Earthquakes - Causes, Seismic Waves and Seismic Zones of India for UPSC

Earthquakes for UPSC: Causes, Waves, Scales, Distribution and Indian Context

Earthquakes are a core topic in physical geography and disaster management. A clear grasp of their causes, types of waves, measurement scales and distribution patterns is essential for understanding Earth’s dynamic processes and for disaster preparedness. This note provides a balanced overview tailored to the UPSC syllabus, from the basics of earthquake mechanics to the seismotectonics of India, notable historical events, hazards and mitigation strategies.

1. Causes and Earthquake Mechanics

Most earthquakes are caused by the slow build‑up and sudden release of elastic strain along faults related to plate tectonics. Important causes include:

• Tectonic movement: Plates diverge, converge or slide past each other. When friction locks faults, strain accumulates. Failure along the fault causes a slip and releases energy. The elastic rebound theory explains how rocks deform elastically and then rebound when frictional resistance is overcome.
• Volcanic activity: Magma movement can fracture rock, producing earthquakes. Such quakes are generally localized around volcanoes but can foreshadow eruptions.
• Collapse events: Sudden collapse of roofs in caves or mines, or sinkholes in karst terrain, can generate minor tremors.
• Human‑induced: Construction of large reservoirs, deep fluid injection (geothermal, waste disposal), hydraulic fracturing and mining can alter stress and trigger earthquakes (e.g., the Koyna dam earthquake of 1967).

Plate boundaries determine the nature of earthquakes:

• Divergent boundaries (constructive): Tension leads to normal faulting and shallow quakes along mid‑ocean ridges and rift valleys.
• Convergent boundaries (destructive): Compression generates reverse or thrust faults. Subduction zones produce powerful shallow to deep earthquakes and tsunamis (e.g., Sumatra–Andaman, 2004). