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An earthquake is a tremor of the earth's surface usually triggered by the release of underground stress along fault lines. This release causes movement in masses of rock and resulting shock waves. In spite of extensive research and sophisticated equipment, it is impossible to predict an earthquake, although experts can estimate the likelihood of an earthquake occurring in a particular region.

In 1935, American seismologist Charles Richter developed a scale that measures the magnitude of seismic waves. Called the Richter scale, it rates earth tremors on a scale from 1 to 9, with 9 being the most powerful and each number representing an increase of ten times the energy over the previous number. According to this scale, any quake that is higher than 4.5 can cause damage to stone buildings; quakes rated a magnitude of 7 and above are considered very severe. A less-known scale, the Mercalli scale, was devised by Italian seismologist Giuseppe Mercalli to measure the severity of an earthquake in terms of its impact on a particular area and its inhabitants and buildings.

Some earthquakes are too small to be felt but can cause movement of the earth, opening up holes and displacing rocks. Shock waves from a very powerful earthquake can trigger smaller quakes hundreds of miles away from the epicenter. Approximately 1,000 earthquakes measuring 5.0 and above occur yearly. Earthquakes of the greatest intensity happen about once a year and major earthquakes (7.0-7.9) occur about 18 times a year. Strong earthquakes (6.0-6.9) occur about 10 times a month and moderate earthquakes (5.0-5.9) happen more than twice daily. Most earthquakes are not even noticed by the general public, since they happen either under the ocean or in unpopulated areas. Sometimes an earthquake under the ocean can be so severe, it will cause a tsunami, responsible for far greater damage.

The greatest danger of an earthquake comes from falling buildings and structures and flying glass, stones and other objects.

If you live in an earthquake-prone area, here are some steps that can be taken to minimize risks:

  • Affix bookcases, cabinets, refrigerators and furniture to the walls.
  • Fit cabinets with "childproof locks," so doors will remain closed and items won't fly out.
  • California and Japan sell silicone putty kits that can be used to stick dishes and other breakables to the walls.
  • Have a backpack prepared and attached to the bed, containing shoes, a flashlight and batteries, keys, money, first-aid supplies and medicines, a knife, food, water, ID and insurance information. Attaching the pack to the bed helps to insure that it will not be thrown around during an earthquake.
  • Keep shoes next to your bed, so you can put them on as soon as a quake begins.
  • Have a family evacuation plan including phone numbers and a safe place to which to evacuate.
  • Establish escape routes from each room in the house.

If you are in an earthquake:

  • If you are indoors, find a secure location to wait out the quake, such as under a heavy table or desk, or in an interior hallway where you can brace yourself between two walls. Doorways are among the safest places to stand, thanks to the strong beams overhead. However, watch out for swinging doors. Stay away from windows.
  • If you are outdoors, try to get into an open area, away from falling buildings, power lines, trees, etc.
  • If you are in a crowded public area, crouch down, with your hands protecting your head and neck.
  • If you are in your car, pull over to the side, away from power lines and overpasses, and stay inside the car until the shaking has subsided.
  • Be sure to put on shoes immediately, to avoid injury from stepping on broken glass and objects.
  • Check yourself and others for injuries.
  • Check for gas and water leaks and damage to electrical wires. Only turn off gas lines if there is damage; it may take a while for technicians to get to your area to turn gas and power back on.
  • Survey the exterior of your home for structural damage to the chimney, roof, foundation and walls.
  • Do NOT use your automobile unless there is an emergency.
  • If you must leave the area, try to leave word where you can be contacted.

REMEMBER that there may be aftershocks, which can also cause great damage to your surroundings. Be prepared!

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Dictionary:

earthquake

  (ûrth'kwāk') pronunciation
n.

A sudden movement of the earth's crust caused by the release of stress accumulated along geologic faults or by volcanic activity. Also called seism, temblor.


 
 
Sci-Tech Encyclopedia: Earthquake

The sudden movement of the Earth caused by the abrupt release of accumulated strain along a fault in the interior. The released energy passes through the Earth as seismic waves (low-frequency sound waves), which cause the shaking. Seismic waves continue to travel through the Earth after the fault motion has stopped. Recordings of earthquakes, called seismograms, illustrate that such motion is recorded all over the Earth for hours, and even days, after an earthquake.

