Earthquakes, Tsunamis, and Slope Failures

1. Earthquakes: The Shuddering Earth

Earthquakes are sudden releases of energy in the Earth's crust that create seismic waves.

Key Concepts:

• Focus (Hypocenter): The actual point of origin deep underground.
• Epicenter: The point on the surface directly above the focus; usually where damage is most severe.
• Shadow Zone: Areas where specific seismic waves (P or S) are not detected due to refraction/reflection at the Earth's core.

Measurement:

1. Richter Scale: Measures Magnitude (Energy released). It is logarithmic; each whole number increase represents a 10-fold increase in amplitude and ~32 times more energy.
2. Mercalli Scale: Measures Intensity (Observable damage). It ranges from I to XII.

2. Seismic Zonation of India

The Bureau of Indian Standards (BIS) has divided India into four seismic zones (Zone I was merged into Zone II):

• Zone V (Very High Risk): Entire North-East India, parts of J&K, Himachal, Uttarakhand, Rann of Kutch (Gujarat), and Andaman & Nicobar Islands. Most vulnerable due to the Indian plate subducting under the Eurasian plate.
• Zone IV (High Risk): Remaining parts of J&K, Delhi, Sikkim, parts of Bihar and UP.
• Zone III (Moderate Risk): Kerala, Goa, Lakshadweep, remaining parts of UP, Gujarat.
• Zone II (Low Risk): The stable Peninsular shield (most of Southern India).

3. Tsunami: The Harbour Wave

A Tsunami is a series of waves caused by a massive displacement of water.

Mechanism:

• Generation: Primarily by underwater earthquakes (Magnitude > 7.5), volcanic eruptions, or submarine landslides.
• Shoaling Effect: In the deep ocean, tsunami waves travel fast (up to 800 km/h) but have low height (< 1m). As they reach shallow coastal waters, friction slows them down, but the energy "piles up" the water, increasing the wave height (amplitude) dramatically (up to 30m).

Early Warning: India has established the Indian Tsunami Early Warning Centre (ITEWC) at INCOIS, Hyderabad. It uses a network of DART (Deep-ocean Assessment and Reporting of Tsunamis) buoys and tide gauges.

4. Landslides: The Gravity-Driven Threat

Landslides are the downward movement of a mass of rock, debris, or earth.

Causes in India:

• Natural: Heavy rainfall (the #1 trigger), earthquake vibrations, and steep slopes.
• Anthropogenic: Unscientific construction in hills, deforestation (roots hold soil), and "Road-cutting" which destabilizes the toe of the slope.

Regional Differences:

• Himalayas: High vulnerability due to fragile, tectonically active geology and heavy rain.
• Western Ghats: High vulnerability due to steep slopes and intense monsoon rainfall, despite Being a stable shield geologically.

5. Avalanches: Snow and Debris

Avalanches are common in the higher reaches of the Himalayas (J&K, Himachal, Uttarakhand).

• Triggers: Heavy snowfall, steep slopes (>30 degrees), and temperature fluctuations that create weak layers in the snowpack.
• Management: The Snow and Avalanche Study Establishment (SASE) under DRDO is the nodal agency for monitoring and warning.

6. Mitigation: Building for Resilience

A. For Earthquakes:

• Retrofitting: Strengthening existing weak structures.
• Base Isolation: Installing shock absorbers at the foundation of buildings.
• Enforcement: Strict adherence to National Building Codes (NBC).

B. For Landslides:

• Retaining Walls: Physical barriers to hold soil.
• Surface Drainage: Diverting water away from vulnerable slopes.
• Afforestation: Planting deep-rooted trees.

7. Bio-Shields and Natural Defense

For Tsunamis and Coastal storms, nature provides the best defense. Mangroves and coral reefs act as "Bio-shields" that dissipate the energy of waves. The 2004 Tsunami showed that areas with intact Mangroves (like parts of Tamil Nadu) suffered significantly less damage.

8. Role of NDRF in Geophysical Rescue

The NDRF specializes in CSSR (Collapsed Structure Search and Rescue). They use specialized equipment like victim location cameras, acoustic sensors, and canine squads to find people trapped under rubble after earthquakes.

9. Critical Analysis for Mains

Disasters in hill stations (like Joshimath or Shimla) are often the result of ignoring the "Carrying Capacity" of the land. For UPSC, emphasize that Land Use Zonation is the most effective "non-structural" mitigation tool. Constructing heavy concrete buildings on fragile slopes is a recipe for disaster.

10. Summary and Sendai Alignment

India's geophysical risk management must align with Priority 1 of the Sendai Framework: "Understanding Disaster Risk." Without accurate micro-zonation and community-level drills, even the best engineering cannot prevent casualties.