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Earthquake

2007 Schools Wikipedia Selection. Related subjects: Natural Disasters

   An earthquake is a phenomenon that results from and is powered by the
   sudden release of stored energy in the crust that propagates seismic
   waves. At the Earth's surface, earthquakes may manifest themselves by a
   shaking or displacement of the ground and sometimes tsunamis, which may
   lead to loss of life and destruction of property.

   Earthquakes may occur naturally or as a result of human activities. In
   its most generic sense, the word earthquake is used to describe any
   seismic event—whether a natural phenomenon or an event caused by
   humans—that generates seismic waves.
   Global earthquake epicenters, 1963–1998
   Enlarge
   Global earthquake epicenters, 1963–1998

Types of earthquakes

Naturally occurring earthquakes

   Most naturally occurring earthquakes are related to the tectonic nature
   of the Earth. Such earthquakes are called tectonic earthquakes. The
   Earth's lithosphere is a patchwork of plates in slow but constant
   motion caused by the heat in the Earth's mantle and core. Plate
   boundaries grind past each other, creating frictional stress. When the
   frictional stress exceeds a critical value, called local strength, a
   sudden failure occurs. The boundary of tectonic plates along which
   failure occurs is called the fault plane. When the failure at the fault
   plane results in a violent displacement of the Earth's crust, the
   elastic strain energy is released and seismic waves are radiated, thus
   causing an earthquake. This processes of strain, stress, and failure is
   referred to as the Elastic-rebound theory. It is estimated that only 10
   percent or less of an earthquake's total energy is radiated as seismic
   energy. Most of the earthquake's energy is used to power the earthquake
   fracture growth and is converted into heat. Therefore, earthquakes
   lower the Earth's available potential energy, though these losses are
   negligible.

   Earthquakes occurring at boundaries of tectonic plates are called
   interplate earthquakes, while the less frequent events that occur in
   the interior of the lithospheric plates are called intraplate
   earthquakes.

   The majority of tectonic earthquakes originate at depths not exceeding
   a few tens of kilometers. In subduction zones, where older and colder
   oceanic crust descends beneath another tectonic plate, earthquakes may
   occur at much greater depths (up to hundreds of kilometers). These
   seismically active areas of subduction are known as Wadati-Benioff
   zones. Deep focus earthquakes are another phenomenon associated with a
   subducting slab. These are earthquakes that occur at a depth at which
   the subducted lithosphere should no longer be brittle, due to the high
   temperature and pressure. A possible mechanism for the generation of
   deep focus earthquakes is faulting caused by olivine undergoing a phase
   transition into a spinel structure.

   Earthquakes may also occur in volcanic regions and are caused by the
   movement of magma in volcanoes. Such quakes can be an early warning of
   volcanic eruptions.

   A recently proposed theory suggests that some earthquakes may occur in
   a sort of earthquake storm, where one earthquake will trigger a series
   of earthquakes each triggered by the previous shifts on the fault
   lines, similar to aftershocks, but occurring years later, and with some
   of the later earthquakes as damaging as the early ones. Such a pattern
   was observed in the sequence of about a dozen earthquakes that struck
   the North Anatolian Fault in Turkey in the 20th Century, the half dozen
   large earthquakes in New Madrid in 1811-1812, and has been inferred for
   older anomalous clusters of large earthquakes in the Middle East and in
   the Mojave Desert.

Induced earthquakes

   Some earthquakes have anthropogenic sources, such as extraction of
   minerals and fossil fuel from the Earth's crust, the removal or
   injection of fluids into the crust, reservoir-induced seismicity,
   massive explosions, and collapse of large buildings. Seismic events
   caused by human activity are referred to by the term induced
   seismicity. They however are not strictly earthquakes and usually show
   a different seismogram than earthquakes that occur naturally.