Earthquakes are not distributed randomly over the globe but tend to occur in narrow, continuous belts of activity. Approximately 90% of all earthquakes occur in these belts, which define the boundaries of the Earth's plates. The plates are in continuous motion with respect to one another at rates on the order of centimeters per year; this plate motion is responsible for most geological activity.

Plate motion occurs because the outer cold, hard skin of the Earth, the lithosphere, overlies a hotter, soft layer known as the asthenosphere. Heat from decay of radioactive minerals in the Earth's interior sets the asthenosphere into thermal convection. This convection has broken the lithosphere into plates which move about in response to the convective motion. As the plates move past each other, little of the motion at their boundaries occurs by continuous slippage; most of the motion occurs in a series of rapid jerks. Each jerk is an earthquake. This happens because, under the pressure and temperature conditions of the shallow part of the Earth's lithosphere, the frictional sliding of rock exhibits a property known as stick-slip, in which frictional sliding occurs in a series of jerky movements, interspersed with periods of no motion—or sticking. In the geologic time frame, then, the lithospheric plates chatter at their boundaries, and at any one place the time between chatters may be hundreds of years. See also Plate tectonics.

The periods between major earthquakes is thus one during which strain slowly builds up near the plate boundary in response to the continuous movement of the plates. The strain is ultimately released by an earthquake when the frictional strength of the plate boundary is exceeded. See also Fault and fault structures.

Most great earthquakes occur on the boundaries between lithospheric plates and arise directly from the motions between the plates. These may be called plate boundary earthquakes. There are many earthquakes, sometimes of substantial size, that cannot be related so simply to the movements of the plates. At many plate boundaries, earthquakes occur over a broad zone—often several hundred miles wide—adjacent to the plate boundary. These earthquakes, which may be called plate boundary-related earthquakes, are secondarily caused by the stresses set up at the plate boundary. Some earthquakes also occur, although infrequently, within plates. These earthquakes, which are not related to plate boundaries, are called intraplate earthquakes. The immediate cause of intraplate earthquakes is not understood.

In addition to the tectonic types of earthquakes described above, some earthquakes are directly associated with volcanic activity. These volcanic earthquakes result from the motion of undergound magma that leads to volcanic eruptions.

Earthquakes often occur in well-defined sequences in time. Tectonic earthquakes are often preceded, by a few days to weeks, by several smaller shocks (foreshocks), and are nearly always followed by large numbers of aftershocks. Foreshocks and aftershocks are usually much smaller than the main shock. Volcanic earthquakes often occur in flurries of activity, with no discernible main shock. This type of sequence is called a swarm.

Earthquakes range enormously in size, from tremors in which slippage of a few tenths of an inch occurs on a few feet of fault, to the greatest events, which may involve a rupture many hundreds of miles long, with tens of feet of slip.

The size of an earthquake is given by its moment: average slip times the fault area that slipped times the elastic constant of the Earth. The units of seismic moment are dyne-centimeters. An older measure of earthquake size is magnitude, which is proportional to the logarithm of moment. Magnitude 2.0 is about the smallest tremor that can be felt. Most destructive earthquakes are greater than magnitude 6; the largest shock known was the 1960 Chile earthquake, with a moment of 1030 dyne-centimeters (1023 newton-meters) or magnitude 9.5. It involved a fault 600 mi (1000 km) long slipping 30 ft (10 m).

The intensity of an earthquake is a measure of the severity of shaking and its attendant damage at a point on the surface of the Earth. The same earthquake may therefore have different intensities at different places. The intensity usually decreases away from the epicenter (the point on the surface directly above the onset of the earthquake), but its value depends on many factors and generally increases with moment. Intensity is usually higher in areas with thick alluvial cover or landfill than in areas of shallow soil or bare rock. Poor building construction leads to high intensity ratings because the damage to structures is high. Intensity is therefore more a measure of the earthquake's effect on humans than an innate property of the earthquake.

Many additional effects may be produced by earthquake shaking, including landslides and tsunamis. See also Landslide; Tsunami.

