AskDefine | Define earthquake

Dictionary Definition

earthquake

Noun

1 shaking and vibration at the surface of the earth resulting from underground movement along a fault plane of from volcanic activity [syn: quake, temblor, seism]
2 a disturbance that is extremely disruptive; "selling the company caused an earthquake among the employees"

User Contributed Dictionary

English

Pronunciation

  • IPA: /ˈɜː(r)θˌkweɪk/
  • SAMPA: /"3:(r)T%kweIk/

Noun

  1. A shaking of the Earth, caused by volcanic activity or movement around geologic faults.
  2. A natural disaster.

Synonyms

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Extensive Definition

An earthquake is the result of a sudden release of energy in the Earth's crust that creates seismic waves. Earthquakes are recorded with a seismometer, also known as a seismograph. The moment magnitude of an earthquake is conventionally reported, or the related and mostly obsolete Richter magnitude, with magnitude 3 or lower earthquakes being mostly imperceptible and magnitude 7 causing serious damage over large areas. Intensity of shaking is measured on the modified Mercalli scale.
At the Earth's surface, earthquakes manifest themselves by a shaking and sometimes displacement of the ground. When a large earthquake epicenter is located offshore, the seabed sometimes suffers sufficient displacement to cause a tsunami. The shaking in earthquakes can also trigger landslides and occasionally volcanic activity.
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. Earthquakes are caused mostly by rupture of geological faults, huge amounts of gas migration, mainly methane deep within the earth, but also by volcanic activity, landslides, mine blasts, and nuclear experiments.
An earthquake's point of initial rupture is called its focus or hypocenter. The term epicenter means the point at ground level directly above this.

Naturally occurring earthquakes

Tectonic earthquakes will occur anywhere within the earth where there is sufficient stored elastic strain energy to drive fracture propagation along a fault plane. In the case of transform or convergent type plate boundaries, which form the largest fault surfaces on earth, they will move past each other smoothly and aseismically only if there are no irregularities or asperities along the boundary that increase the frictional resistance. Most boundaries do have such asperities and this leads to a form of stick-slip behaviour. Once the boundary has locked, continued relative motion between the plates leads to increasing stress and therefore, stored strain energy in the volume around the fault surface. This continues until the stress has risen sufficiently to break through the asperity, suddenly allowing sliding over the locked portion of the fault, releasing the stored energy. This energy is released as a combination of radiated elastic strain seismic waves, frictional heating of the fault surface, and cracking of the rock, thus causing an earthquake. This process of gradual build-up of strain and stress punctuated by occasional sudden earthquake 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 or is converted into heat generated by friction. Therefore, earthquakes lower the Earth's available elastic potential energy and raise its temperature, though these changes are negligible compared to the conductive and convective flow of heat out from the Earth's deep interior.

Earthquakes away from plate boundaries

Where plate boundaries occur within continental lithosphere, deformation is spread out a over a much larger area than the plate boundary itself. In the case of the San Andreas fault continental transform, many earthquakes occur away from the plate boundary and are related to strains developed within the broader zone of deformation caused by major irregularities in the fault trace (e.g. the “Big bend” region). The Northridge earthquake was associated with movement on a blind thrust within such a zone. Another example is the strongly oblique convergent plate boundary between the Arabian and Eurasian plates where it runs through the northwestern part of the Zagros mountains. The deformation associated with this plate boundary is partitioned into nearly pure thrust sense movements perpendicular to the boundary over a wide zone to the southwest and nearly pure strike-slip motion along the Main Recent Fault close to the actual plate boundary itself. This is demonstrated by earthquake focal mechanisms.
All tectonic plates have internal stress fields caused by their interactions with neighbouring plates and sedimentary loading or unloading (e.g. deglaciation). These stresses may be sufficient to cause failure along existing fault planes, giving rise to intra-plate earthquakes.

Deep focus earthquakes

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, Deep focus earthquakes may occur at much greater depths (up to seven hundred kilometers). These seismically active areas of subduction are known as Wadati-Benioff zones. 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 and volcanic activity

Earthquakes also often occur in volcanic regions and are caused there, both by tectonic faults and by the movement of magma in volcanoes. Such earthquakes can serve as an early warning of volcanic eruptions.

