Microzonation of Earthquake Hazards
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 the future. The hazards associated with earthquakes are referred to as seismic hazards. The practise 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. It is defined as the zonation concerning ground-motion characteristics taking into account source and site conditions. Making improvements on the conventional macrozonation maps and regional hazard maps, the micro zonation of the ar region 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 behaviour for engineering design and land-use planning.
The basis of microzonation is to model the rupture mechanism at the source of an earthquake, evaluate the propagation of waves through the earth to the top of bedrock, determine the effect of the 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 earthquake-prone area into zones concerning some geological and geophysical characteristics of the sites such as ground shaking, liquefaction susceptibility, landslide and rockfall 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.
Effect of Site Conditions on Earthquake Ground Motion
It has long been recognized that the intensity of ground shaking during earthquakes and the associated damage to structures are significantly influenced by local geologic and soil conditions. Unconsolidated sediments are found to amplify ground motion during earthquakes and are hence more prone to earthquake damage than ground with hard strata. Modern cities built on soft sediments are especially vulnerable to damage caused by amplified ground motions.
Dynamic characteristics of the site such as predominant period, amplification factor, shear wave velocity, standard penetration test values can be used for seismic microzonation purpose. Microtremor observations are easy to perform, inexpensive and can be applied to places with low seismicity as well; hence, microtremor measurements can be used conveniently for microzonation.In a nutshell, microzonation is the need of the hour to deal with such terrible and devastating disasters.
WRITTEN BY: Biswajit Sahoo and Debasish Beria