Earthquake prediction research has been going on for nearly a century. Unfortunately, successful earthquake predictions are extremely rare. There are two basic categories of earthquake predictions: forecasts (months to years in advance) and short-term predictions (hours or days in advance). Forecasts are based a variety of research, including the history of earthquakes in a specific region, the identification of fault characteristics (including length, depth, and segmentation), and the identification of strain accumulation. Data from these studies are used to provide rough estimates of earthquake sizes and recurrence intervals.

 
Thesaurus: earthquake

noun

    A shaking of the earth: quake, seism, temblor, tremblor, tremor. Informal shake. See move/halt, repetition.

 
Geography Dictionary: earthquake

A sudden and violent movement, or fracture, within the earth followed by the series of shocks resulting from this fracture. The point of origin of an earthquake is known as the focus (but see epicentre). Earthquakes occur in narrow, continuous belts of activity which correspond with the junction of plates.

The scale of the shock of an earthquake is known as the magnitude; the most commonly used scale is the Richter scale, while the intensity of an earthquake is measured by the Mercalli scale.

Earthquake waves are of three basic types: P, primary, push waves travel from the focus by the displacement of surrounding particles and are transmitted though solids, liquids, and gases. S, secondary or shake waves travel through solids. L, long or surface waves travel on the earth's surface. The monitoring of these waves indicates that the earth's core is molten since S waves do not pass through it. see seismic waves, seismology.

Fully credible earthquake predictions are not yet available; one of the most hopeful avenues entails the application of dilatancy theory.

 
Britannica Concise Encyclopedia: earthquake

Sudden shaking of the ground caused by a disturbance deeper within the crust of the Earth. Most earthquakes occur when masses of rock straining against one another along fault lines suddenly fracture and slip. The Earth's major earthquakes occur mainly in belts coinciding with the margins of tectonic plates. These include the Circum-Pacific Belt, which affects New Zealand, New Guinea, Japan, the Aleutian Islands, Alaska, and the western coasts of North and South America; the Alpide Belt, which passes through the Mediterranean region eastward through Asia; oceanic ridges in the Arctic, Atlantic, and western Indian oceans; and the rift valleys of East Africa. The "size," or magnitude, of earthquakes is usually expressed in terms of the Richter scale, which assigns levels from 1.0 or lower to 8.0 or higher. The largest quake ever recorded (Richter magnitude 9.5) occurred off the coast of Chile in 1960. The "strength" of an earthquake is rated in intensity scales such as the Mercalli scale, which assigns qualitative measures of damage to terrain and structures that range from "not felt" to "damage nearly total." The most destructive quake of modern times occurred in 1976, when the city of Tangshan, China, was leveled and more than 250,000 people killed. See also seismic wave; seismology.

For more information on earthquake, visit Britannica.com.

 
US History Encyclopedia: Earthquakes

Earthquakes occur when the lithospheric plates that compose the surface of the earth shift in relation to one another. Earthquakes are happening constantly all over the world, but major quakes seem to occur only once every two or three years. The size of an earthquake is generally described in terms of intensity and magnitude. The Modified Mercalli scale gauges earthquake intensity by assessing the effect of the quake on the inhabitants of an area. Intensity assessments do not depend on seismographic instruments, but are subjective appraisals of (1) human and animal reaction to shaking and, (2) damage to structures of human origin and to the ground surface. Seismologists use the scale to assign to each earthquake an intensity ranking from I (felt by only a few people under favorable conditions) to XII (total damage).

Magnitude of energy released by an earthquake at its point of origin is a strictly quantitative measure based upon data from seismographs that record maximum wave amplitude (the extreme range of vibrations—or shock waves—caused by the sudden movement of the earth's crust). Charles Richter developed the first magnitude scale in 1935, but a variety of magnitude scales are used today. The Richter magnitude scale has no upper or lower numerical limits; some very small earthquakes are actually given negative numbers. The scale is logarithmic, meaning that each increase of one Richter number represents a tenfold increase in the magnitude of the earthquake. An earthquake of magnitude 5 releases energy equivalent to that released by 1,000 tons of TNT. Recently, seismologists and earthquake engineers have begun to use a measure called "seismic moment" to estimate the size of seismic sources. Moment magnitude measures the leverage of the forces (couples) across the whole area of the fault slip rather than just wave motion, which is affected by fracture and friction in the rocks.

