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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, 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

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 release to space of the heat in the Earth's mantle and core. The heat causes the rock in the Earth to become flow on geological timescales, so that the plates move slowly but surely. Plate boundaries lock as the plates move 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, 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 convection flow of heat out from the Earth's deep interior.^[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, 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.^[2]

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.

Some earthquakes occur in a sort of earthquake storm, where earthquake strike a fault in clusters, each triggered by the previous shifts on the fault lines, similar to aftershocks, but occurring on adjacent segments of fault, sometimes 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.

Size and frequency of occurrence

Small 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, the Azores in Portugal, New Zealand, Greece and Japan.^[3] Large 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 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, although this is likely a statistical fluctuation. 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.

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.

Tsunamis

Undersea earthquakes and earthquake-triggered landslides into the sea, can cause 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

• Earthquake preparedness
• Household seismic safety
• HurriQuake nail (for resisting hurricanes and earthquakes)
• Seismic retrofit
• Seismic hazard
• Mitigation of seismic motion
• Earthquake prediction

Specific fault articles

Major earthquakes

Main article: List of earthquakes

Pre-20th century

• Pompeii (62).
• 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).
• Dubrovnik earthquake (1667). Disastrous earthquake in Dubrovnik, Croatia killed about 3/5 of the population.
• The great Sicilian earthquake (1693). As many as 100,000 may have died.
• 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 Caribbean.
• Calabria earthquake (1783). Series of 6 earthquakes in Calabria, Italy killed 50,000.^[8]
• New Madrid Earthquake (1811), and another tremor (1812) that also struck the small Missouri town, was reportedly the strongest ever in North America and made the Mississippi River temporarily change its direction and permanently altered its course in the region.
• Fort Tejon Earthquake (1857). Estimated Richter Scale above 8, said the strongest earthquake in Southern California history.
• 1872 Lone Pine 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.

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.
• Messina Earthquake (1908). Killed about 60,000 people.
• Gansu earthquake (1920). Killed 200,000 in Gansu province, China.^[9]
• Great Kantō earthquake (1923). On the Japanese island of Honshū, killing over 140,000 in Tokyo and environs.
• 1931 Hawke's Bay earthquake. Occurred in the Hawkes Bay in the North Island of New Zealand leaving 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.
• Ashgabat earthquake (1948). Earthquake in Ashgabat, Soviet Union measuring 7.3 on the Richter scale killed over 110,000 (2/3 the population of the city).^[10]
• Assam earthquake of 1950 (1950). Earthquake in Assam, India measures 8.6M.
• Kamchatka earthquakes (1952 and 1737), measuring >9.0.
• 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,^[11] 9.5 on Moment magnitude scale, and generated tsunamis throughout the Pacific ocean.
• 1960 Agadir earthquake, Morocco with around 15,000 casualties.
• 1963 Skopje earthquake, 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 second biggest earthquake recorded,^[12] 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 earthquake (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).
• Newcastle, NSW Australia earthquake 1989 {FLEMO}
• Armenian earthquake (1988). Killed over 25,000.
• Loma Prieta earthquake (1989). Severely affecting Santa Cruz, San Francisco, San Jose 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.
• Iran Earthquake (1990). 7.7 on the Richter scale. Killed over 35,000 in Gilan Province, southwest of Caspian sea.^[13]
• 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.
• August 1993 Guam Earthquake, measuring 8.2 on the Richter scale and lasting 60 seconds.
• Northridge, California earthquake (1994). Damage showed seismic resistance deficiencies in modern low-rise apartment construction.
• Sakhalin earthquake (1995). Measuring 7.6 on the Richter scale, killing over 2,000 people in Sakhalin, Russia.^[14]
• Great Hanshin earthquake (1995). Killed over 6,400 people in and around Kobe, Japan.
• Afghanistan earthquake (1998). 6.9 on the Richter scale. Some 125 villages were damaged and 4000 people killed.^[15]
• Athens earthquake (1999). 5.9 on the Richter scale, it hit Athens on September 7. Epicentered 10 miles north of the Greek capital, it claimed 143 lives.
• 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).

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.
• Chūetsu earthquake (2004).
• Sumatra-Andaman Earthquake (26 December 2004). By some estimates, the second largest earthquake in recorded history (estimates of magnitude vary between 9.1^[16] and 9.3). Epicentered off the coast of the Indonesian island of Sumatra, this massive earthquake triggered a series of gigantic tsunamis that smashed onto the shores of a number of nations, causing more than 285,000 fatalities.
• Sumatran (Nias) Earthquake (2005).
• Fukuoka earthquake (2005).
• Northern Chile Earthquake (2005). 7.9 (13 June). Killed only 15 people, but left many poor families homeless.
• 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).
• 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).
• 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).
• 2007 Guatemala Earthquake 6.7 magnitude (2007)
• July 16, 2007, 6.6 magnitude, Niigata prefecture, Japan (2007)
• 2007 Peru earthquake 8.0 magnitude, August 15 (2007)^[17]
• September 2007 Sumatra earthquakes 8.0 magnitude September 12 (2007)^[18]
• September 30, 2007, 6.8 magnitude, south of Mariana Islands (2007).
• September 30, 2007, 7.3 magnitude, northwest of Auckland Island, New Zealand (2007).

