Earthquakes – Mechanism, Distribution & Preparedness – Explained Pointwise

The recent twin earthquakes that struck Venezuela, measuring 7.2 and 7.5 in magnitude within seconds of each other, caused widespread devastation and renewed global attention to seismic hazards. The US geological Survey described the disaster as a “seismic doublet”. This disaster underscores the need to understand the mechanism, distribution, and impacts of earthquakes, while strengthening preparedness, resilience, and disaster-management strategies.

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 Earth’s outer layer (the lithosphere) is not one solid shell; it is broken up into giant puzzle pieces called tectonic plates. These plates are constantly, very slowly drifting. As they move, they grind against, pull apart from, or collide into each other. The edges of these plates are rough, so they often get stuck while the rest of the plate continues to move. This builds up immense pressure and stress over decades or even centuries. Eventually, the stress becomes too great, and the stuck rocks suddenly break and snap past each other along a crack in the Earth’s crust called a fault line. This sudden movement releases the stored-up energy in the form of seismic waves.
• Important terms related to earthquake:
  • FOCUS (also known as the HYPOCENTER) = The point inside the earth where the energy is released is called the FOCUS.
  • EPICENTRE = The point on the surface, nearest to the focus, is called EPICENTRE.
  • SEISMIC WAVES (aka EARTHQUAKE WAVES) = The waves of energy that travel through the Earth’s layers, causing the ground to shake.
  • FAULT: The fracture or zone along which the slip occurs.

What are SEISMIC 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 move both vertically and horizontally in a vertical plane pointed in the direction in which the waves are travelling.

What are the TYPES OF FAULTS ASSOCIATED WITH EARTHQUAKES?

• There are 3 main types of faults:
  1. Normal Fault
  2. Reverse (Thrust) Fault
  3. Strike-slip Fault
• 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.

What are the different TYPES OF EARTHQUAKES?

What are the different ways in which earthquakes are measured?

Earthquakes are measured in two principal ways: Magnitude & Intensity:

How are Earthquakes distributed across the earth’s surface?

What is a SEISMIC DOUBLET?

• A seismic doublet is a rare phenomenon in which two major earthquakes of similar magnitude strike close together in both time and geographic location.
• Key Characteristics of Seismic Doublet:
  • Similar Magnitudes: Unlike aftershocks (which are typically one or more magnitudes smaller than the main earthquake), the two events in a doublet are nearly equal in magnitude.
  • Rapid Time Separation: The twin quakes occur one after another with no time for the initial tectonic stress to settle. This can happen seconds apart.
  • Distinct Yet Linked Ruptures: The quakes originate from separate but contiguous or adjacent fault segments. The rupture of the first fault transfers stress to a nearby fault, rapidly triggering the second event.
  • Shared Waveform Characteristics: Because both events originate from the same general rupture zone and stress field, seismograms often display nearly identical seismic waveforms.
  • High Destructiveness: Doublets are vastly more dangerous than individual earthquakes. The second major shock frequently strikes before structural assessments can be made, causing structurally compromised buildings to collapse during prolonged shaking.

Why Are They So Dangerous?

1. Compounded Structural Damage: Buildings, bridges, and dams that managed to survive the first earthquake with minor, invisible structural cracks are frequently brought down entirely by the second, equally powerful shock.
2. Hazard to Rescue Teams: First responders and rescue workers are often actively searching through the rubble of the first event when the second major shock hits, creating extreme peril for emergency operations.
3. Psychological Impact: Because communities expect aftershocks to get progressively weaker, a second massive shock creates severe panic and disrupts emergency sheltering plans.

What are some notable examples of seismic doublets?

How SEISMICALLY VULNERABE is 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 the 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.

What are the different 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 to mitigate the impact of earthquakes?

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 National Centre for Seismology (NCS) has aggressively expanded its tracking footprint. The number of national seismic observatories has grown from just 80 in 2014 to 168, vastly improving the accuracy and speed of detecting tremors.
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.
7. Seismic Microzonation: Major metro areas (like Delhi, Mumbai, and Bengaluru) are progressively undergoing microzonation, which maps soil behavior and seismic hazards down to specific neighborhoods, ensuring safer local zoning laws. 
8. Mass Mock Drills: Regular large-scale simulation exercises are carried out across regions. Initiatives like the integrated Exercise Suraksha Chakra simulate massive earthquakes across multiple locations in the Delhi-NCR zone to test the coordination between local police, hospitals, and disaster teams. 

What should be the 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. Mandatory Retrofitting of 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. 
8. Traditional & Light Architecture in Hill States: In highly vulnerable eco-sensitive zones like the Himalayas and Northeast India, heavy concrete construction must be discouraged. Traditional, flexible, and lighter architecture (like the Khasi houses of Meghalaya or Dhajji Dewari of Kashmir) should be modernized and incentivized. 
9. Deepening Microzonation: Complete the seismic microzonation of all Tier-1 and Tier-2 cities in high-risk zones. This data must be seamlessly integrated into Geographic Information Systems (GIS) used by urban planning authorities to forbid high-rise zoning on unstable or liquefaction-prone soil.

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.