Earthquake storms

Sometimes a series of earthquakes occur in a sort of earthquake storm, where the earthquakes strike a fault in clusters, each triggered by the shaking or stress redistribution of the previous earthquakes. Similar to aftershocks but on adjacent segments of fault, these storms occur over the course of years, 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.

Size and frequency of occurrence

Minor earthquakes occur nearly constantly around the world in places like California and Alaska in the U.S., as well as in Chile, Peru, Indonesia, Iran, Pakistan the Azores in Portugal, Turkey, New Zealand, Greece, Italy, and Japan, Larger earthquakes occur less frequently, the relationship being exponential; for example, 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 - 4.6 every year
  • an earthquake of 4.7 - 5.5 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. In fact, in recent years, the number of major earthquakes per year has actually decreased, although this is likely a statistical fluctuation. 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 Himalayan Mountains.
With the rapid growth of mega-cities such as Mexico City, Tokyo or Tehran, in areas of high seismic risk, some seismologists are warning that a single quake may claim the lives of up to 3 million people.

Effects/impacts of earthquakes

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

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.
Ground rupture is a visible breaking and displacement of the earth's surface along the trace of the fault, which may be of the order of few metres in the case of major earthquakes. Ground rupture is a major risk for large engineering structures such as dams, bridges and nuclear power stations and requires careful mapping of existing faults to identify any likely to break the ground surface within the life of the structure.

Landslides and avalanches

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

Fires

Following an earthquake, fires can be generated by break of the electrical power or gas lines. In the event of water mains rupturing and a loss of pressure, it may also become difficult to stop the spread of a fire once it has started.

Soil liquefaction

Soil liquefaction occurs when, because of the shaking, water-saturated granular material temporarily loses its 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.

Tsunami

Undersea earthquakes and earthquake-triggered landslides into the sea, can cause Tsunami. See, for example, the 2004 Indian Ocean earthquake.

Floods

Floods may be a secondary effects of earthquakes, if dams are damaged.
Earthquakes may cause landslips to dam rivers, which then collapse and cause floods.

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.
The most significant human impact is loss of life

Specific fault articles

Major earthquakes

Pre-20th century

20th century

21st century

Earthquakes in mythology and religion

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.
In Greek mythology, Poseidon was the god of earthquakes.

See also

References

External links

earthquake in Dutch: Aardbeving
earthquake in Japanese: 地震
earthquake in Norwegian: Jordskjelv
earthquake in Norwegian Nynorsk: Jordskjelv
earthquake in Oromo: Chocho'a lafa
earthquake in Uzbek: Zilzila
earthquake in Low German: Eerdbeven
earthquake in Polish: Trzęsienie ziemi
earthquake in Portuguese: Sismo
earthquake in Romanian: Cutremur
earthquake in Romansh: Terratrembel
earthquake in Quechua: Pacha kuyuy
earthquake in Russian: Землетрясение
earthquake in Sicilian: Tirrimotu
earthquake in Simple English: Earthquake
earthquake in Slovak: Zemetrasenie
earthquake in Slovenian: Potres
earthquake in Silesian: Třyńśyńy źymji
earthquake in Serbian: Земљотрес
earthquake in Serbo-Croatian: Potres
earthquake in Finnish: Maanjäristys
earthquake in Swedish: Jordbävning
earthquake in Tagalog: Lindol
earthquake in Tamil: நிலநடுக்கம்
earthquake in Telugu: భూకంపం
earthquake in Thai: แผ่นดินไหว
earthquake in Vietnamese: Động đất
earthquake in Tajik: Заминларза
earthquake in Turkish: Deprem
earthquake in Ukrainian: Землетрус
earthquake in Urdu: زلزلہ کي تاريخ
earthquake in Venetian: Teremoto
earthquake in Walloon: Tronnmint d' tere
earthquake in Yiddish: ערדציטערניש
earthquake in Contenese: 地震
earthquake in Chinese: 地震

Synonyms, Antonyms and Related Words

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