Earthquakes – Mechanism, Distribution & Preparedness – Explained Pointwise

Recently, several earthquake tremors were felt in the National Capital Region as reported by National Centre for Seismology. Increased frequency of such tremors is a wake up call for India’s seismic vulnerability & particularly highlights the vulnerability of Delhi which currently houses 33.5mn people & over 5,000 high rise buildings, where over 80% buildings, fail to comply with seismic codes.

What is an EARTHQUAKE?

• An earthquake (also known as a quake, tremor or temblor) is a type of diastrophic movement which involves shaking of the Earth surface, resulting from the sudden release of energy in the Earth’s lithosphere that creates seismic waves.
• The release of energy occurs along a fault. A fault is a sharp break in the crustal rocks. Rocks along a fault tend to move in opposite directions. As the overlying rock strata press them, the friction locks them together. However, their tendency to move apart at some point of time overcomes the friction. As a result, the blocks get deformed and eventually, they slide past one another abruptly. This causes a release of energy, and the energy waves travel in all directions.
• The point inside the earth where the energy is released is called the FOCUS (also known as the HYPOCENTER). The point on the surface, nearest to the focus, is called EPICENTRE.

EARTHQUAKE WAVES:

• Earthquake shaking and damage is the result of 2 basic types of seismic waves:
  1. Body Waves = Travel through the interior of the Earth.:
     1. P-waves = Longitudinal (compressional) waves
     2. S-waves = Transverse (shear) waves
  2. Surface Waves = Travel along the Earth’s surface (cause most destruction):
     1. L-waves = Horizontal shear motion (side-to-side)
     2. R-waves = Rolling motion (like ocean waves) – both vertical and horizontal
• P-waves = The faster of these body waves is called the primary or P wave. Its motion is the same as that of a sound wave in that, as it spreads out, it alternately pushes (compresses) and pulls (dilates) the rock. These P waves are able to travel through both solid rock, such as granite mountains, and liquid material, such as volcanic magma or the water of the oceans.
• S-waves = The slower wave through the body of rock is called the secondary or S wave. As an S wave propagates, it shears the rock sideways at right angles to the direction of travel. If a liquid is sheared sideways or twisted, it will not spring back, hence S waves cannot propagate in the liquid parts of the earth, such as oceans and lakes.
• L-waves i.e. Love waves = Its motion is essentially that of S waves that have no vertical displacement; it moves the ground from side to side in a horizontal plane but at right angles to the direction of propagation. The horizontal shaking of Love waves is particularly damaging to the foundations of structures.
• R-waves i.e. Rayleigh waves = They are like rolling ocean waves. Rayleigh waves wave move both vertically and horizontally in a vertical plane pointed in the direction in which the waves are travelling.

TYPES OF FAULTS ASSOSCIATED WITH EARTHQUAKES:

• There are 3 main types of faults: Normal, Reverse (Thrust) and Strike-slip.
• Normal and reverse faulting are examples of dip-slip, where the displacement along the fault is in the direction of dip and movement on them involves a vertical component.
• Normal faults occur mainly in areas where the crust is being extended such as a divergent boundary.
  Reverse faults occur in areas where the crust is being shortened such as at a convergent boundary.
  Strike-slip faults are steep structures where the two sides of the fault slip horizontally past each other. Transform boundaries are a particular type of strike-slip fault.
• Reverse faults, particularly those along convergent plate boundaries are associated with the most powerful earthquakes, megathrust earthquakes, including almost all of those of magnitude 8 or more.

