What are Earthquakes? How are they caused? Give their distribution.
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Answer:
Earthquake, any sudden shaking of the ground caused by the passage of seismic waves through Earth’s rocks. Seismic waves are produced when some form of energy stored in Earth’s crust is suddenly released, usually when masses of rock straining against one another suddenly fracture and “slip.” Earthquakes occur most often along geologic faults, narrow zones where rock masses move in relation to one another. The major fault lines of the world are located at the fringes of the huge tectonic plates that make up Earth’s crust. (See the table of major earthquakes.)
Building knocked off its foundation by the January 1995 earthquake in Kōbe, Japan.
Building knocked off its foundation by the January 1995 earthquake in Kōbe, Japan.
Dr. Roger Hutchison/NGDC
Residents of an earthquake-damaged neighbourhood of Port-au-Prince, Haiti, seeking safety in a sports field, Jan. 13, 2010. The magnitude-7.0 earthquake struck the region the day before.
Residents of an earthquake-damaged neighbourhood of Port-au-Prince, Haiti, seeking safety in a sports field, Jan. 13, 2010. The magnitude-7.0 earthquake struck the region the day before.
Petty Officer 2nd Class Sondra-Kay Kneen/U.S. Coast Guard
Earthquake
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John Michell
Boris Borisovich, Prince Golitsyn
Charles F. Richter
Arthur L. Day
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Seismic belt
Little was understood about earthquakes until the emergence of seismology at the beginning of the 20th century. Seismology, which involves the scientific study of all aspects of earthquakes, has yielded answers to such long-standing questions as why and how earthquakes occur.
earthquake epicentres
earthquake epicentres
Global seismic centres for earthquakes of magnitude 5.5 and greater occurring between 1975 and 1999.
Encyclopædia Britannica, Inc.
TOP QUESTIONS
Why is an earthquake dangerous?
What are earthquake waves?
How is earthquake magnitude measured?
Where do earthquakes occur?
About 50,000 earthquakes large enough to be noticed without the aid of instruments occur annually over the entire Earth. Of these, approximately 100 are of sufficient size to produce substantial damage if their centres are near areas of habitation. Very great earthquakes occur on average about once per year. Over the centuries they have been responsible for millions of deaths and an incalculable amount of damage to property.
San Francisco earthquake of 1906
San Francisco earthquake of 1906
Crowds watching the fires set off by the earthquake in San Francisco in 1906, photo by Arnold Genthe.
Library of Congress, Washington, D.C.
The Nature Of Earthquakes
Causes of earthquakes
Earth’s major earthquakes occur mainly in belts coinciding with the margins of tectonic plates. This has long been apparent from early catalogs of felt earthquakes and is even more readily discernible in modern seismicity maps, which show instrumentally determined epicentres. The most important earthquake belt is the Circum-Pacific Belt, which affects many populated coastal regions around the Pacific Ocean—for example, those of New Zealand, New Guinea, Japan, the Aleutian Islands, Alaska, and the western coasts of North and South America. It is estimated that 80 percent of the energy presently released in earthquakes comes from those whose epicentres are in this belt. The seismic activity is by no means uniform throughout the belt, and there are a number of branches at various points. Because at many places the Circum-Pacific Belt is associated with volcanic activity, it has been popularly dubbed the “Pacific Ring of Fire.”
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A second belt, known as the Alpide Belt, passes through the Mediterranean region eastward through Asia and joins the Circum-Pacific Belt in the East Indies. The energy released in earthquakes from this belt is about 15 percent of the world total. There also are striking connected belts of seismic activity, mainly along oceanic ridges—including those in the Arctic Ocean, the Atlantic Ocean, and the western Indian Ocean—and along the rift valleys of East Africa. This global seismicity distribution is best understood in terms of its plate tectonic setting.
Natural forces
Earthquakes are caused by the sudden release of energy within some limited region of the rocks of the Earth. The energy can be released by elastic strain, gravity, chemical reactions, or even the motion of massive bodies. Of all these the release of elastic strain is the most important cause, because this form of energy is the only kind that can be stored in sufficient quantity in the Earth to produce major disturbances. Earthquakes associated with this type of energy release are called tectonic earthquakes.
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Explanation:
Earthquakes occur when tension/energy is released from inside the crust. Plates do not always move smoothly alongside each other and sometimes get stuck. This results a sudden release of stored energy in the Earth's crust that creates seismic waves.
Earthquakes are not randomly distributed around the earth, rather they are located in distinct zones which can be related to the margins of tectonic plates on the Earth's surface.