What is microzonation? Why is it necessary?

Many earthquakes in the past have left many lessons to be learned which are very essential to plan infrastructure and even to mitigate such calamities in future. The hazards associated with earthquakes arereferred to as seismic hazards. The practice of earthquake engineering involves the identification and mitigation of seismic hazards. Microzonation has generally been recognized as the most accepted tool in seismic hazard assessment and risk evaluation and it is defined as the zonation with respect to groundmotion characteristics taking into account source and site conditions. Makingimprovements on the conventional macrozonation maps and regional hazard maps, microzonation of ar egion generates detailed maps that predict the hazard at much smaller scales. Seismic microzonation is the generic name for subdividing a region into individual areas having different potentials hazardous earthquake effects, defining their specific seismic behavior for engineering design and land-use planning.

The basis of microzonation is to model the rupture mechanism at the source of anearthquake, evaluate the propagation of waves through the earth to the top of bed rock, determine the effect of local soil profile and thus develop a hazard map indicating the vulnerability of the area to potential seismic hazard. Seismic microzonation will also help in designing buried lifelines such as tunnels, water and sewage lines, gas and oil lines, and power and communication lines.

Seismic microzonation is defined as the process of subdividing a potential seismic or earthquake prone area into zones with respect to some geological and geophysical characteristics of the sites such as ground shaking,liquefaction susceptibility, landslide and rock fall hazard, earthquake-related flooding, so that seismic hazards at different locations within the area can correctly be identified. Microzonation provides the basis for site-specific risk analysis, which can assist in the mitigation of earthquake damage. In most general terms, seismic microzonation is the process of estimating the response of soil layers under earthquake excitations and thus the variation of earthquake characteristics on the ground surface.

Regional geology can have a large effect on the characteristics of ground motion. The site response of the ground motion may vary in different locations of the city according to the local geology. A seismic zonation map for a whole country may, therefore, be inadequate for detailed seismic hazard assessment of the cities. This necessitates the development of microzonation maps for big cities for detailed seismic hazard analysis. Microzonation maps can serve as a basis for evaluating site-specific risk analysis, which is essential for critical structures like nuclear power plants, subways, bridges, elevated highways, sky trains and dam sites. Seismic microzonation can be considered as the preliminary phase of earthquake risk mitigation studies. It requires multi-disciplinary contributions as well as comprehensive understanding of the effects of earthquake generated ground motions on man made structures. Many large cities around the world have put effort into developing microzonation maps for the better understanding of earthquake hazard within the cities

As part of the national level microzonation programme, Department of Science and Technology, Govt. of India has initiated microzonation of 63 cities in India . Some of them arefinished and some of them are ongoing. As an initial experiment, seismic hazard analysis and microzonation was taken up for Jabalpur city in Madhaya Pradesh. Further, for many other cities such as Sikkim, Mumbai, Delhi, North East India, Gauwhati, Ahmedabad, Bhuj, Dehradun and Chennai anattempt has been made to carryout microzonation considering geomorphological features and detailed
geotechnical studies.