Earthquakes & Landslides: Tectonic and Slope Hazards

1. EARTHQUAKE RISK & SEISMIC MICROZONATION

Cue Words	Notes
BIS Zonation & Vulnerability Share	**BIS Zonation Map**: Bureau of Indian Standards divides India into four seismic zones — Zone II (Low), III (Moderate), IV (High), V (Very High); Zone I is merged into Zone II. **Exposure**: **61%** of India's landmass is vulnerable to moderate-to-severe earthquakes (Zones III–V); Zone V covers Northeast India, J&K, Himachal Pradesh, Uttarakhand, Rann of Kutch, and parts of Bihar.
Monitoring & Microzonation	**National Center for Seismology (NCS)**: Nodal body under the Ministry of Earth Sciences, monitoring seismic activity via a real-time grid of **150+ stations** and deploying early-warning systems. **Seismic Microzonation**: Subdividing a municipal territory into micro-zones based on local soil density and rock characteristics to identify liquefaction/amplification risk, customising building bye-laws for high-density metros. **Plate Tectonics Driver**: The ongoing northward collision of the Indian Plate with the Eurasian Plate accumulates massive strain along Himalayan fault lines, producing recurring high-magnitude earthquakes; over **80%** of residential structures in mountain towns are self-built, using heavy concrete roofs on weak masonry columns — high vulnerability to lateral seismic forces. **NBC 2016 Structural Fixes**: Mandates ductile detailing (IS 13920 steel reinforcement configurations in concrete joints) so structures bend rather than fail brittlely; proposes compulsory third-party structural audits of foundation/soil load-bearing capacity before municipal clearance; non-structural mitigation includes anchoring heavy interior equipment and ceiling fixtures to prevent falling debris during tremors.

> **Summary**: With over 60% of India in moderate-to-severe seismic zones, NCS-led monitoring and fine-grained microzonation are the twin pillars of pre-quake risk reduction.

2. LANDSLIDE DYNAMICS: HIMALAYAS VS. WESTERN GHATS

Cue Words

Notes

Comparative Trigger Mechanisms

**Extent**: **12.6%** of national territory (~0.42 Million sq km) is classified as landslide-susceptible. **Himalayas**: Geologically young, active fold mountains; triggered by both intense rainfall and seismic shocks; high vertical relief, loose shale; human stressors include road-cutting (Char Dham widening), tunnel blasting, and dam construction. **Western Ghats**: Geologically ancient, stable block mountains; triggered almost exclusively by extreme monsoonal rainfall; human stressors include tea/coffee plantations, resort construction, and quarrying. Thick laterite soils overlying hard basalt liquefy under heavy monsoon bursts once plantation-cutting, mining, and resort construction block natural drainage. **Landslide Hazard Zonation (LHZ)**: High-resolution remote sensing classifies slopes into Very High/High/Moderate/Low hazard classes based on rock type, slope angle, vegetation density, and hydrological conditions — used to demarcate "red zones" where construction should be barred.

> **Summary**: Himalayan landslides are a "rainfall-plus-seismicity" hazard compounded by infrastructure stress, while Western Ghats landslides are a purely monsoon-driven hazard compounded by land-use change — the mitigation strategy must therefore differ regionally.

3. NDMA GUIDELINES FOR GEOLOGICAL RISK MITIGATION

Cue Words	Notes
Structural Interventions	**Slope Stabilisation**: Retaining walls, rock bolting, and soil nailing. **Bio-Engineering**: Sowing soil-binding species (e.g., Vetiver grass) to anchor loose slopes. **Building Codes**: Mandatory earthquake-resistant construction for public assets (schools, hospitals).
Non-Structural Interventions	**NLSM Program**: Geological Survey of India's National Landslide Susceptibility Mapping to restrict slope construction. **LEWS (Early Warning)**: Real-time satellite rainfall sensors predicting landslide events 24–48 hours in advance. **Drainage & Retaining Structures**: Interceptor drains and catch-pits along hill highways channelise rainwater away from unstable slip zones; gabion walls (rock-filled wire baskets) plus soil nailing/rock bolting physically tie unstable rock blocks to stable bedrock. **Hill Road Engineering**: Eco-sensitive alternatives to explosive blasting — half-tunnelling and bio-turfing — reduce slope destabilisation during road widening (e.g., Char Dham corridor). **2025-26 Push**: GSI has been expanding automated Landslide Early Warning Systems in Uttarakhand and Himachal Pradesh following repeated monsoon slope failures along the Char Dham corridor, alongside stricter hill-cutting regulation debates post the Wayanad (2024) and subsequent Himalayan landslide events.

> **Summary**: NDMA's structural (retaining walls, bio-engineering) and non-structural (NLSM mapping, LEWS) toolkit is being scaled up in the most fragile Himalayan corridors, though unscientific infrastructure expansion continues to outpace mitigation.

UPSC Mains PYQs

• Landslides & Himalayan Vulnerability (2016): "Landslides are a major hazard in the Himalayan region, which has been exacerbated by unscientific infrastructure development." Critically analyze the natural and anthropogenic causes of landslides in the Himalayas and compare them with the Western Ghats. Discuss the key recommendations of NDMA Guidelines for landslide risk management. (15 Marks, 250 Words)
• Seismic Microzonation: Discuss the significance of seismic microzonation in reducing earthquake risk in Indian cities. What are the challenges in its implementation? (10 Marks, 150 Words)