Earthquake

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.

An earthquake's underground point of origin is called its focus or hypocenter. The better-known term epicenter means the point at ground level directly above this.

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 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.

Measuring earthquakes

Main article: Seismic scale

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.

Seismic maps

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.^[3] 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.^[4] 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.^[5]

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.^[6][7] Massive earthquakes tend to occur along other plate boundaries, too, such as along the Himalayan Mountains.

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.

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.

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.

Tsunamis

See, for example, the 2004 Indian Ocean earthquake.

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.

Preparation for earthquakes

• Emergency preparedness
• Household seismic safety
• Seismic retrofit
• Seismic hazard
• Mitigation of seismic motion
• Earthquake prediction

Specific fault articles

• Alpine Fault
• Calaveras Fault
• Cascadia subduction zone
• Geology of the Death Valley area
• Great Glen Fault
• Great Sumatran fault
• Hayward Fault Zone
• Highland Boundary Fault
• Hope Fault
• Liquiñe-Ofqui Fault
• North Anatolian Fault Zone
• New Madrid Fault Zone
• San Andreas Fault

Major earthquakes

Pre-20th Century

• Aleppo Earthquake (1138).
• Basel earthquake (1356). Major earthquake that struck Central Europe in 1356.
• Carniola earthquake (1511). A major earthquake that shook a large portion of South-Central Europe. Its epicenter was around the town of Idrija, in today's Slovenia. It caused great damage to structures all over Carniola, including Ljubljana, and minor damage in Venice, among other cities.
• Shaanxi Earthquake (1556). Deadliest known earthquake in history, estimated to have killed 830,000 in China.
• Dover Straits earthquake of 1580 (1580).
• Cascadia Earthquake (1700).
• Kamchatka earthquakes (1737 and 1952).
• Lisbon earthquake (1755), one of the most destructive and deadly earthquakes in history, killing between 60,000 and 100,000 people and causing a major tsunami that affected parts of Europe, North Africa and the Carribean.
• New Madrid Earthquake (1811) and another tremor (1812), both struck the small Missouri town, reportedly to been the strongest ever in North America and made the Mississippi River temporarily change its course.
• Fort Tejon Earthquake (1857). Estimated Richter Scale above 8, said the strongest earthquake in Southern California history.
• Owens Valley earthquake (1872). Might been strongest ever measured in California with an estimated Richter Scale of 8.1 said seismologists.
• Charleston earthquake (1886). Largest earthquake in the southeastern United States, killed 100.
• Ljubljana earthquake (14. IV. 1895), a series of powerful quakes that ultimately had a vital impact on the city of Ljubljana, being a catalyst of its urban renewal.
• Assam earthquake of 1897 (1897). Large earthquake that destroyed all masonry structures, measuring more than 8 on the Richter scale.

14 Total

20th Century

• San Francisco Earthquake (1906). Between 7.7 and 8.3 magnitudes; killed approximately 3,000 people and caused around $400 million in damage; most devastating earthquake in California and U.S. history.
• Great Kantō earthquake (1923). On the Japanese island of Honshū, killing over 140,000 in Tokyo and environs.
• Napier earthquake (1931). 256 dead.
• 1933 Long Beach earthquake
• 1935 Balochistan earthquake at Quetta, Pakistan measuring 7.7 on the Richter scale. Anywhere from 30,000 to 60,000 people died
• 1939 Erzincan earthquake at Erzincan, Turkey measuring 7.9 on the Richter scale.
• Assam earthquake of 1950 (1950). Earthquake in Assam, India measures 8.6M.
• Kamchatka earthquakes (1952 and 1737).
• Great Kern County earthquake (1952). This was second strongest tremor in Southern California history, epicentered 60 miles North of Los Angeles. Major damage in Bakersfield, California and Kern County, California, while it shook the Los Angeles area.
• Quake Lake (1959) Formed a lake in southern Montana, USA
• Great Chilean Earthquake (1960). Biggest earthquake ever recorded, 9.5 on Moment magnitude scale, and generated tsunamis throughout the Pacific ocean.
• Skopje earthquake of 1963, measuring 6.1 on the Richter scale kills 1,800 people, leaves another 120,000 homeless, and destroys 80% of the city.
• Good Friday Earthquake (1964) In Alaska, it was the third biggest earthquake recorded, measuring 9.2M. and generated tsunamis throughout the Pacific ocean.
• 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.
• Managua earthquake (1972), which killed more than 10,000 people and destroyed 90% of the city. The earthquake took place on December 23 1972 at midnight.
• Friuli earthquake (1976), Which killed more than 2.000 people in Northeastern Italy on the 6th of May
• 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). Causing 23,000 deaths, 77,000 injuries and the destruction of more than 250,000 homes.
• Coalinga, California earthquake (1983). 6.5 on the Richter scale on a section of the San Andreas Fault. Six people killed, downtown Coalinga, California devastated and oil field blazes.
• Great Mexican Earthquake (1985). Killed over 6,500 people (though it is believed as many as 30,000 may have died, due to missing people never reappearing.)
• Great San Salvador Earthquake (October 10, 1986). Killed over 1,500 people.
• 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 Game 3 of the 1989 World Series was just getting underway at Candlestick Park in San Francisco. Revealed necessity of accelerated seismic retrofit of road and bridge structures.
• Luzon Earthquake (1990). On 16 July 1990, an earthquake measuring 7.7 on the Richter scale struck the island of Luzon, Philippines.
• Landers, California earthquake (1992). Serious damage in the small town of Yucca Valley, California and was felt across 10 states in Western U.S. Another tremor measured 6.4 struck 3 hours later and felt across Southern California.
• 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.
• Chi-Chi earthquake (1999) Also called the 921 earthquake. Struck Taiwan on September 21 1999. Over 2,000 people killed, destroyed or damaged over ten thousand buildings. Caused world computer prices to rise sharply.
• Armenia, Colombia (1999) 6.2 on the Richter scale, Killed over 2,000 in the Colombian Coffee Grown Zone.
• 1999 İzmit earthquake measuring 7.4 on the Richter scale and killed over 17,000 in northwestern Turkey.
• Hector Mine earthquake (1999). 7.1 on the Richter scale, epicentered 30 miles east of Barstow, California, widely felt in California and Nevada.
• 1999 Düzce earthquake at Düzce, Turkey measuring 7.2 on the Richter scale.
• Baku earthquake (2000).

