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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 moment magnitude scale, Richter scale, and the modified 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. An earthquake on one fault can trigger the sudden release of stress on a nearby fault and even cause a complex earthquake that propagates through several neighboring faults under the appropriate geological conditions. Volcanic eruptions, rockfalls, landslides, and explosions can also cause a quake, but most of these are of only local extent. Earthquakes have also been associated with hydraulic fracturing, or fracking, a drilling technique that injects slurry under pressure into a well, and with the disposal of fracking wastewater in old wells, most notably in N and central Oklahoma, where injection of wastewater in disposal wells has led to a significant increase in the number and strength of earthquakes since 2009. 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, travel fastest, and are the least destructive; and S, or secondary, waves, which are transverse, i.e., they cause the earth to vibrate perpendicularly to the direction of their motion. P and S waves are known as body waves because they propograte within the body of the earth. Surface waves consist of two main types, Love and Rayleigh waves; they move more slowly than P and S waves. Love waves cause horizontal shifting at the surface, and Rayleigh waves, the slowest moving, have a rolling motion similar to that of an ocean wave and vertically displace the earth's surface. 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 seismic 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, and soil with a significant water content can destabilize, liquefy, and even flow, causing structures and infrastructure to sink or be displaced. Bedrock areas receive fewer effects. The worst damage occurs in densely populated urban areas where structures are not built to withstand intense shaking. There, surface 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.

Earthquake Warning Systems

Modern earthquake warning systems have been developed by several nations, and may be deployed nationwide or regionally. They depend on the detection of P waves by seismometers, and the transmission of that detection to affected populations by various means. Because P waves travel faster and are much less destructive than the later-arriving seismic waves an earthquake also produces, their detection can provide early warning, of a few seconds to several tens of seconds, of the arrival of the destructive shaking associated with an earthquake. If an earthquake's epicenter is close to a given location, however, the several types of waves will arrive closer together and little or no early warning will be possible.

Warning can be provided through several means including sirens and public loudspeakers, radio and television broadcasts, text messaging, and smartphone apps including dedicated earthquake-alert apps. More sophisticated systems can give some indication of the duration and severity of the earthquake. Early warning, even though measured in seconds, may permit people to take protective measures to prevent or limit injury, allow authorities to halt trains and subways and stop traffic from entering bridges and tunnels, and alert emergency responders to take measures to prevent their vehicles from becoming inaccessible or damaged.

Major Earthquakes

On average about 1,000 earthquakes with intensities of 5.0 or greater are recorded each year. Great earthquakes (magnitude 8.0 or higher) occur once a year, major earthquakes (magnitude 7.0–7.9) occur 18 times a year, strong earthquakes (magnitude 6.0–6.9) 10 times a month, and moderate earthquakes (magnitude 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. Moderate to strong earthquakes can cause more significant destruction if they occur closer to the earth's surface. 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); S Alaska (1964); 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); NW Sumatra, Indonesia (2004); Sichuan, China (2008); S Haiti (2010); Chile (2010); South Island, New Zealand (2010, 2011); NE Japan (2011); and W Sulawesi, Indonesia (2018). The Lisbon, Chilean, Alaskan, Sumatran, NE Japan, and Sulawesi earthquakes were accompanied by significant tsunamis.

Twelve of the twenty largest earthquakes in the United States have occurred in Alaska. Most of the largest in the continental United States have occurred in California or elsewhere along the Pacific Coast, but the three New Madrid earthquakes (1811–12) also were among the largest continental events, as was the Charleston, S.C., earthquake (1886). On Good Friday 1964, one of the most severe North American earthquakes ever recorded struck near Anchorage, Alaska, measuring 8.4 to 8.6 in magnitude. 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. Other recent earthquakes that have 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 magnitude 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.


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.

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What does it mean when you dream about an earthquake?

Dreams about natural disasters often occur during life crises—during major “shake-ups.” The earth represents the material basis of life, so an earthquake can be an especially appropriate symbol of financial upheaval. Dreams about earthquakes may also occur during life-threatening illnesses or in the recovery period following life-threatening accidents.

The Dream Encyclopedia, Second Edition © 2009 Visible Ink Press®. All rights reserved.


A 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.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.


a sudden release of energy in the earth's crust or upper mantle, usually caused by movement along a fault plane or by volcanic activity and resulting in the generation of seismic waves which can be destructive
Collins Discovery Encyclopedia, 1st edition © HarperCollins Publishers 2005


(IBM) The ultimate real-world shock test for computer hardware. Hackish sources at IBM deny the rumor that the San Francisco Bay Area quake of 1989 was initiated by the company to test quality-assurance procedures at its California plants.
This article is provided by FOLDOC - Free Online Dictionary of Computing (


Unlike some of the other dreams about natural disasters, earthquakes usually symbolize parts of the dreamer’s physical reality rather than his emotional life. The earthquake in the dream may be representing financial difficulties, health issues, or any number of other problems that could occur in daily life. An experience that is “shaking” you up, and changing your daily life, could be creeping into your dream state and showing up as an earthquake.
Bedside Dream Dictionary by Silvana Amar Copyright © 2007 by Skyhorse Publishing, Inc.
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