Earthquake – Mechanism, Measurement, Distribution, Destruction & Mitigation

More than 30,000 earthquakes are strong enough to be felt yearly. About 75 of these are significant. An earthquake is essentially the shaking of the Earth caused by the sudden release of energy from the Earth’s interior. This energy radiates outward from the focus, or hypocenter, where the earthquake starts.

Mechanism of Earthquakes

  1. Tectonic Forces: Tectonic forces slowly deform rocks on either side of a fault line.
  2. Elastic Energy: As the rocks bend, they store elastic energy.
  3. Rupture: Eventually, the stress exceeds the rock’s strength, causing slippage and displacement along the fault.
  4. Vibrations: The rocks snap back to their original shape, releasing the stored energy as vibrations, which we feel as earthquakes.

Intensity and Magnitude of Earthquakes

  • Focus and Epicenter: The focus, or hypocenter, is the earthquake’s starting point inside the Earth. The epicenter is the point on the Earth’s surface directly above the focus where the seismic waves first hit.
  1. Intensity:
    • Definition: Intensity measures how strong the shaking is at different locations, decreasing with distance from the epicenter.
    • Mercalli Scale: Developed by Giuseppe Mercalli in 1897, it rates intensity from I (not felt) to XII (total destruction). This scale indicates the damage and human perception of the earthquake.
  2. Magnitude:
    • Definition: Magnitude measures the total energy released at the focus. It is consistent regardless of where you are.
    • Richter Scale: Introduced by Charles F. Richter in 1935, it measures magnitude on a scale from 1 to 9, with each whole number representing a tenfold increase in amplitude of the seismic waves and about 31 times more energy release.

Earthquake Waves

Earthquake waves travel through the Earth’s layers, which a seismograph can record. These waves are categorized into two main types:

  1. Body Waves:
    • P-Waves (Primary Waves): These are the fastest waves, traveling through solids, liquids, and gases. They compress and expand the material they move through, similar to sound waves.
    • S-Waves (Secondary Waves): These arrive after P-waves and move the ground up and down or side to side. They only travel through solids, which helps scientists understand the Earth’s inner structure.

2. Surface Waves:

      • Love Waves: Move side to side, causing horizontal shaking. They are faster than Rayleigh waves but slower than P-waves and S-waves. They can be very damaging to structures.
      • Rayleigh Waves: Create a rolling motion, similar to ocean waves, causing both vertical and horizontal displacement. They are the slowest of the surface waves but are very destructive due to their rolling motion.

Propagation of Earthquake Waves

When an earthquake occurs, waves spread from the focus in all directions. The behavior of these waves changes as they travel through different materials:

  • Reflection: Waves bounce back when they hit boundaries between different materials.
  • Refraction: Waves change direction and speed as they pass through materials of different densities.

Shadow Zones

Shadow zones are areas on the Earth’s surface where seismic waves are not detected or are significantly weakened due to their interaction with the Earth’s layers. This phenomenon helps scientists understand the Earth’s interior:

  1. P-Wave Shadow Zone:
    • Cause: P-waves can travel through both solids and liquids. When they pass through the liquid outer core, they are refracted (bent).
    • Location: Between approximately 104° and 140° from the earthquake’s focus, P-waves are not detected directly due to this refraction.
  2. S-Wave Shadow Zone:
    • Cause: S-waves only travel through solids and are stopped by the liquid outer core.
    • Location: Extends from 104° to 180° from the focus. This zone is devoid of S-waves, confirming the outer core’s liquid state.

Importance of Shadow Zones

  1. Core Composition: Shadow zones reveal the Earth’s layered structure, including a solid mantle and a liquid outer core.
  2. Core-Mantle Boundary: The abrupt change in wave behavior at this boundary supports the theory of a liquid outer core surrounding a solid inner core.
  3. Core Size and Shape: Analyzing shadow zones helps estimate the core’s size and shape, suggesting a roughly spherical core with solid and liquid regions.
  4. Temperature and Pressure: The behavior of waves in shadow zones provides insights into the Earth’s internal temperature and pressure conditions.

