The point within the earth where are the Waves originate is called focus and and from the focusand from the book off the vibration spread in all direction they reach the surface first eight point immediately above the focus in this point is called epicenter.
It is at epicenter where a shockof earthquake is first experienced.it is, however, evident that no earthquake can possibly originate at a meer point alone. Earthquake occurs beneath the surface of the earth. Where the rocks and suddenly, of course, after prolonged build- up of stresses. They are often associated with fault- lines, which provided is zone of fracture and easy yielding. Earthquake emerges various depth, which may be anywhere in the crossed or age 700 kilometre down into mantal.
Due to sudden yielding of Rock to stress,wave of energy are sent out through the earth. These waves of energy are called seismic waves. Sesmicwave emergs in the focus of an earthquake and and radiated outward, like ripples produced when stone is thrown into a pool of water , gradually losing energy.it is known that the seismic waves pass through Earth is mainly because of its being an elastic body. When stress is applied become the form more and more,what to do when the stress is removed it returns to its original shape. If the stress applied is to create the elastic body yeild permanently. The sesmicwave to been considered to be the outcome of elastic deformation of Rock.since, since all elastic bodies can be subjected to types of deformation comparison and shear the sesmicwave are also related to these.
Primary waves-these are longitudinal waves similar to sound waves. Rocks vibrate parallel to the direction of Propagation. In the same direction as wave moving. This kind of motion leads to the longitudinal compression and refraction of the substances.
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