Earthquake

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This article is about the natural seismic phenomenon. For other uses, see Earthquake (disambiguation).

An earthquake is a natural phenomenon that results from the sudden release of stored energy in the Earth's crust that creates seismic waves. At the Earth's surface, earthquakes may manifest themselves by a shaking or displacement of the ground and sometimes cause tsunamis, which may lead to loss of life and destruction of property. An earthquake is caused by tectonic plates getting stuck and putting a strain on the ground. The strain becomes so great that rocks give way and fault lines occur.

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
Global earthquake epicenters, 1963–1998
Global earthquake plate tectonic movement
Global earthquake plate tectonic movement

Contents

[edit] Types of earthquakes

[edit] Naturally occurring earthquakes

Plate tectonics wave types
Plate tectonics wave types

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 planetary core. The heat causes the rock under the earth to become liquid magma, which the plates are able to move around on, slowly but surely. 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 process 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.[1]

The majority of tectonic earthquakes originate at depths not exceeding 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.[2]

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.

[edit] 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.[3]

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 precise location of a nuclear 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.

[edit] Measuring earthquakes

Main article: Seismic scale
Richter Magnitude Scale of the Sumatra-Andaman earthquake and Asian Tsunami by date.
Richter Magnitude Scale of the Sumatra-Andaman earthquake and Asian Tsunami by date.

Because seismologists cannot directly observe rupture in the Earth's interior, they rely on seismographs, geodetic measurements, and numerical modeling to analyze seismic waves and accurately assess the size and other physical characteristics of earthquakes. The size of an earthquake can be expressed quantitatively as a magnitude and the local strength of shaking as an intensity. The inherent size of an earthquake is expressed using a magnitude.

The empirically-defined Richter scale is a famous (and the original) example of a such a scale. However, the Richter scale is not well-suited to accurately measure earthquakes with magnitudes over approximately 6.8, and was furthermore originally defined by Charles Richter to apply to earthquakes only in southern California. Most researchers (and increasingly the media) now calculate and report magnitudes using the moment magnitude scale. The seismic moment and its associated moment magnitude scale are based on the fundamental faulting parameters of best fit planar fault area, average fault slip, and the rigidity of the surrounding medium.

The use of intensities has largely been superseded by the development and widespread deployments of strong-motion seismometers capable of recording ground accelerations that are an appreciable fraction of g. However, intensity estimates based on common effects of strong shaking are still useful for assessing pre-instrumental earthquakes. The Mercalli intensity scale, which measures the effects of the seismic waves, is a commonly referenced intensity scale.

[edit] 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.
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.
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.

[edit] 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.[4] 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. In the (low seismicity) United Kingdom, for example, it has been calculated that the average recurrences are:

  • 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 seismic stations has increased from about 350 in 1931 to many thousands today. As a result, many more earthquakes are reported than in the past because of the vast improvement in instrumentation (not because the number of earthquakes has increased). 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.[5] 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.[6]

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.[7][8] Massive earthquakes tend to occur along other plate boundaries, too, such as along the Himalayan Mountains.

[edit] Effects/impacts of earthquakes

Chuetsu earthquake.
Chuetsu earthquake.
Smoldering after the 1906 San Francisco earthquake.
Smoldering after the 1906 San Francisco earthquake.
Man walking around in Ruins after Tsunami.
Man walking around in Ruins after Tsunami.

There are many effects of earthquakes including, but not limited to the following:

[edit] Shaking and ground rupture

Shaking and ground rupture are the main effects created by earthquakes, principally resulting in more or less severe damage to buildings or other rigid structures. The severity of the local effects depends on the complex combination of the earthquake magnitude, the distance from epicenter, and the local geological and geomorphological conditions, which may amplify or reduce wave propagation. The ground-shaking is measured by ground acceleration.

Specific local geological, geomorphological, and geostructural features can induce high levels of shaking on the ground surface even from low-intensity earthquakes. This effect is called site or local amplification. It is principally due to the transfer of the seismic motion from hard deep soils to soft superficial soils and to effects of seismic energy focalization owing to typical geometrical setting of the deposits.

[edit] Landslides and avalanches

Earthquakes can cause landslides and avalanches, which may cause damage in hilly and mountainous areas.

[edit] Fires

Following an earthquake, fires can be generated by break of the electrical power or gas lines.

[edit] Soil liquefaction

Soil liquefaction occurs when, because of the shaking, water-saturated granular material temporally loses their strength and transforms from a solid to a liquid. Soil liquefaction may cause rigid structures, as buildings or bridges, to tilt or sink into the liquefied deposits.

[edit] Tsunamis

See, for example, the 2004 Indian Ocean earthquake.

[edit] Human impacts

Earthquakes may result in disease, lack of basic necessities, loss of life, higher insurance premiums, general property damage, road and bridge damage, and collapse of buildings or destabilization of the base of buildings which may lead to collapse in future earthquakes.

[edit] Preparation for earthquakes

[edit] Specific fault articles

[edit] Wikipedia articles on major earthquakes

[edit] Pre-20th Century

The 1988 Spitak earthquake which claimed more than 25,000 peoples lives and left more than 500,000 homeless.
The 1988 Spitak earthquake which claimed more than 25,000 peoples lives and left more than 500,000 homeless.

14 Total

[edit] 20th Century

33 Total

[edit] 21st Century

[edit] Earthquakes in mythology

In Norse mythology, earthquakes were explained as the violent struggling of the god Loki. When Loki, god of mischief and strife, murdered Baldr, god of beauty and light, he was punished by being bound in a cave with a poisonous serpent placed above his head dripping venom. Loki's wife Sigyn stood by him with a bowl to catch the poison, but whenever she had to empty the bowl the poison would drip on Loki's face, forcing him to jerk his head away and thrash against his bonds, causing the earth to tremble.[9] Another myth is in Greece. The God Zeus created lightning and struck the Earth whenever something bad happened.

[edit] See also

Look up earthquake in Wiktionary, the free dictionary.

[edit] References

  1. ^ Error on call to Template:cite web: Parameters url and title must be specifiedSpence, William; S. A. Sipkin, G. L. Choy (1989). . United States Geological Survey. Retrieved on 2006-11-03.
  2. ^ Greene, H. W.; Burnley, P. C. (26 October 1989). "A new self-organizing mechanism for deep-focus earthquakes". Nature 341: 733-737. DOI:10.1038/341733a0. Retrieved on 2006-11-03. 
  3. ^ BALLIN! S C Bhatia, M Ravi Kumar and H K Gupta. A Probabilistic Seismic Hazard Map of India and Adjoining Regions. Global Seismic Hazard Assessment Program. Retrieved on 2006-08-14.
  4. ^ Earthquake Hazards Program. USGS. Retrieved on 2006-08-14.
  5. ^ Common Myths about Earthquakes. USGS. Retrieved on 2006-08-14.
  6. ^ Earthquake Facts and Statistics: Are earthquakes increasing?. USGS. Retrieved on 2006-08-14.
  7. ^ Historic Earthquakes and Earthquake Statistics: Where do earthquakes occur?. USGS. Retrieved on 2006-08-14.
  8. ^ Visual Glossary - Ring of Fire. Retrieved on 2006-08-14.
  9. ^ Prose Edda by Snorri Sturluson

[edit] External links

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