Seismic Zones of India

India is divided into seismic zones based on the level of earthquake risk:

  1. Zone II (Low Risk):
    • Description: Lowest seismic activity with minimal damage expected.
    • Regions: Parts of central and southern India, including the Deccan plateau and eastern coastal regions.
  2. Zone III (Moderate Risk):
    • Description: Moderate seismic activity with potential for minor damage.
    • Regions: Western and central India, the Indo-Gangetic plains (including Delhi), and coastal regions of Maharashtra, Orissa, and Andhra Pradesh.
  3. Zone IV (High Risk):
    • Description: High seismic activity with potential for significant damage.
    • Regions: The Himalayan region (Jammu & Kashmir, Himachal Pradesh, Uttarakhand), northern Bihar and West Bengal, and Delhi-NCR.
  4. Zone V (Very High Risk):
    • Description: Highest seismic activity with potential for severe damage.
    • Regions: Northeastern states (Assam, Arunachal Pradesh, Meghalaya, Mizoram, Tripura, Nagaland, Manipur), parts of Jammu & Kashmir, the western and central Himalayas, the Rann of Kutch in Gujarat, and parts of North Bihar.

Seismic Zoning Significance

  1. Building Codes: Seismic zones help establish building codes and construction practices to ensure safety in high-risk areas.
  2. Disaster Preparedness: Understanding seismic zones aids in preparing for earthquakes, including early warning systems and emergency response plans.
  3. Infrastructure Development: Guides the design of critical infrastructure like bridges and dams to withstand earthquakes.

Earthquake Mitigation and Guidelines in India

  1. Building Codes and Construction Guidelines:
    • National Building Code (NBC): Rules for earthquake-resistant design, stricter standards for high-risk areas (Zones IV and V).
    • Seismic Design Codes:
      • IS 1893: Guidelines for designing earthquake-resistant structures.
      • IS 4326: Recommendations for making buildings earthquake-resistant.
    • Construction Practices: Use reinforced concrete and steel, ensure proper foundations, and conduct regular inspections.
  2. Disaster Preparedness and Response:
    • Emergency Plans: Set by the National Disaster Management Authority (NDMA) and local authorities. Includes planning for response and recovery.
    • Training and Drills: Regular drills and training for responders and the public.
  3. Public Awareness and Education:
    • Awareness Campaigns: Educate the public about earthquake risks and safety practices.
    • Community Involvement: Engage communities in emergency planning and training.
  4. Infrastructure Planning and Development:
    • Urban Planning: Avoid high-risk areas for new buildings and retrofit old structures to meet current standards.
    • Critical Infrastructure: Ensure essential facilities like hospitals and schools can withstand earthquakes.
  5. Research and Development:
    • Seismic Research: Support studies on earthquake behavior and building technologies.
    • Technology Integration: Use seismic monitoring and early warning systems to detect and alert people about earthquakes.

By following these guidelines, India aims to reduce earthquake risks, enhance safety, and improve preparedness.

Examples and data related to EARTHQUAKE:

Recent Earthquake Data:

  • 2023: In April 2023, a magnitude 6.2 earthquake struck parts of North India, including Jammu & Kashmir, causing damage to infrastructure and buildings.
  • 2022: In February 2022, a magnitude 5.9 earthquake affected the Uttarkashi region in Uttarakhand, causing minor damage and several injuries.
  • 2021: In May 2021, magnitude 6.1 earthquakes hit the Andaman and Nicobar Islands, with minimal damage reported.

Earthquake Belts:           

95% of EQs are along plate boundaries the greatest amount of energy is released along the circum-Pacific belt, another major concentration is along the Mediterranean Sea, Iran through the Himalayan Complex.

Earthquake Depths:       

90% of EQs occur at depths less than 100 km and almost all are very damaging.

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