BharatNotes What is Seismic Microzonation?
Seismic microzonation divides a region — usually a city — into small zones that are expected to behave similarly during an earthquake. While the BIS code IS 1893 assigns whole regions to broad seismic zones (macrozonation), the actual shaking felt at a site depends heavily on local conditions: soft alluvial soil amplifies ground motion, saturated sandy soil can liquefy, and steep slopes can fail. Microzonation captures these local site effects on detailed maps, helping engineers and planners design safer buildings and choose safer sites.
In India the work is led by the National Centre for Seismology (NCS) under the Ministry of Earth Sciences (MoES), established in August 2014. Maps are typically produced at a 1:50,000 regional scale and a finer 1:10,000 urban scale.
Macrozonation vs Microzonation
| Aspect | Macrozonation (IS 1893) | Microzonation |
|---|---|---|
| Scale | Country/regional | City/site (1:50,000–1:10,000) |
| Basis | Broad seismic history, intensity | Local soil, geology, water table |
| Output | Four zones (II–V) | Fine hazard sub-zones within a city |
| Use | Building code zone factor | Site-specific design, land-use planning |
India's macrozonation has four zones with zone factors (Z): Zone II = 0.10g, Zone III = 0.16g, Zone IV = 0.24g and Zone V = 0.36g. About 59% of India's land area falls in the higher-hazard Zones III, IV and V (as per NDMA).
Key Parameters Mapped
A microzonation study typically integrates:
- Site response / amplification — how soil layers magnify bedrock shaking.
- Surface peak ground acceleration (PGA) and spectral accelerations.
- Liquefaction potential — using shear-wave velocity (Vs), SPT-N values and water-table data.
- Shear-wave velocity profiles and geotechnical borehole data.
These layers are combined in a GIS framework to produce composite hazard maps.
Current Status in India (as of MoES, 2024)
According to MoES (Rajya Sabha reply, 08-Feb-2024), seismic microzonation has been completed for cities and areas including Delhi, Bengaluru, Kolkata, Guwahati, Jabalpur, Dehradun, Ahmedabad, Gandhidham, Chennai, Coimbatore, Bhubaneswar, Mangalore and the State of Sikkim. Cities such as Patna, Varanasi, Lucknow, Kanpur, Meerut, Agra, Dhanbad and Amritsar were reported to be in advanced stages of completion. Results are shared with state and national disaster management authorities and the Bureau of Indian Standards (BIS) to inform codes and planning.
Why It Matters
Microzonation turns a coarse "this city is in Zone IV" warning into actionable, neighbourhood-level guidance — flagging which localities need stronger foundations, which are prone to liquefaction, and where high-occupancy buildings should be restricted. For rapidly urbanising, earthquake-prone Indian cities, it is among the most cost-effective pre-disaster mitigation tools available, directly supporting the Sendai Framework's emphasis on understanding disaster risk.
