An earthquake is defined as a movement of the ground surface which can range from a quaint, negligible tremor to a wild shaking capable of great destruction. Geographically, it can be understood as “a form of energy wave motion transmitted through the surface layer of earth in widening circles from a point of sudden energy release called the focus”. The intensity or the magnitude of the earthquake is measured on the Richter Scale. The world’s most intensive earthquake measured until now is 8.9 on the Richter Scale. As said earlier, the place of the origin or the place of the energy release is called the focus which is usually below the surface. The place perpendicular to the focus on the ground surface is called the epicenter. The seismic waves move away from the epicenter. Hence, an instrument to record the seismic waves is called seismograph.
The main reason behind the occurrence of an earthquake is the disequilibrium in the earth’s crust. This disequilibrium can be caused by volcanic eruptions, faulting and folding, hydrostatic pressure of man-made water bodies such as reservoirs and lakes, and by the movement of the plates. The earthquakes in India along the Himalayan range and its foothills are caused by the shifting of plates. The Asiatic plate is moving southwards whereas the Indian plate is moving northwards. This causes the Indian plate to be subducted below the Asiatic plate. This collision and folding causes earthquake in North of India as well as Tibet and Nepal. This earthquake belt encompasses the Sulaiman and Kirthar Shear Zones in the west to the Himalayas in the north and the Burmese arc in the east.
The intensity of an earthquake is determined by the scale reading on the Richter or Mercalli scales. An earthquake becomes a disastrous one only when it occurs in an inhabitant area. Many a times an earthquake is not as disastrous as other things as landslides and floods which are triggered by it. Earthquakes have a disastrous effect on human beings as buildings, bridges, roads, dams; factories, railway lines etc get collapsed. The earthquakes sometimes wipe out towns and cities; entail great loss of human life, animals, property etc. Conditions get worse if the occurring earthquake, along with it, stimulates violent fires, landslides, floods etc. Two major earthquakes in Bihar and Nepal border in 1984 and 1988 stand a testimony to the kind of destruction it can cause, especially to human structures. A similar example can be dried of the earthquake that had struck Mexico City in 1985 and the one which occurred in 1990 in Iran. The strong vibrations caused by earthquakes shake the buildings which can cause fires in houses and factories due to overturning of gas cylinders and other electrical appliances can be very dangerous during such times. Sometimes severe earthquake causes deformation of ground surface due to the rise and subsidence of the ground surface. A10-15 mt ground displacement was caused in the Alaskan earthquake in 1964. The 1993 earthquake in Lattur.Maharashtra, the Bhuj, Gujarat earthquake in January 2001 and the most recent one in Jammu and Kashmir are examples of earthquakes as hazards and natural weapons of destruction.
The earthquakes in seas generate high velocity waves—Tsunamis, causing large scale devastation along the coastal regions. The Tsunami that struck the coastal regions of south and South-east Asia in December 2005 brought about severe damage along the coastal areas in these regions.