India has released an updated seismic zonation map under the revised Bureau of Indian Standards (BIS) Earthquake Design Code.

Key Features of the Revised Zonation Map

• Introduction of Zone VI: The new seismic map introduces Zone VI, the highest-risk seismic classification, and places the entire Himalayan arc stretching from Jammu & Kashmir and Ladakh to Arunachal Pradesh within this zone.
• Enhanced Geographic Coverage:
  • The revised map now classifies 61% of India’s landmass under moderate to high hazard zones, an increase from 59% in earlier versions.
    • 75% of India’s population is now in seismically active regions
  • The southern peninsula shows only minor refinements, maintaining a stable hazard profile due to relatively stable tectonic conditions.
• Boundary Rule Enhancement:
  • Under the updated rules, any town or settlement located on the boundary between two seismic zones is automatically placed in the higher-risk zone.
  • This change ensures zoning decisions reflect geological risk rather than administrative boundaries.
• Scientific Methodology: 
• • The updated seismic hazard map is based on Probabilistic Seismic Hazard Assessment (PSHA), an internationally recognised methodology.
  • It incorporates factors such as ground-shaking attenuation with distance, regional tectonic regimes, and the lithological characteristics of underlying rock formations.
• Mandatory Safety Measures:
  • The new code mandates safety norms for parapets, ceilings, overhead tanks, façades, HVAC systems, and similar elements, especially when their weight exceeds 1% of the building’s total load.
  • In addition, buildings located near active fault lines must be designed to withstand pulse-like ground motions, which are characteristic of near-fault earthquakes.
• Southward Rupture Modelling: 
  • The revised map acknowledges that ruptures along the Himalayan Frontal Thrust (HFT) can propagate southwards, affecting densely populated foothill settlements such as Dehradun (near Mohand).
  • This inclusion reflects a better understanding of rupture dynamics that extend beyond traditional fault-line mapping.
• Exposure Mapping:
  • The map now integrates population density, infrastructure concentration, and socioeconomic vulnerability through the PEMA (Population Exposure Mapping and Analysis) method.
  • This ensures seismic zoning considers hazard + human exposure, crucial for rapidly urbanising regions.

Why the Himalayas Are in the Highest Danger Zone?

• Active Plate Collision: The Himalayas sit on one of the world’s most active tectonic collision zones, where the Indian Plate pushes into the Eurasian Plate at 5 cm per year, continually building immense stress.
• Geologically Young and Unstable Terrain: Being geologically young, the Himalayas experience active folding, fracturing, and uplift, making the region inherently unstable.
• Multiple Major Fault Systems: Faults such as the Main Frontal Thrust, Main Boundary Thrust, and Main Central Thrust are capable of generating large, destructive earthquakes.
• Seismic Gaps and Stored Energy: Long seismic gaps with centuries without major quakes indicate significant accumulated strain, heightening earthquake potential.

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Implications for Infrastructure and Planning

• Stricter Building Standards: Engineers and planners must adopt more rigorous design codes for buildings, bridges, and critical infrastructure in vulnerable zones.
• Retrofitting Urgency: Existing structures in high-risk zones must undergo retrofitting to withstand potential high-magnitude earthquakes.
• Restrictions on Hazardous Zones: The guidelines call for halting infrastructure expansion on soft sediments, active faults, and liquefaction-prone areas.

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