Earthquake

From Free net encyclopedia

For other senses of this word, see earthquake (disambiguation).

Image:Quake epicenters 1963-98.png An earthquake is a sudden and sometimes catastrophic movement of a part of the Earth's crust. Earthquakes result from the dynamic release of elastic strain energy that radiates seismic waves. Earthquakes typically result from the movement of faults, planar zones of deformation within the Earth's upper crust. The word earthquake is also widely used to indicate the source region itself. The Earth's lithosphere is a patch work of plates in slow but constant motion (see plate tectonics). Earthquakes occur where the stress resulting from the differential motion of these plates exceeds the strength of the crust. The highest stress (and possible weakest zones) are most often found at the boundaries of the tectonic plates and hence these locations are where the majority of earthquakes occur. Events located at plate boundaries are called interplate earthquakes; the less frequent events that occur in the interior of the lithospheric plates are called intraplate earthquakes (see, for example, New Madrid Seismic Zone). Earthquakes related to plate tectonics are called tectonic earthquakes. Most earthquakes are tectonic, but they also occur in volcanic regions and as the result of a number of anthropogenic sources, such as reservoir induced seismicity, mining and the removal or injection of fluids into the crust. Seismic waves including some strong enough to be felt by humans can also be caused by explosions (chemical or nuclear), landslides, and collapse of old mine shafts, though these sources are not strictly earthquakes. These sources will also show a different seismogram than earthquakes.

1 Characteristics
2 Measuring earthquakes
3 Causes
4 Preparation for earthquakes
5 Specific fault articles
6 Specific earthquake articles
7 See also
8 External links

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Characteristics

Large numbers of earthquakes occur on a daily basis on Earth, but the majority are detected only by seismometers and cause no damage.

Most earthquakes occur in narrow regions around plate boundaries at depths down to a few tens of kilometres in the earth's crust where it is rigid enough to support elastic strain. Where the crust is thicker and colder, they will occur at greater depths, whereas the opposite applies in portions of the crust that are hot. Along subduction zones, places where plates descend into the mantle, earthquakes have been recorded at depths of up to 600 km, although these deep earthquakes are caused by different mechanisms than the more common, shallow events. Some deep earthquakes may be due to the transition of olivine to spinel, which is more stable in the deep mantle. 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.[1][2]

Large earthquakes can cause serious destruction and massive loss of life through a variety of agents of damage, including fault rupture, vibratory ground motion (shaking), inundation (tsunami, seiche, or dam failure), various kinds of permanent ground failure (liquefaction, landslides), 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; nonetheless, for most earthquakes shaking is the dominant and most widespread cause of damage. There are four types of seismic waves that are all generated simultaneously and can be felt on the ground. Responsible for the shaking hazard, they are P-waves (primary waves), S-waves (secondary or shear waves) and two types of surfaces waves, (Love waves and Rayleigh waves). Image:SanFranHouses06.JPG Image:EarthquakeFreewayCa1989.jpg

Most large earthquakes are accompanied by other, smaller ones that can occur either before or after the main shock; these are called foreshocks and aftershocks, respectively. While almost all earthquakes have aftershocks, foreshocks occur in only about 10% of events. The power of an earthquake is always distributed over a significant area, but in large earthquakes, it can even spread over the entire planet. Ground motions caused by very distant earthquakes are called teleseisms. The Rayleigh waves from the Sumatra-Andaman Earthquake of 2004 caused ground motion of over 1 cm even at seismometers that were located far from it, although this displacement was abnormally large. Using such ground motion records from around the world, seismologists can identify a point from which the earthquake's seismic waves apparently originated. That point is called its focus or hypocenter and usually coincides with the point where the fault slip started. The location on the surface directly above the hypocenter is known as the epicenter. The total length of the section of a fault that slips, the rupture zone, can be as long as 1,000 km for the biggest earthquakes.

Earthquakes that occur below sea level and have large vertical displacements 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 landslides directly or indirectly triggered by the quake.

