Earthquake

: Global earthquake [[epicenters, 1963–1998]] An earthquake is a trembling or a shaking movement of the Earths 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 Earths lithosphere is 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.

Characteristics

Earthquakes occur every day on Earth, but the majority of them are minor and cause no damage(those less than 5 on ritcher scale). 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. 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.

Intensity

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 shindo 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.]] 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.

Causes

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. Another possibility to explain earthquakes is related to gas movement in the earth's interior, mainly methane (see related topics http://www.people.cornell.edu/pages/tg21/Earthq.html]; [http://www.people.cornell.edu/pages/tg21/eyewit.html).

Preparation for earthquakes

Specific fault articles

Specific earthquake articles

See also List of earthquakes

Two other earthquakes

Bam, Tehran Earthquake (2003) Tehran city earthquake of 9 March 2003 at 22:50:23 (GMT) a moderate earthquake, M=4.1 (IGTU) struck north east of Tehran. Its epicenter has been estimated at 35.74N and 51.46E (IGTU) up to now,the following stations have recorded the strong motion due to this event: Abbasspur University, Jamshidiyeh Park, Ministry of Agriculture (20Th floor), among which the maximum acceleration is 0.055g. Northern Iran earthquake magnitude 6.3, 2004 28 May 12:38:46 UTC, Preliminary Earthquake Report, U.S. Geological Survey, National Earthquake Information Center, World Data Center for Seismology, Denver, A strong earthquake occurred at 12:38:46 (UTC) on Friday, 28 May 2004. The magnitude 6.3 event has been located in Northern Iran. Category:Seismology af:Aardbewing be:Землятрус ca:Terratrèmol cs:Zemětřesení cy:Daeargryn da:Jordskælv de:Erdbeben et:Maavärin es:Sismo eo:Tertremo fa:زمین‌لرزه fr:Tremblement de terre gu:ધરતીકંપ ko:지진 id:Gempa bumi is:Jarðskjálfti it:Terremoto he:רעידת אדמה ms:Gempa bumi nl:Aardbeving ja:地震 nb:Jordskjelv nn:Jordskjelv pl:Trzęsienie ziemi pt:Terremoto ro:Cutremur ru:Землетрясение simple:Earthquake sk:Zemetrasenie sl:Potres fi:Maanjäristys sv:Jordbävning th:แผ่นดินไหว vi:Động đất tr:Deprem zh:地震

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