A movement of the earth's crust which originates naturally and below the surface. It sometimes causes a permanent change of level at the surface but often the damage done by the shaking provides the only lasting visible effects. It may be produced by a volcanic explosion. earthquake in fact are common in most volcanic areas and often precede eruptions. Its is more likely however to be of tectonic origin and probably due to the existence of a fault.
At least three distinct sets of waves are setup by an earthquake. and at a considerable distance from the place of origin these are felt separately close to the place of origin, however they all reach an observer at approximately the same time. As these waves pass a place, the ground may be felt to rock and building sway backwards and forward. Maximum damage is not done always at the epicenter, where the movement is up and down, but at the place where the waves reaches the surface obliquely yet which are still close enough to the origin for it to have lost little of its force.
An earthquake which originates below or near to the sea causes great disturbance of the water and sometimes large waves emanate from it and travel a considerable distance on occasion these waves have caused a greater loss of life by flooding coastal regions than the earthquake itself.
In the main earthquake regions, many of which have active volcanoes in their midst. An earthquake of some kind takes place practically every day. There are extensive areas on the earth's, however where earthquakes are very rare.