Earthquakes

An earthquake is the sudden shaking of the Earth’s surface, caused by a rapid release of energy within the Earth’s lithosphere. This energy travels as seismic waves, radiating outward from the point of origin and causing vibrations, tremors, and sometimes massive destruction.

Understanding earthquakes is crucial for India, especially because several zones—like the Himalayan region, the northeast, and parts of Gujarat—fall under high-risk seismic zones.

What Causes an Earthquake?

Earthquakes occur due to the release of stored energy in the Earth’s crust. This usually happens along fault lines, which are fractures or zones of weakness in the rock layers.

The pressure builds up due to plate movements, and when the force exceeds the friction holding the rocks together, the energy is suddenly released, causing an earthquake.

Major Causes of Earthquakes

• Tectonic movements (most common)
• Volcanic activity
• Faulting and folding
• Human-induced factors like mining, reservoir-induced stress, and nuclear testing

Important Terms in Earthquake Studies

• Seismic Waves: Waves of energy that travel through the Earth due to an earthquake.
• Hypocentre: The point inside the Earth where the earthquake originates.
• Epicentre: The point directly above the hypocentre on the surface of the Earth.
• Seismograph: Instrument used to detect and record seismic waves.
• Richter Scale: Measures the magnitude (energy released) of an earthquake.
• Mercalli Scale: Measures the intensity (damage caused) on a scale of 1 to 12.

Types of Seismic Waves

1. Body Waves

These travel through the Earth’s interior.

• Primary (P) Waves:
  • Fastest seismic waves
  • Move through solids, liquids, and gases
  • Longitudinal in nature (push-pull movement)
• Secondary (S) Waves:
  • Slower than P-waves
  • Travel only through solid rock
  • Transverse in nature (up-down or side-to-side motion)

2. Surface Waves

These travel along the Earth’s surface and are generally more destructive.

• Love Waves: Move the ground side-to-side.
• Rayleigh Waves: Cause rolling movements – up-down and side-to-side. Most surface shaking is due to these.

How Does an Earthquake Occur?

• Faults in the Earth’s crust act as zones of weakness.
• Rocks on either side of the fault accumulate strain due to tectonic forces.
• Once the accumulated energy exceeds friction, the rocks slip suddenly.
• This release of energy radiates in all directions as seismic waves.

Types of Faults

• Normal Faults: Occur due to extension (pulling apart) of the crust; common at divergent boundaries.
• Reverse Faults: Result from compression, often seen at convergent plate margins.
• Strike-Slip Faults: Horizontal displacement; occur at transform boundaries (e.g., San Andreas Fault).

Classification of Earthquakes

Based on Cause

• Tectonic Earthquakes – Due to movement of tectonic plates (most common).
• Volcanic Earthquakes – Caused by volcanic eruptions and magma movement.
• Plutonic Earthquakes – Originate at great depths (>300 km), linked with subduction zones.
• Isostatic Earthquakes – Caused by vertical movement due to adjustments in Earth’s crust.
• Human-Induced Earthquakes – Caused by mining, dam construction, petroleum extraction, or nuclear tests.
• Collapse Earthquakes – Result from underground mine collapse.
• Explosion Earthquakes – Result from man-made explosions.
• Reservoir-Induced Earthquakes – Linked to large artificial water bodies (e.g., Koyna Dam, 1967).

Based on Depth of Focus

• Shallow-focus: 0–70 km depth (most destructive)
• Intermediate-focus: 70–300 km depth
• Deep-focus: 300–700 km depth

Based on Casualties

• Moderate: < 50,000 deaths
• Highly Hazardous: 50,000–100,000 deaths
• Most Hazardous: >100,000 deaths

Earthquake-Prone Zones in India

• India is divided into four seismic zones: Zone II, III, IV, and V.
• Zone V: Highest risk – Includes the Himalayas, parts of Northeast India, Kutch region in Gujarat, and Andaman-Nicobar Islands.
• Zone IV: High risk – Includes Delhi, northern Punjab, Haryana, parts of Uttar Pradesh and Bihar.
• Zone III: Moderate risk – Covers parts of central India and coastal areas.
• Zone II: Low risk – Southern Peninsular region, though not completely safe.

Notably, even the so-called stable peninsular India has experienced destructive earthquakes (e.g., Latur 1993, Koyna 1967), often due to intraplate seismicity or reservoir-induced stress.

Impacts of Earthquakes

• Loss of Human Life
• Destruction of Infrastructure
• Displacement and Homelessness
• Economic Damage
• Land Degradation
• Floods and Tsunamis (especially if the epicentre is underwater)

Earthquake Disaster Management in India

The goal of earthquake management is to minimize loss and ensure efficient recovery.

1. Risk Identification

• Mapping of seismic zones and identifying vulnerable regions.
• Development of earthquake hazard maps.

2. Early Warning Systems

• Though predicting earthquakes is still challenging, advances in seismic monitoring and real-time alerts are being developed.

3. Structural Solutions

• Promoting earthquake-resistant building designs, especially in high-risk areas.
• Retrofitting existing structures, bridges, and dams.
• Implementing building codes as per Bureau of Indian Standards (BIS).

4. Policy and Governance

Disaster Management Act, 2005

• Laid the foundation for institutional response mechanisms in India.
• Establishes National Disaster Management Authority (NDMA), State Disaster Management Authorities (SDMAs), and District DMAs.
• Emphasizes preparedness, prevention, and mitigation over reactive relief measures.

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Subject: Geography

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