   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. Earthquakes have also been
   known to be caused by the removal of natural gas from subsurface
   deposits, for instance in the northern Netherlands. The world’s largest
   reservoir-induced earthquake occurred on December 10, 1967 in the Koyna
   region of western Maharashtra in India. It had a magnitude of 6.3 on
   the Richter scale. However, the U.S. geological survey reported the
   magnitude of 6.8.

   The detonation of powerful explosives, such as nuclear explosions, can
   cause low-magnitude ground shaking. Thus, the 50-megaton nuclear bomb
   code-named Ivan detonated by the Soviet Union in 1961 created a seismic
   event comparable to a magnitude 7 earthquake, producing the seismic
   shock so powerful that it was measurable even on its third passage
   around the Earth. In an effort to promote nuclear non-proliferation,
   the International Atomic Energy Agency uses the tools of seismology to
   detect illicit activities such as nuclear weapons tests. The nuclear
   nations routinely monitor each other's activities through networks of
   interconnected seismometers, which allow to precisely locate the source
   of an explosion.

   Sports games have been known to inadvertently produce microearthquakes.
   This phenomenon was first seen in 1988 with the Earthquake Game at
   Louisiana State University, in which fans stamped their feet and jumped
   up and down vigorously enough to have the effect register on the campus
   seismograph.

   Earthquakes happen every day around the world, but most of them go
   unnoticed and cause no damage. Large earthquakes however can cause
   serious destruction. They may be caused by the ground shaking, a tidal
   wave or tsunami, fire or by gas or petrol leaks. Most large earthquakes
   are accompanied by other, smaller ones that can occur either before or
   after the 'main shock'. The power of an earthquake covers a large area,
   but in a very large earthquake, it can even cover the whole planet.
   Scientists can locate the point from which the earthquake started. That
   point is called its 'focus' or 'hypocenter'. The location on the
   surface of the earth directly above the hypocenter is known as the
   'epicenter'.

Measuring earthquakes

   Since seismologists cannot directly observe rupture in the Earth's
   interior, they rely on geodetic measurements and numerical experiments
   to analyze seismic waves and accurately assess severity of earthquakes.
   The severity of an earthquake can be measured in terms of magnitude and
   intensity. For that seismologists use two fundamentally different but
   equally important types of scales. The original force or energy of an
   earthquake is measured on a magnitude scale. The Richter scale is a
   well known example of a magnitude scale. The second type of scale
   measures the intensity of shaking occurring at any given point on the
   Earth's surface. These scales are referred to as intensity scales. The
   Mercalli intensity scale, which measures the effects of the seismic
   waves, is an example of a commonly used intensity scale.

   The non-specialized media will often refer to the magnitudes of
   earthquakes as being reported on the Richter scale. However, the
   magnitudes reported nowadays are actually on the moment magnitude
   scale. This is because the older Richter scale is not well-suited to
   accurately measure earthquakes with magnitudes over 6.8.

   The analyses of earthquake severity allow scientists to estimate the
   locations and likelihoods of future earthquakes, helping identify areas
   of greatest hazard and ensure safety of people and infrastructure
   located in such areas.

Seismic maps

   An isoseismal map created by the Pacific Northwest Seismograph Network
   showing the instrument-recorded intensities of the 2001 Nisqually
   earthquake of February 28, 2001.
   Enlarge
   An isoseismal map created by the Pacific Northwest Seismograph Network
   showing the instrument-recorded intensities of the 2001 Nisqually
   earthquake of February 28, 2001.
   A Community Internet Intensity Map generated by the USGS showing the
   intensity of shaking felt by humans during the Nisqually earthquake;
   locality divisions are by ZIP Code.
   Enlarge
   A Community Internet Intensity Map generated by the USGS showing the
   intensity of shaking felt by humans during the Nisqually earthquake;
   locality divisions are by ZIP Code.

