Earthquakes occur sudden by with little or no warning. However, following a major earthquake, the after-shocks may sometimes indicate the likelihood of a further earthquake. On some occasions, an earthquake may be preceded by a less intense tremors or foreshocks. The basic characteristics of an earthquake are the following:
1. It is not yet possible to predict magnitude, time and place of occurrence of an earthquake.
2. The onset is usually sudden.
3. Earthquake prone areas are generally well identified and well known on the basis of geological features and past occurrences of earthquakes.
4. Major effects arise mainly from ground movement and fracture or slippage of rocks underground. The obvious effects include damage (usually very severe) to buildings and infrastructures along with considerable casualties.
5. About 200 large magnitude earthquake (M>6.0) occur in a decade.
6. The world's earthquake problem seems to be increasing with the increased population, high rise buildings and crowded cities.
The exact spot underneath the surface of the earth at which an earthquake originates is known as “focus” while the point lying on the ground surface vertically above the focus is defined as “epicenter" of the earthquake. The seismic shocks originating at a depth of about. 50 km or less below the surface is termed as shallow focus earthquakes; otherwise, these are known as deep focus earthquakes.