An earthquake is a trembling or shaking movement of the Earth's surface. Earthquakes typically result from the movement of faultss, 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 vast majority of them are minor and cause no damage. Large earthquakes can cause serious destruction and massive loss of life via 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 hazardous materials release. 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 earthquakes shaking is the dominant and most widespread cause of damage.

Most large earthquakes are accompanied by other, smaller ones, known as foreshocks when they occur before the principal or mainshock and aftershocks when they occur following it. The source of an earthquake is distributed over a significant area -- in the case of the very largest earthquakes, in excess of a thousand kilometres -- but it is usually possible to identify a point from which the earthquake waves appear to emanate. That point is called its "focus" and usually proves to be the point at which fault rupture was initiated. The position of the focus is known as the "hypocentre" and the location on the surface directly above it is the "epicenter." Earthquakes, especially those that occur beneath sea- or ocean-covered areas, 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.

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 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. The press sometimes mistakenly reports such values as "Richter magnitude", and this has given rise to public confusion.

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.

Interstate 880 in Oakland, California following the Loma Prieta earthquake, ca. 1989

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 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 from the Earth's crust (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 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, elastic-rebound theory

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