Scientists have used intensity and magnitude data to prepare seismic risk maps of the United States. One map places locales in one of four zones: Zone 0, such as Florida, is an area where no damage is expected; Zone 3 is one in which a quake intensity of VIII and higher is expected, as in parts of California. The western United States exhibits the greatest seismic activity in the country—especially Alaska, California, Nevada, Utah, and Montana—although the upper part of the Mississippi embayment, southwest Kentucky, southern Illinois, and southeastern Missouri are also seismically active.

The historical record of earthquakes in the United States goes back to 1638 in New England and to about 1800 in California. One of the earliest major earthquakes to affect the colonies occurred in the Three Rivers area north of Quebec, along the lower Saint Lawrence River, on 5 February 1663. It caused chimneys to break as far away as Massachusetts Bay. In the early nineteenth century, the Midwest was hit with a series of earthquakes that began in New Madrid, Missouri. The largest of the shocks from these quakes, which occurred in 1811 and 1812, were felt over an area of about 950,250 square miles. Nor has the southern part of the United States been spared. An unpredicted earthquake occurred near Charleston, South Carolina, on 31 August 1886 that did considerable damage in Charleston (much of which was built on filled land) and killed, by some estimates, more than one hundred people. It was the largest seismic event in recorded history on the eastern seaboard. Tremors were felt as far away as New York, Boston, Cuba, and Bermuda. The most notorious earthquake in U.S. history was the one that hit San Francisco on 18 April 1906. It was associated with a rupture of the San Andreas fault from the vicinity of Point Delgada to a point in San Benito County near San Juan, a distance of more than 250 miles. The shock hit at 5 A.M. and, almost instantly, building after building crumbled to the ground. Thousands of fires ignited and burned out of control for three days fed by severed electrical wires, overturned coal burners, ruptured gas mains, broken water lines that prevented fighting the fires, and bungled efforts of troops trying to create backfires with dynamite. The earthquake and fire caused extensive damage throughout northern California, but in San Francisco it obliterated 500 city blocks, caused nearly $500 million in damages, and killed more than 3,000 people.

California was hit again by major earthquakes in 1925 and 1933, but it was almost sixty years before the United States experienced another quake of the magnitude of the 1906 San Francisco earthquake. That event occurred during the late afternoon of 27 March 1964, at 5:36 P.M. local time. An earthquake of magnitude 8.6 on the Richter scale occurred in the sparsely inhabited mountainous area of northern Prince William Sound in south central Alaska. It caused serious damage within an area of approximately 7,500 square miles, creating large changes in land levels and vertical displacements of nearly thirty-six feet in places along the continental margin. Three hundred people were killed, some from the effects of the quake itself and others by drowning in the seismic sea-wave (tsunami, or tidal wave) caused by the quake.

During the last third of the twentieth century, California again rocked from seismic activity. On 9 February 1971, an earthquake of magnitude 6.5 on the Richter scale struck the San Fernando Valley. This earthquake demonstrated the extent of damage that can occur from a moderate shock centered in a large metropolitan area (the Los Angeles Basin, with a population of 5 million). It caused sixty-five deaths, and damage was estimated to exceed $500 million. Southern California experienced an earthquake measuring 6.4 on the Richter scale in 1979. Eight years later, another quake in the area measured 5.9. In October 1989, the Loma Prieta earthquake struck the San Francisco Bay area, killing at least sixty-three people and collapsing several elevated highways, including a section of the bridge between San Francisco and Oakland. Damages from this earthquake, that registered 7.1 on the Richter scale, reached $6–7 billion. In 1992, a quake measuring 7.4 on the Richter scale struck the desert east of Los Angeles, with one fatality. That same year, a quake of 6.9 struck northern California, with no fatalities. And in 1994, a major quake struck the Los Angeles area, with its epicenter in the city's Northridge section. This quake, measuring 6.6 on the Richter scale, damaged many structures in the city, including freeways, and killed at least fifty-one people. Property losses exceeded $4 billion. Scientists have not yet determined how to predict the precise onset of an earthquake; however, since the 1960s, engineers have developed earthquake-resistant building techniques that can reduce the impact of ground shaking. Regardless, public acceptance of earthquake probability estimates and mandated hazard abatement measures often has been slow.

