The ministry of Earth sciences has said that the impact of high-intensity earthquakes can be greatly reduced through seismic hazard microzonation of vulnerable areas and putting in place adequate mitigation measures. The ministry has put in place a project that is aimed at seismic hazard microzonation of major Indian cities and areas falling under high-seismic Zones IV and V. Such areas vulnerable to earthquakes include all states in the Northeast, Delhi, the Kutch region in Gujarat and the Andaman and Nicobar Islands.

An ambitious plan

The ministry is planning a phased seismic hazard microzonation of important cities with a population of at least 5 lakh people. They will include some state capitals and some other cities in vulnerable areas.

This is expected to help the authorities understand which areas within a city are more prone to seismic hazards and adopt mitigation measures accordingly.

So far, micro-seismic mapping of Sikkim and eight cities - Bengaluru, Delhi, Kolkata, Guwahati, Jabalpur, Ahmedabad, Dehradun and Gandhidham - has been completed.

Work is in progress in Coimbatore, Chennai, Bhubaneswar and Mangaluru.

Micro-seismic mapping of Agra, Amritsar, Lucknow, Kanpur, Varanasi, Patna, Dhanbad and Meerut will be taken up soon after.

Seismic microzonation

Seismic microzonation is sub-dividing a region into smaller areas having different potential for hazardous earthquake effects. The earthquake effects depend on ground geomorphological attributes consisting of geological, geomorphology and geotechnical information. The parameters of geology and geomorphology, soil coverage / thickness and rock outcrop / depth are some of the important geomorphological attributes.

Other attributes are the earthquake parameters, which are estimated by hazard analysis and effects of local soil for a hazard (local site response for an earthquake).

The peak ground acceleration, amplification / site response, predominant frequency, liquefaction and landslide due to earthquakes are some of the important seismological attributes. Weight of the attributes depends on the region and decision-maker, for example flat terrain has weight of ‘0’ value for landslide, and deep soil terrain has highest weight for site response or liquefaction.

Seismic zoning of India

The Indian subcontinent has a long history of devastating earthquakes, partially due to the fact that India is driving into Asia at a rate of approximately 47 mm / year. More than 50 per cent area of the subcontinent is vulnerable to earthquakes.

According to the IS 1893:2002 (the latest code of Bureau of Indian Standards or BIS, which lays down the criteria for quake-resistant design of structures), India has been divided into four seismic zones -  Zone II, III, IV and V unlike its previous version, which consisted of five zones. After some revisions in the previous zoning, Zone I was removed.

This zoning has been done on the basis of MSK-64 scale and a IS code zone factor has been assigned by the BIS to each of them.

The zone factor of 0.36 is indicative of effective (zero period) peak horizontal ground acceleration of 0.36 g (36 per cent of gravity) that may be generated during MCE (maximum considered event) level quake in this zone.

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