   To show the extent of various levels of seismic effects within a
   particular locality, seismologists compile special maps called
   isoseismal maps. An isoseismal map uses contours to outline areas of
   equal value in terms of ground shaking intensity, ground surface
   liquefaction, shaking amplification, or other seismic effects.
   Typically, these maps are created by combining historical
   instrument-recorded data with responses to postal questionnaires that
   are sent to each post office near the earthquake and to a sparser
   sample of post offices with increasing distance from the earthquake.
   This way of preparing a seismic hazard map can take months to complete.
   In contrast to the old method, a newer method of information collection
   takes advantage of the Internet to generate initial hazard maps almost
   instantly. Data are received through a questionnaire on the Internet
   answered by people who actually experienced the earthquake, reducing
   the process of preparing and distributing a map for a particular
   earthquake from months to minutes.

   Seismic hazard maps have many applications. They are used by insurance
   companies to set insurance rates for properties located in
   earthquake-risky areas, by civil engineers to estimate the stability of
   hillsides, by organizations responsible for the safety of nuclear waste
   disposal facilities, and also by building codes developers as the basis
   of design requirements.

   In building codes, the shaking-hazard maps are converted into seismic
   zone maps, which are used for seismic analysis of structural components
   of buildings. The seismic zone maps depict seismic hazards as zones of
   different risk levels. Such zones are typically designated as Seismic
   Zone 0, Seismic Zone 1, Seismic Zone 2 and so on. The seismic zone maps
   usually show the severity of expected earthquake shaking for a
   particular level of probability, such as the levels of shaking that
   have a 1-in-10 chance of being exceeded in a 50-year period. Buildings
   and other structures must be designed with adequate strength to
   withstand the effects of probable seismic ground motions within the
   Seismic Zone where the building or structure is being constructed.

Size and frequency of occurrence

   Small earthquakes occur every day all around the world, and often
   multiple times a day in places like California and Alaska in the U.S.,
   as well as Indonesia, Azores in Portugal and Japan. Large earthquakes
   occur less frequently, the relationship being exponential; namely,
   roughly ten times as many earthquakes larger than magnitude 4 occur in
   a particular time period than earthquakes larger than magnitude 5. For
   example, it has been calculated that the average recurrence for the
   United Kingdom can be described as follows:
     * an earthquake of 3.7 or larger every year
     * an earthquake of 4.7 or larger every 10 years
     * an earthquake of 5.6 or larger every 100 years.

   The number of earthquake reporting stations increased from about 350 in
   1931 to about 4,000 today. As a result, many more earthquakes are
   reported than in the past -- currently, about 35 per day worldwide.
   This does not necessarily mean that the number of earthquakes has
   increased, however. The USGS estimates that, since 1900, there have
   been an average of 18 major earthquakes (magnitude 7.0-7.9) and one
   great earthquake (magnitude 8.0 or greater) per year, and that this
   average has been relatively stable. In fact, in recent years, the
   number of major earthquakes per year has actually decreased. More
   detailed statistics on the size and frequency of earthquakes is
   available from the USGS.

   Most of the world's earthquakes (90%, and 81% of the largest) take
   place in the 40,000 km-long, horseshoe-shaped zone called the
   circum-Pacific seismic belt, also known as the Pacific Ring of Fire,
   which for the most part bounds the Pacific Plate.Massive earthquakes
   tend to occur along other plate boundaries, too, such as along the
   Himalaya Mountains.

Effects/impacts of earthquakes

   There are many effects of earthquakes including, but not limited to the
   following:
     * Broken windows
     * Collapse of buildings
     * Fires, as seen in the 1906 San Francisco earthquake (Although many
       fires were deliberately started by residents to claim off the
       insurance, as they were not covered against earthquake damage)
     * Tsunamis, as seen in the 2004 Sumatran earthquake
     * Landslides
     * Destabilisation of the base of some buildings which may lead to
       collapse in a future earthquake
     * Disease
     * Lack of basic necessities
     * Human loss of life
     * Higher insurance premiums

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