An earthquake is a sudden and temporary vibration set up on the earth’s surface, ranging from a faint tremor to a wild motion, due to the sudden release of energy stored in the rocks beneath the earths surface. Earthquake is a form of energy of wave motion which originates in a limited region and then spreads out in all directions from the source of disturbance.
Earthquakes usually last for a few seconds to a minute. Sometimes, the vibrations are so feeble that we cannot feel them, whereas the violent earthquakes result in huge material loss and the loss of human lives.
The point within the earth where earthquake-waves originate is called the focus and from the focus the vibrations spread in all directions. They reach the surface first at the point immediately above the focus and this point is called the epicentre. It is at the epicentre where the shock of the earthquake is first experienced. It is, however, evident that no earthquake can possibly originate at a mere point alone.
Earthquakes occur beneath the surface of the earth, where the rocks yield suddenly, of course, after porolonged build-up of stresses. They are often associated with fault-lines, which provide a zone of fracture and easy yielding. Earthquake emerges at various depths, which may be anywhere in the crust or as far as 700 km down into the mantle.
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Due to the sudden yielding of rocks to stresses, waves of energy are sent out through the earth. These waves of energy are called seismic- waves. The seismic waves emerge in the focus of an earthquake and radiate outward, like ripples produced when a stone is thrown into a pool of water, gradually losing energy.