Bibliography

Bolt, Bruce A. Earthquakes. New York: Freeman, 1999.

Bolt, Bruce A. Earthquakes and Geological Discovery. New York: Scientific American Library, 1993.

Coffman, Jerry L., and Carl A. von Hake, eds. Earthquake History of the United States. Boulder, Colo.: Environmental Data Service, 1973.

Geschwind, Carl-Henry. California Earthquakes: Science, Risk, and the Politics of Hazard Mitigation. Baltimore: Johns Hopkins University Press, 2001.

Hansen, Gladys C., and Emmet Condon. Denial of Disaster: The Untold Story and Photographs of the San Francisco Earthquake and Fire of 1906. San Francisco: Cameron, 1989; 1990.

Steinberg, Theodore. Acts of God: The Unnatural History of Natural Disaster in America. New York: Oxford University Press, 2000.

—Bruce A. Bolt

 
Spotlight: earthquake

From our Archives: Today's Highlights, January 23, 2005

Earthquakes! Recorded history's most devastating earthquake occurred on this date in 1556 in Shaanxi, China. Estimated to be between 8.0 and 8.3 on the Richter Scale, the earthquake devastated 98 counties and eight provinces of Central China. The destruction spanned an area of 500 miles, and some 830,000 lives were lost.
 
Columbia Encyclopedia: earthquake,
trembling or shaking movement of the earth's surface. Most earthquakes are minor tremors. Larger earthquakes usually begin with slight tremors but rapidly take the form of one or more violent shocks, and end in vibrations of gradually diminishing force called aftershocks. The subterranean point of origin of an earthquake is called its focus; the point on the surface directly above the focus is the epicenter. The magnitude and intensity of an earthquake is determined by the use of scales, e.g., the Richter scale and the Mercalli scale.

Causes of Earthquakes

Most earthquakes are causally related to compressional or tensional stresses built up at the margins of the huge moving lithospheric plates that make up the earth's surface (see lithosphere). The immediate cause of most shallow earthquakes is the sudden release of stress along a fault, or fracture in the earth's crust, resulting in movement of the opposing blocks of rock past one another. These movements cause vibrations to pass through and around the earth in wave form, just as ripples are generated when a pebble is dropped into water. Volcanic eruptions, rockfalls, landslides, and explosions can also cause a quake, but most of these are of only local extent. Shock waves from a powerful earthquake can trigger smaller earthquakes in a distant location hundreds of miles away if the geologic conditions are favorable.

See also plate tectonics.

Seismic Waves

There are several types of earthquake waves including P, or primary, waves, which are compressional and travel fastest; and S, or secondary, waves, which are transverse, i.e., they cause the earth to vibrate perpendicularly to the direction of their motion. Surface waves consist of several major types and are called L, or long, waves. Since the velocities of the P and S waves are affected by changes in the density and rigidity of the material through which they pass, the boundaries between the regions of the earth known as the crust, mantle, and core have been discerned by seismologists, scientists who deal with the analysis and interpretation of earthquake waves (see earth). Seismographs (see seismology) are used to record P, S, and L waves. The disappearance of S waves below depths of 1,800 mi (2,900 km) indicates that at least the outer part of the earth's core is liquid.

Damage Caused by Earthquakes

The effects of an earthquake are strongest in a broad zone surrounding the epicenter. Surface ground cracking associated with faults that reach the surface often occurs, with horizontal and vertical displacements of several yards common. Such movement does not have to occur during a major earthquake; slight periodic movements called fault creep can be accompanied by microearthquakes too small to be felt. The extent of earthquake vibration and subsequent damage to a region is partly dependent on characteristics of the ground. For example, earthquake vibrations last longer and are of greater wave amplitudes in unconsolidated surface material, such as poorly compacted fill or river deposits; bedrock areas receive fewer effects. The worst damage occurs in densely populated urban areas where structures are not built to withstand intense shaking. There, L waves can produce destructive vibrations in buildings and break water and gas lines, starting uncontrollable fires.