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Measuring earthquakes

The severity of an earthquake can be expressed in terms of both intensity and magnitude. However the two terms are quite different, they are often confused. Intensity indicated the local effects and damages produced by an earthquake on the Earth's surface, humans, animals, and structures. Intensities are usually expressed in roman numerals, each numeral representing the severity of an earthquake. The magnitude is a number, usually an Arabic numeral, that characterizes the relative size of an earthquake. It is a measure of the amount of energy released. Since the earthquake effects vary with distance from its epicentre, each earthquake can be described by only one magnitude, but many intensities.

Two fundamentally different but equally important scales are used by seismologists to describe earthquakes. The original force or energy of an earthquake is measured on a magnitude scale, while the shaking and quaking that occurs at the surface producing damaging effects is measured on an intensity scale.

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Seismic intensity scales

Numerous intensity scales have been developed over the last several hundred years to evaluate the effects of earthquakes. The scale currently used in the United States is the Modified Mercalli Intensity Scale, while a number of other scales used in different parts of the world: the Shindo scale in Japan, the European Macroseismic Scale in the European Union, the Rossi-Forel scale in Latin America, the MSK-64 in Russia and the CIS.

An intensity scale consists of a series of certain key responses such as people awakening, movement of furniture, damage to chimneys, collapse of structures, and total destruction. For example, the intensity level VI on the MM scale is defined as follows:

Everyone feels movement. People have trouble walking. Objects fall from shelves. Pictures fall off walls. Furniture moves. Plaster in walls might crack. Trees and bushes shake. Damage is slight in poorly built buildings. No structural damage.

Image:Nisqually Earthquake ShakeMAp Mon 13 2003.jpg

Image:Nisqually Community Internet Intensity Map for the Nisqually Earthquake FEB 2281854 ciim.gif

To show the extent of various levels of intensity within a particular locality, seismologists compile special maps called isoseismal maps. An isoseismic map uses contours to outline areas of equal seismic intensity.

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Magnitude scales

The first attempt to qualitatively define a single, absolute value to describe the size of earthquakes was the magnitude scale (the name being taking from similarly formulated scales used to represent the brightness of stars). In the 1930s, a California seismologist named Charles F. Richter devised a simple numerical scale (later called magnitude) to describe the relative sizes of earthquakes in Southern California. This is known as the “Richter scale”, “Richter Magnitude” or “Local Magnitude” (M_L). It is obtained by measuring the maximum amplitude of a recording on a Wood-Anderson torsion seismometer (or one calibrated to it) at a distance of 600 km from the earthquake. Other more recent Magnitude measurements include: body wave magnitude (m_b), surface wave magnitude (M_s) and duration magnitude (M_D). Each of these is scaled to give values similar to those given by the Richter scale; but because each is based on a measurement of one part of the seismogram, they do not measure the overall power of the source and can be negatively affected by saturation at higher magnitude values—meaning that they fail to report higher magnitude values for larger events. Further, since these scales too are empirical, they provide no values that are meaningful from a physics perspective. This does not mean, though, that they are useless. They are because they can be rapidly calculated, catalogues of them dating back many years are available, and the public is familiar with them.

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The moment-magnitude scale

Because of the limitations of the magnitude scales, a new, more uniformly applicable extension of the magnitude scale, known as moment magnitude, or MW, was developed. In particular, for very large earthquakes moment magnitude gives the most reliable estimate of earthquake size. This is because seismic moment is derived from the concept of moment in physics and therefore provides clues to the physical size of an earthquake—the size of fault rupture and accompanying displacement and length of slippage—as of as well as the amount of energy released. So while seismic moment, too, is calculated from seismograms, it can also be obtained by working backwards from geologic estimates of the size of the fault rupture and displacement. The values of moments for different earthquakes range over several orders of magnitude, and because they are not influenced by variables such as local circumstances, the results obtained make it easy to objectively compare the sizes of different earthquakes. These characteristics, plus the seismic moment's immunity to saturation at higher magnitudes and compatibility with other magnitude scales, led Tom Hanks and Hiroo Kanamori to introduce in 1979 the moment magnitude (M_W) scale for representing the absolute size of earthquakes.

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Earthquake size and frequency of occurrence

Larger earthquakes occur less frequently than smaller earthquakes, 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.