33 Total

21st Century

• Nisqually Earthquake (2001).
• El Salvador earthquakes (2001). 7.9 (13 January) and 6.6 (13 February) magnitudes, killed more than 1,100 people.
• Gujarat Earthquake (26 January 2001).
• Hindu Kush earthquakes (2002). Over 1.100 killed.
• Molise earthquake (2002) 26 killed.
• Bam Earthquake (2003). Over 40,000 people are reported dead.
• 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).
• Sumatra-Andaman Eathquake (26 December 2004). Second largest earthquake in recorded history, registering a moment magnitude of 9.3. Epicentered off the coast of the Indonesian island of Sumatra, this massive temblor triggered a series of gigantic tsunamis that smashed onto the shores of a number of nations, causing more than 229,000 fatalities.
• Sumatran (Nias) Earthquake (2005).
• Fukuoka earthquake (2005).
• Kashmir earthquake (2005) (also known as the Great Pakistan earthquake). Killed over 79,000 people; and many more injured.
• Lake Tanganyika earthquake (2005).
• May 2006 Java earthquake (2006).
• July 2006 7.7 magnitude Java earthquake which triggered tsunamis (2006).
• July 2006 6.3 magnitude Java earthquake (2006).
• July 2006 6.6 magnitude Celebes earthquake (2006).
• August 2006 5.9 magnitude Michoacan earthquake (2006).
• September 2006 6.0 magnitude Gulf of Mexico earthquake (2006).
• October 2006 6.6 magnitude Kona, Hawaii earthquake (2006).
• November 2006 8.1 magnitude north of Japan (2006).
• December 26, 2006, 7.2 magnitude, southwest of Taiwan (2006).
• January 13, 2007, 8.2 magnitude, north of Japan (2007).
• February 12, 2007, 6.0 magnitude, southwest of Cape St. Vincent, Portugal (2007).
• Sumatra Earthquakes March 06, 2007, 6.4 and 6.3 magnitude, Sumatra, Indonesia (2007).
• March 25, 2007, 6.9 magnitude, off the west coast of Honshū, Japan (2007).
• April 1, 2007, 8.1 magnitude, Solomon Islands (2007).

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.^[8] Another myth is in Greece. The God Zeus created lightning and struck the Earth whenever something bad happened.

See also

• Catastrophe modeling
• Cryoseism
• Earthquake insurance
• Earthquake lights
• Earthquake weather
• Earthquake (1974 disaster film)
• Earthquakes in literature
• Elastic-rebound theory
• Geophysics
• HurriQuake environment nail
• Interplate earthquake
• Intraplate earthquake
• List of earthquakes
• List of tectonic plates
• Megathrust earthquake
• Mercalli intensity scale
• Moonquake
• Plate tectonics
• Richter magnitude scale
• Seismic scale
• Seismic wave
• Seismogenic layer
• Seismograph
• Seismology
• Shock (mechanics)
• Submarine earthquake
• Tsunami
• The VAN method to predict earthquakes

References

Wikipedia information about Earthquake This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Earthquake". It may not have been reviewed by professional editors (see full disclaimer). Help contribute to Americola and edit this article.