Why Earthquake occurs ?

By Umang Rajpurohit

What is a Earthquake ?

An Earthquake is the shaking of the earth from the sudden energy releasing from the surface of the earth.The place of the origin of an earthquake called focus which is always hidden inside the earth

but it's depth varies from place to place.

The deepest earthquake may have it's focus at a depth of even 70 km below the ground surface but some of the major himalayan earthquake, such as the bihar-nepal earthquake of august 21,1988 have their focus around 20-30 km deep. The place on the ground surface, which is perpendicular to the burried 'focus' or 'hypocentre', recording the seismic waves for the first time is called epicentre.

The waves generated an earthquake are called 'seismic waves' which recorded by instrument called seismograph or siesmometer at the epicentre.The science, that deal with seismic waves,

is called 'seismology'.

Causes of Earthquake

Earthquakes are caused mainly due to disequilibrium in any part of the crust of the earth number of causes have been assigned to cause disequilibrium or isostatic imbalance in the earth's crust such as volcanic erruptions, faulting and folding, upwarping and downwarping, gaseous expansion and contraction inside the earth, hydrostatic pressure of man-made water bodies like reservoirs and lakes, and plate movements.

If we look at the world distribution of earthquakes appears that the earthquake belts are closely associated with the weaker zones and isostatically disturbed areas of the globe. It was generally believed that isostatically balanced and old and stable rigid masses were free from seismic events but the devastating earthquake of Koyna on 11 December, in Satara district of Maharashtra, latur-Kilari

earthquake of september 30, 1993 of maharashtra, disproved this old connotation and made us believe that no part of the earth is immune from seismic events.A host of possible causes have been suggested to cause disequilibrium in the earth's crust which trigger earth trimors of various sorts.

Following are the causes of earthquake:

1.Faulting

2.Vulcanicity

3.Folding

4.Human activities( like Tunnel construction,building construction,nuclear explosions, etc.)

Source: Physical geography by Savindra singh(Book)

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