Among the different natural hazards, earthquakes happen to be the most feared and dangerous because of their sudden impact and the devastation they cause in a matter of few seconds. Earthquake occurs mainly due to fracturing of the rocks in the earth's crust. Large and small internal forces e.g. contraction, expansion, uplifting, depression, folding and faulting bring earth movements.
The most occurring earthquake in the world are tectonic earthquakes which originate at depth not more than tens of kilometers. An earthquake is called shallow focus earthquake if it occurs at a depth of less than 70 km. if the range of focal depth is from 70 to 300 km them it is known as mid focus or intermediate depth earthquake. At greater depth ranging from 300 km to 700 km deep focus earthquakes occur.
Earthquakes also occur in volcanic regions due to tectonic fault as well as by the movement of magna in volcanoes. This type of earthquake gives an early warning of volcanic eruption, sometimes a series of earthquakes occur in a sort of earthquake storm, where the earthquakes strike a fault in clusters, each triggered by the stress distribution of the previous earthquake.