Earthquake

Context:

The 1950 Great Assam Earthquake, with a magnitude of 8.6, marks its 75th anniversary, prompting reassessment of seismic risks in the Eastern Himalayan region.

Earthquake

Sudden shaking or trembling of the Earth's surface caused by release of energy from the Earth’s crust.
Primary causes:
- Tectonic plate movement
- Volcanic activity
- Artificial triggers (e.g., mining explosions, reservoir-induced seismicity)

Causes of Earthquakes

Tectonic:
- Movement along faults due to stress build-up at plate boundaries.
- Example: Himalayan earthquakes due to Indian Plate subducting under Eurasian Plate.
Volcanic
- Triggered by magma movement beneath the Earth’s crust.
- Example: Icelandic volcanic quakes.
Collapse
- Due to cavern collapse (mining, karst regions).
Explosion
- Due to man-made explosions (nuclear tests, large industrial blasts).
Reservoir-induced
- Due to filling of large dams (changes in pressure on crustal faults).

Seismic Terms

Focus (Hypocentre): Actual point of energy release inside the crust.
Epicentre: Point on the Earth’s surface directly above the focus.
Seismic Waves:
- P-waves (Primary) – fastest, travel through solids & liquids.
- S-waves (Secondary) – slower, travel only through solids.
- Surface waves – cause most damage.
Magnitude: Quantitative measure of energy released (Richter scale).
Intensity: Qualitative measure of shaking & damage (MSK/Mercalli scale).

Earthquake Vulnerability in India

59% of land at moderate–severe seismic risk (MSK VII & above).
Three main tectonic sub-regions:
- Himalayan belt – high magnitude (>8.0) potential.
- Alluvial plains – extensions of Himalayan seismicity.
- Peninsular India – intraplate earthquakes due to ancient faults.

Historic major earthquakes:

Shillong (1897, M 8.7), Kangra (1905, M 8.0), Bihar–Nepal (1934, M 8.3), Assam–Tibet (1950, M 8.6).

Seismic Zoning in India (BIS Classification)

Zone	MSK Intensity	Risk Level	Examples
V	IX & above	Very High	Kashmir valley, NE States, A&N Islands, Rann of Kutch
IV	VIII	High	Delhi, parts of J&K, Himachal, Uttarakhand, Sikkim
III	VII	Moderate	Kerala, Goa, parts of MP, Odisha, AP
II	VI or less	Low	Remaining parts of Rajasthan, MP, TN

Impacts of Earthquakes

Structural Damage – Building collapse, >90% casualties in India due to this.
Surface Rupture & Ground Displacement – Disrupts roads, pipelines, railways.
Tsunamis – Triggered by undersea earthquakes (esp. subduction zones).
Liquefaction – Water-saturated soils lose strength during shaking.
Fires – Due to ruptured fuel pipes & electrical failures.
Livelihood Disruption – Economic slowdown, loss of employment.

Mitigation Strategies

Structural: Earthquake-resistant building codes, retrofitting old structures.
Planning: Avoid high-risk areas for settlements.
Awareness: Drills, school safety programs.
Preparedness: Emergency kits, evacuation routes.
Technological: Seismic microzonation, early warning systems.

India’s Preparedness Initiatives

National Centre for Seismology (NCS) – Monitors quakes via 115 observatories.
National Earthquake Risk Mitigation Project (NERMP) – Focus on Zones IV & V.
Seismic Microzonation – Mapping vulnerability of large cities.
NDMA Campaigns – Public awareness & preparedness.
BIS Building Codes – Guidelines for earthquake-resistant structures.
Earthquake Disaster Risk Index – Prioritises high-risk cities for preparedness.

Challenges

High Seismic Activity – Costly retrofitting in active zones.
Unplanned Urbanisation – Violations of safety codes (Delhi: ~80% non-compliance).
Tourism in Risk Zones – Increased exposure in Himalayan belt.
Lax Regulation Enforcement – Weak inspection & penalties.
Low Public Awareness – Poor safety literacy.
Financial Constraints – Limited funds for seismic safety programs.