India Updates Seismic Zonation Map, Entire Himalayas Classified in Highest-Risk Zone VI

India has unveiled a revised seismic zonation map under the updated Earthquake Design Code by the Bureau of Indian Standards (BIS), placing the entire Himalayan arc in the newly introduced highest-risk Zone VI for the first time. This update reflects one of the most significant changes in seismic hazard assessment in decades, showing that 61% of the country now falls under moderate to high hazard zones.

• Five-Zone Classification for Earthquake Risk
The updated code classifies India into five earthquake zones: II, III, IV, V, and VI. Previously, the Himalayan belt was fragmented across Zones IV and V, despite uniform tectonic threats. The new classification ensures consistent risk recognition across the region.

• Himalayan Faults and Tectonic Activity
The Himalayas are in Zone VI due to their position along the highly active collision boundary between the Indian and Eurasian Plates, with the Indian Plate pushing northward at roughly five centimetres per year. Long-unruptured fault segments in the central Himalayas, which have not experienced major surface-rupturing events for nearly 200 years, were previously underestimated. The updated map recognises that ruptures in the outer Himalayas are likely to propagate southward until reaching the Himalayan Frontal Thrust near Mohand, Dehradun.

• Scientific Basis and Methodology
The BIS said the revised map is based on internationally accepted probabilistic seismic hazard assessment (PSHA) methods, incorporating data on active faults, maximum potential magnitudes, ground-shaking behaviour, tectonic regimes, and underlying lithology. Previous maps primarily relied on past epicentres, damage reports, and broad soil features, which often failed to account for fault-specific seismic potential.

• Policy Implications
Under the new rules, towns located along boundaries between two zones will be classified in the higher-risk category, ensuring stricter design and safety standards aligned with actual geological hazards rather than administrative boundaries.

Source: TOI