major damage and great loss of life, but for most of the earthquakes shaking is the dominant and most widespread cause of damage.

Section of collapsed freeway after the 1989 Loma Prieta earthquake.

Most large earthquakes are accompanied by other, smaller ones, that can occur either before or after the principal quake — these are known as foreshocks or aftershocks, respectively. The power of an earthquake is distributed over a significant area, but in the case of large earthquakes, it can spread over the entire planet. Ground motions caused by very distant earthquakes are called teleseisms. It is usually possible to identify a point from which the earthquake's seismic waves appear to originate. That point is called its "focus" and usually proves to be the point at which the fault slip was initiated. The position of the focus is known as the "hypocenter" and the location on the surface directly above it is the "epicenter". The fault may slip well beyond its epicenter, though. Just as a large loudspeaker can produce a greater volume of sound than a smaller one, large faults are capable of higher magnitude earthquakes than smaller faults are.

Earthquakes, especially those that occur beneath oceans or seas, can give rise to tsunamis, either as a direct result of the deformation of the sea bed due to the earthquake, or as a result of submarine landslips or "slides" indirectly triggered by it.

There are four types of seismic waves all that are generated simultaneously. They arrive in the following order: first the body waves P waves (primary or pressure waves) then S waves (secondary or shear waves), next the surface waves (Love waves) then Rayleigh waves.


A class of earthquakes known as silent earthquakes are thought to be caused by very slow slippage. They are of extremely low intensity but can last for days or weeks releasing as much energy as large earthquakes.

In the 1930s, a California seismologist named Charles F. Richter devised a simple numerical scale (which he called the magnitude) to describe the relative sizes of earthquakes, which has come to be called the Richter scale. Since Richter, seismologists have developed a number of magnitude scales. Most of the scales in use in the Western world (such as the moment magnitude scale) are mutually consistent to a sufficient extent that the term "Richter scale" is routinely used in reporting these numbers to the public. Other scales (and other ways of describing the size of earthquakes) are used in some non-Western countries, and by earthquake specialists. For example, the Japanese scale for measuring the force of earthquakes measures horizontal movement. The press sometimes mistakenly reports such values as "Richter magnitude", and this has given rise to public confusion.

A Shakemap recorded by the Pacific Northwest Seismograph Network that shows the instrument recorded intensity of the shaking of the Nisqually earthquake on February 28, 2001.
A Community Internet Intensity Map generated by the USGS that shows the intensity felt by humans by ZIP code of the shaking of the Nisqually earthquake on February 28, 2001.

Earthquake effects are described in terms of intensity, a scale which attempts to quantify the severity of shaking at a given location. A number of intensity scales are in use, and there is a significant degree of consistency amongst them. The best known is the Mercalli (or Modified Mercalli, MM) scale, but the more consistent and analytical European Macroseismic Scale (EMS) is now increasingly widely used. In Japan the Japan Meterological Agency seismic intensity scale (JMA) is used.


Most earthquakes are powered by the release of the stresses that accumulate over time, typically, at the boundaries of the plates that make up the Earth's lithosphere. The most severe of these earthquakes are located along compressional and translational plate boundaries. Deep focus earthquakes are possibly generated as subducted lithospheric material catastrophically undergoes a phase transition at depths greater than 600 km. Some earthquakes are also caused by the movement of magma in volcanoes, and such quakes can be an early warning of volcanic eruptions. A rare few earthquakes have been associated with the build-up of large masses of water behind dams, such as the Kariba Dam in Zambia, Africa, and with the injection or extraction of fluids into the Earth's crust (e.g. at certain geothermal power plants and at the Rocky Mountain Arsenal). Such earthquakes occur because the strength of the Earth's crust can be modified by fluid pressure. Finally, earthquakes (in a broad sense) can also result from the detonation of explosives. Thus scientists have been able to monitor, using the tools of seismology, nuclear weapons tests performed by governments that were not disclosing information about these tests along normal channels. Earthquakes such as these, that are caused by human activity, are referred to by the term induced seismicity.

Preparation for earthquakes

Specific fault articles

Specific earthquake articles

See also List of earthquakes

Related articles

External links

Earthquake is a video game character. See Earthquake (video game character).

Earthquake is a 1974 movie starring Charlton Heston, Ava Gardner and others. See: Earthquake (movie).

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An earthquake is the sudden, sometimes violent movement of the earth's surface from the release of energy in the earth's crust.
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