An earthquake can be defined as a movement or tremor 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 effect.
It can be produced by a volcanic explosion; earthquakes, in fact, are common in most volcanic areas, and often precede or accompany eruptions. It 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 set up by an earthquake, and at a considerable distance from 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 buildings sway backwards and forwards. Maximum damage is not done always at the Epicenter, where the movement is up and down, but at places 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. A large earthquake is usually followed by a series of other shocks. 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. It is called Tsunami.
In the main earthquake regions, many of which have active volcanoes in their midst, an earthquake of some kind takes place practically every day; on Hawaii, for instance, hundreds of minor shocks are recorded during the year. There are extensive areas on the earth’s surface, however, where earthquakes are rare. The three great regions where earthquakes have taken place more or less frequently are;
1. The west coast area of North and South America.
2. A belt across southern Europe and south Asia.
3. A belt in the Pacific Ocean which includes Japan, the Philipines, and most of the East Indies. Of the thousands of earthquakes which are recorded annually, only a hundred or so cause damage.
Seismograph is an instrument for recording earthquake shocks. The principle of its operation or action consists in the disturbance of a portion of the apparatus by earth tremors, the suitable amplification of the motion thus produced, and then its recording. The vibrations used to be recorded by a pen tracing on a revolving drum, but in more modern instruments a photographic record is obtained on a moving film. Many seismographs are so sensitive that they will record vibrations due to an earthquake thousands of miles away, and its distance and also its direction may be approximately calculated. A properly equipped seismological observatory has three different seismographs; two of these will record the horizontal components of the earth’s vibrations, one in an east-west and the other in a north-south direction, while the third will record the vertical components.
A common way t o measure the size of earthquakes was invented by Charles Richter in 1935. The Richter scale uses numbers from one to ten [1-10] to express the energy released by an earthquake. Each number represents about thirty-two times the energy released by an earthquake one number lower. For, example, an earthquake rated 5 on the Richter scale releases thirty-two times as much energy as one rated at 4. Any earthquake with a rating of 6 or more on the Richter scale may cause damage to buildings.