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===Naturally occurring earthquakes=== ===Naturally occurring earthquakes===


Most naturally occurring earthquakes are related to the tectonic nature of the ]. Such earthquakes are called '']''. The Earth's ] is a patch work of plates in slow spaz but constant motion caused by the heat in the Earth's ] and ]. Plate boundaries glide past each other, creating frictional ]. 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 '']''. When the failure at the fault plane results in a violent displacement of the Earth's ], the elastic ] energy is released and elastic waves are radiated, thus causing an earthquake. It is estimated that only 10 percent or less of an earthquake's total energy is ultimately radiated as seismic energy, while most of the earthquake's energy is used to power the earthquake ] growth and is eventually converted into heat. Therefore, earthquakes lower the Earth's available potential energy and thermal energy, though these losses are negligible. To describe the physical boob process of occurrence of an earthquake, seismologists use the ]. Most naturally occurring earthquakes are related to the tectonic nature of the ]. Such earthquakes are called '']''. The Earth's ] is a patch work of plates in slow but constant motion caused by the heat in the Earth's ] and ]. Plate boundaries glide past each other, creating frictional ]. 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 '']''. When the failure at the fault plane results in a violent displacement of the Earth's ], the elastic ] energy is released and elastic waves are radiated, thus causing an earthquake. It is estimated that only 10 percent or less of an earthquake's total energy is ultimately radiated as seismic energy, while most of the earthquake's energy is used to power the earthquake ] growth and is eventually converted into heat. Therefore, earthquakes lower the Earth's available potential energy and thermal energy, though these losses are negligible. To describe the physical process of occurrence of an earthquake, seismologists use the ].


The majority of tectonic earthquakes originate at depths not exceeding a few tens of kilometers. Earthquakes occurring at boundaries of tectonic plates are called '']s'', while the less frequent events that occur in the interior of the lithospheric plates are called '']s''. The majority of tectonic earthquakes originate at depths not exceeding a few tens of kilometers. Earthquakes occurring at boundaries of tectonic plates are called '']s'', while the less frequent events that occur in the interior of the lithospheric plates are called '']s''.

Revision as of 13:51, 7 September 2006

For other uses, see the natural seismic phenomenon.

An earthquake is a phenomenon that results from and is powered by the sudden release of stored energy that radiates 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

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 patch work of plates in slow but constant motion caused by the heat in the Earth's mantle and core. Plate boundaries glide 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 elastic waves are radiated, thus causing an earthquake. It is estimated that only 10 percent or less of an earthquake's total energy is ultimately radiated as seismic energy, while most of the earthquake's energy is used to power the earthquake fracture growth and is eventually converted into heat. Therefore, earthquakes lower the Earth's available potential energy and thermal energy, though these losses are negligible. To describe the physical process of occurrence of an earthquake, seismologists use the Elastic-rebound theory.

The majority of tectonic earthquakes originate at depths not exceeding a few tens of kilometers. 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.

Where the crust is thicker and colder, earthquakes occur at greater depths of hundreds of kilometers along subduction zones where plates descend into the Earth's mantle. These types of earthquakes are called deep focus earthquakes. They are possibly generated when subducted lithospheric material catastrophically undergoes a phase transition (e.g., olivine to spinel), releasing stored energy—such as elastic strain, chemical energy or gravitational energy—that cannot be supported at the pressures and temperatures present at such depths.

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 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

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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.

Characteristics

Damage from the 1906 San Francisco earthquake.
Section of collapsed freeway after the 1989 Loma Prieta earthquake.

Earthquakes occur on a daily basis around the world, most detected only by seismometers and causing no damage. Large earthquakes however 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 e.g gas leaks or petrol leaks. 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).

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 epicenter and usually coincides with the point where the fault slip started. The location on the surface directly above the epicenter is known as the hypocenter. The total length of the section of a fault that slips, the rupture zone, can be as long as 250 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.

Measuring earthquakes

Main article: Seismic scale
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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, while the intensity of shaking occurring at any given point on the Earth's surface is measured on an intensity scale.

It is commonly assumed that the Richter scale is the predominant measure of magnitude, but this is actually a poor scale. It saturates at values of about 7.1-7.2. Moment magnitude should be used for a reliable measure of earthquake strength.

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 Nisqually earthquake of February 28 2001.
File:Nisqually Community Internet Intensity Map for the Nisqually Earthquake FEB 2281854 ciim.gif
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 and Japan on the other side of the Pacific. 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, these include, but are not limited to,

  • Broken windows
  • Collapse of buildings
  • Fires, as seen in the 1906 San Francisco earthquake
  • 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
  • Los Angeles becoming a suburb of San Francisco

Preparation for earthquakes

Specific fault articles

Specific earthquake articles

Pre-20th Century

20th Century

21st Century

See also

References

  1. 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 2006-08-14.
  2. "Earthquake Hazards Program". USGS. Retrieved 2006-08-14.
  3. "Common Myths about Earthquakes". USGS. Retrieved 2006-08-14.
  4. "Earthquake Facts and Statistics: Are earthquakes increasing?". USGS. Retrieved 2006-08-14.
  5. "Historic Earthquakes and Earthquake Statistics: Where do earthquakes occur?". USGS. Retrieved 2006-08-14.
  6. "Visual Glossary - Ring of Fire". Retrieved 2006-08-14.

External links

Educational

Seismological data centers

Europe

United States

Seismic scales

Scientific information

Miscellaneous

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