An earthquake is caused by a sudden slip on a fault. Stresses in the earth’s outer layer push the sides of the fault together. Stress builds up and the rocks slips suddenly, releasing energy in waves that travel through the earth’s crust and cause the shaking. An Earthquake occurs when plates grind and scrape against each other. India lies at the northwestern end of the IndoAustralian Plate, which encompasses India, Australia, a major portion of the Indian Ocean and other small countries. This plate is colliding against the huge Eurasian Plate and going under the Eurasian Plate,this process of one tectonic plate getting under another is responsible for making India a earthquake prone country. A number of significant earthquakes occured in and around India over the past century. Some of these occured in populated and urbanized areas and hence caused great damage. Many went unnoticed, as they occurred deep under the Earth’s surface or in relatively un-inhabited places. The varying geology at different locations in the country implies that the likelihood of damaging earthquakes taking place at different locations is different. Thus, a seismic zone map is required to identify these regions.
Bureau of Indian Standards, based on the past seismic history, grouped the country into four seismic zones, Zone – 2,-3,-4and 5. Of these, zone 5 is the most seismically active region, while zone 2 is the least. The Modified Mercalli(MM) intensity, which measures the impact of the earthquakes on the surface of the earth, broadly associated with various zones, is as follows.