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.^[19]

In Greek mythology, Poseidon was the god of earthquakes.^[20]

In Christian mythology, certain saints were invoked as patrons against earthquakes, including Saint Gregory Thaumaturgus, Saint Agatha, Saint Francis Borgia, and Saint Emygdius.^[21]

See also

References

External links

Educational

• How to survive an earthquake - Guide for children and youth
• Guide to earthquakes and plate tectonics
• Earthquakes — an educational booklet by Kaye M. Shedlock & Louis C. Pakiser
• The Severity of an Earthquake
• USGS Earthquake FAQs
• Latest Earthquakes in the World - Past 7 days - View in near-real time all of the recent earthquake events on the planet.
• Earthquake Information from the Deep Ocean Exploration Institute, Woods Hole Oceanographic Institution
• Geo.Mtu.Edu — How to locate an earthquake's epicenter
• Photos/images of historic earthquakes
• earthquakecountry.info Answers to FAQs about Earthquakes and Earthquake Preparedness
• Interactive guide: Earthquakes - an educational presentation by Guardian Unlimited
• Geowall — an educational 3D presentation system for looking at and understanding earthquake data
• Virtual Earthquake - educational site explaining how epicenters are located and magnitude is determined
• HowStuffWorks — How Earthquakes Work
• CBC Digital Archives — Canada's Earthquakes and Tsunamis
• Earthquakes Educational Resources - dmoz

Seismological data centers

Europe

• European-Mediterranean Seismological Centre (EMSC)
• Global Seismic Monitor at GFZ Potsdam
• Global Earthquake Report – chart
• Earthquakes in Iceland during the last 48 hours
• Istituto Nazionale di Geofisica e Vulcanologia (INGV), Italy
• Database of Individual Seismogenic Sources (DISS), Central Mediterranean
• Portuguese Meteorological Institute (Seismic activity during the last month)

United States

• EQNET: Earthquake Information Network
• The U.S. National Earthquake Information Center
• Southern California Earthquake Data Center
• The Southern California Earthquake Center (SCEC)
• Putting Down Roots in Earthquake Country An Earthquake Science and Preparedness Handbook produced by SCEC
• Recent earthquakes in California and Nevada
• Seismograms for recent earthquakes via REV, the Rapid Earthquake Viewer
• Incorporated Research Institutions for Seismology (IRIS), earthquake database and software
• IRIS Seismic Monitor - world map of recent earthquakes
• SeismoArchives - seismogram archives of significant earthquakes of the world

Seismic scales

• The European Macroseismic Scale

Scientific information

• Earthquake Magnitudes and the Gutenberg-Richter Law. SimScience. Retrieved on 2006-08-14.
• Hiroo Kanamori, Emily E. Brodsky (June 2001). "The Physics of Earthquakes". Physics Today 54 (6): 34. 

Miscellaneous

• Kashmir Relief & Development Foundation (KRDF)
• PBS NewsHour - Predicting Earthquakes
• USGS – Largest earthquakes in the world since 1900
• The Destruction of Earthquakes - a list of the worst earthquakes ever recorded
• Los Angeles Earthquakes plotted on a Google map
• the EM-DAT International Disaster Database
• Earthquake Newspaper Articles Archive
• Earth-quake.org
• PetQuake.org- official PETSAAF system which relies on strange or atypical animal behavior to predict earthquakes.
• A series of earthquakes in southern Italy - November 23 1980, Gesualdo
• Recent Quakes WorldWide
• Real-time, worldwide earthquake list for the past 7 days
• Real-time earthquakes on Google Map, Australia and rest of the world
• Earthquake Information - Electricquakes.com Exploring possible links between solar activity and earthquakes with earthquake and solar data streaming sources shown side by side for visual correlation.
• Earthquake Information - detailed statistics and integrated with Google Maps and Google Earth
• Kharita - INGV portal for Digital Cartography - Last earthquakes recorded by INGV Italian Network (with Google Maps)
• Kharita - INGV portal for Digital Cartography - Italian Seismicity by region 1981-2006 (with Google Maps)

•  • [ e] Topics in geotechnical engineering
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