Damage and loss of life sustained during an earthquake result from falling structures and flying glass and objects. Flexible structures built on bedrock are generally more resistant to earthquake damage than rigid structures built on loose soil. In certain areas, an earthquake can trigger mudslides, which slip down mountain slopes and can bury habitations below. A submarine earthquake can cause a tsunami, a series of damaging waves that ripple outward from the earthquake epicenter and inundate coastal cities.

Major Earthquakes

On average about 1,000 earthquakes with intensities of 5.0 or greater are recorded each year. Great earthquakes (intensity 8.0 or higher) occur once a year, major earthquakes (intensity 7.0–7.9) occur 18 times a year, strong earthquakes (intensity 6.0–6.9) 10 times a month, and moderate earthquakes (intensity 5.0–5.9) more than twice a day. Because most of these occur under the ocean or in underpopulated areas, they pass unnoticed by all but seismologists. Notable earthquakes have occurred at Lisbon, Portugal (1755); New Madrid, Mo. (1811 and 1812); Charleston, S.C. (1886); Assam, India (1897 and 1950); San Francisco (1906); Messina, Italy (1908); Gansu, China (1920); Tokyo, Japan (1923); Chile (1960); Iran (1962); Managua, Nicaragua (1972); Guatemala (1976); Hebei, China (1976); Mexico (1985); Armenia (1988); Luzon, Philippines (1990); N Japan (1993); Kobe, Japan (1995); Izmit, Turkey (1999); central Taiwan (1999); Oaxaca state, Mexico (1999); Bam, Iran (2003); and NW Sumatra, Indonesia (2004). The Lisbon, Chilean, and Sumatran earthquakes were accompanied by tsunamis. On Good Friday 1964, one of the most severe North American earthquakes ever recorded struck Alaska, measuring 8.4 to 8.6 in intensity. Besides elevating some 70,000 sq mi (181,300 sq km) of land and devastating several cities, it generated a tsunami that caused damage as far south as California.

Ten of the fifteen largest earthquakes in the United States have occurred in Alaska, and eight of the fifteen largest in the continental United States have occurred in California. Recent earthquakes that affected the United States include the Feb., 1971, movement of the San Fernando fault near Los Angeles. It rocked the area for 10 sec, thrust parts of mountains 8 ft (2.4 m) upward, killed 64 persons, and caused damage amounting to $500 million. In 1989, the Loma Prieta earthquake above Santa Cruz shook for 15 seconds at an intensity of 7.1, killed 67 people, and toppled buildings and bridges. In Jan., 1994, an earthquake measuring 6.6 with its epicenter in N Los Angeles caused major damage to the city's infrastructure and left thousands homeless.

Bibliography

See C. H. Scholz, The Mechanics of Earthquakes and Faulting (1991); C. Lomnitz, Fundamentals of Earthquake Prediction (1994); D. S. Brumbaugh, Earthquakes: Science and Society (1998); B. A. Bolt, Earthquakes (4th ed. 1999). See also bibliography under seismology.

 
Science Dictionary: earthquake

A tremor of the surface of the Earth, sometimes severe and devastating, which results from shock waves generated by the movement of rock masses deep within the Earth, particularly near boundaries of tectonic plates. (See fault, Richter scale, and seismology.)

  • Earthquakes are particularly likely where such plates are sliding past each other, as in the San Andreas Fault.
  • Earthquakes cannot be accurately predicted, although the likelihood of a region's suffering an earthquake can be estimated.

    •  
    Cosmic Lexicon: Earthquake

    Sudden motion or trembling of Earth caused by the abrupt release of slowly accumulated elastic energy in rocks.

     
    Wikipedia: earthquake

    <imagemap>

    Image:Padlock-silver-medium.svg
    This article is about the natural seismic phenomenon. For other uses, see Earthquake (disambiguation).

    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.

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

    Naturally occurring earthquakes

    Fault types
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    Fault 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 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

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

    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

    Specific fault articles

    Major earthquakes

    Main article: List of earthquakes

    Pre-20th century

    The 1988 Spitak earthquake claimed over 25,000 lives and left 500,000-plus homeless.
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    The 1988 Spitak earthquake claimed over 25,000 lives and left 500,000-plus homeless.

    20th century

    21st century