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Causes

Most earthquakes are powered by the release of the elastic strain that accumulates over time, typically, at the boundaries of the plates that make up the Earth's lithosphere via a process called Elastic-rebound theory. The Earth is made up of tectonic plates driven by the heat in the Earth's mantle and core. Where these plates meet stress accumulates. Eventually when enough stress accumulates, the plates move, causing an earthquake. Deep focus earthquakes, at depths of hundreds of kilometres, are possibly generated as subducted lithospheric material catastrophically undergoes a phase transition since at the pressures and temperatures present at such depth elastic strain cannot be supported. 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. Earthquakes have also been known to be caused by the removal of natural gas from subsurface deposits, for instance in the northern Netherlands. Finally, ground shaking 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 type of movement of the Earth is observed by terrestrial spectroscopy. These oscillations of the earth are either due to the deformation of the Earth by tide caused by the Moon or the Sun, or other phenomena.

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.

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Preparation for earthquakes

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Specific fault articles

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Specific earthquake articles

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Pre-20th Century

Shaanxi Earthquake (1556). Deadliest known earthquake in history, estimated to have killed 830,000 in China.
Cascadia Earthquake (1700).
Kamchatka earthquakes (1737 and 1952).
Lisbon earthquake (1755).
New Madrid Earthquake (1811).
Fort Tejon Earthquake (1857).
Charleston earthquake (1886). Largest earthquake in the Southeast and killed 100.

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20th Century

San Francisco Earthquake (1906).
Great Kanto earthquake (1923). On the Japanese island of Honshu, killing over 140,000 in Tokyo and environs.
Kamchatka earthquakes (1952 and 1737).
Quake Lake (1959) 7.5 on Richter scale. Formed a lake in southern Montana, USA
Great Chilean Earthquake (1960). Biggest earthquake ever recorded, 9.5 on Moment magnitude scale.
Good Friday Earthquake (1964) Alaskan earthquake.
Ancash earthquake (1970). Caused a landslide that buried the town of Yungay, Peru; killed over 40,000 people.
Sylmar earthquake (1971). Caused great and unexpected destruction of freeway bridges and flyways in the San Fernando Valley, leading to the first major seismic retrofits of these types of structures, but not at a sufficient pace to avoid the next California freeway collapse in 1989.
Tangshan earthquake (1976). The most destructive earthquake of modern times. The official death toll was 255,000, but many experts believe that two or three times that number died.
Guatemala (1976). 7.5 on the Richter Scale, causing 23,000 deaths, 77,000 injuries and the destruction of more than 250,000 homes.
Great Mexican Earthquake (1985). 8.1 on the Richter Scale, killed over 6,500 people (though it is believed as many as 30,000 may have died, due to missing people never reappearing.)
Whittier Narrows earthquake (1987).
Armenian earthquake (1988). Killed over 25,000.
Loma Prieta earthquake (1989). Severely affecting Santa Cruz, San Francisco and Oakland in California. This is also called the World Series Earthquake. It struck as the World Series was just getting underway. Revealed necessity of accelerated seismic retrofit of road and bridge structures.
Northridge, California earthquake (1994). Damage showed seismic resistance deficiencies in modern low-rise apartment construction.
Great Hanshin earthquake (1995). Killed over 6,400 people in and around Kobe, Japan.
İzmit earthquake (1999) Killed over 17,000 in northwestern Turkey.
Düzce earthquake (1999)
Chi-Chi earthquake (1999)
Baku earthquake (2000).

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21st Century

Nisqually Earthquake (2001).
Gujarat Earthquake (2001).
Dudley Earthquake (2002).
Bam Earthquake (2003).
Parkfield, California earthquake (2004). Not large (6.0), but the most anticipated and intensely instrumented earthquake ever recorded and likely to offer insights into predicting future earthquakes elsewhere on similar slip-strike fault structures.
Chuetsu Earthquake (2004).
Indian Ocean Earthquake (2004). One of the largest earthquakes ever recorded at 9.0. Epicenter off the coast of the Indonesian island Sumatra. Triggered a tsunami which caused nearly 300,000 deaths spanning several countries.
Sumatran Earthquake (2005).
Fukuoka earthquake (2005).
Kashmir earthquake (2005). Killed over 79,000 people. Many more at risk from the Kashmiri winter.
Lake Tanganyika earthquake (2005).