TYPES OF EARTHQUAKES:

MEASURING EARTHQUAKES:

DISTRIBUTION OF EARTHQUAKE:

SEISMIC VULNERABILITY OF INDIA:

• Around 58% of India’s landmass is vulnerable to moderate or severe seismic hazard.
• India’s seismic risk is rooted in the northward drift of the Indian Plate, colliding with the Eurasian Plate at 4-5cm a year.
• Great Himalayan Earthquake = Himalayas are one of the most tectonically active regions of the world. An earthquake of magnitude 8 or higher is long overdue in the region according to various seismic studies. The Himalayan ‘Seismic Gap’ where strain has built since the Kangra earthquake (1906) & Gorkha earthquake (2015), are a ticking clock.
• Factors Increasing Vulnerability:
  • Unplanned urbanization and poor construction practices.
  • High population density in hazard-prone regions.
  • Lack of earthquake-resistant design in many buildings.
  • Low awareness and preparedness.

EARTHQUAKE ZONES IN INDIA:

Since the current division of India into earthquake hazard zones does not use Zone 1, no area of India is classed as Zone 1.

WHAT PREPAREDNESS MEASURES HAVE BEEN UNDERTAKEN?

1. Seismic Zoning Map: The Bureau of Indian Standards (BIS) has classified India into four seismic zones (Zone II, III, IV, and V) based on historical earthquake data and geological features. This map guides earthquake-resistant design. Zone V is the most seismically active, while Zone II is the least.

2. Earthquake-Resistant Building Codes: The National Building Code of India includes stringent guidelines for designing earthquake-resistant structures, especially in high-risk zones.

3. Retrofitting of Buildings: A major focus has been on retrofitting and strengthening existing older buildings, particularly critical infrastructure (hospitals, schools, government buildings) and those in highly vulnerable areas, to withstand seismic events. Financial grants are sanctioned to support these efforts.

4. Expansion of Seismic Observatories: The number of seismic observatories has significantly increased (from 80 in 2014 to 168 by February 2025), enhancing monitoring capabilities.

5. BhooKamp App: Launched by NCS, this mobile application provides real-time earthquake information to users.

6. Earthquake Risk Indexing (EDRI): NDMA’s EDRI project assesses earthquake risks in Indian cities, evaluating hazard, vulnerability, and exposure to guide mitigation efforts. Phase I covered 50 cities, and Phase II targets 16 more.

WAY FORWARD:

1. Strict Compliance and Audits: Implement stricter enforcement mechanisms for existing earthquake-resistant building codes (e.g., IS 1893:2016) for all new constructions. This includes mandatory structural safety certificates and regular, independent structural audits, especially for critical infrastructure, schools, hospitals, and high-rise buildings.

2. Retrofitting Old Structures: Accelerate and expand the retrofitting program for the vast inventory of older, non-compliant buildings, particularly in high-risk seismic zones and densely populated urban centers. This will require significant budgetary allocation, government incentives, and public-private partnerships.

3. Risk-Informed Urban Planning: Enforce stringent land-use regulations to avoid construction in high-risk liquefaction zones and active fault lines. Promote urban planning that includes adequate open spaces for evacuation and resilient infrastructure development.

4. Accelerate EEW System Deployment: Expedite the research, development, and deployment of robust Earthquake Early Warning (EEW) systems, particularly in the Himalayan region and other Zone IV and V areas. Focus on “last-mile connectivity” to ensure timely alerts reach communities effectively.

5. International Collaboration: Strengthen collaboration with earthquake-prone countries (e.g., Japan, Chile) to share best practices, research, and technology in earthquake monitoring and early warning systems.

6. Continuous Awareness Campaigns: Sustain and intensify public awareness campaigns using diverse media to educate citizens on earthquake risks, safe building practices, and the “Drop, Cover, and Hold On” technique.

7. Seismic Insurance and Risk Financing: Explore and promote government-backed earthquake insurance schemes for homes and businesses, potentially with incentives for adopting earthquake-resistant measures, to transfer financial risk and aid faster recovery. 

Conclusion:
The seismic activity intensifying globally as well as regionally, from Greece to Indonesia to Chile-Argentina border & Ecuador signals a dynamic earth. India cannot afford delay & must bridge the enforcement gap to prevent large-scale devastation.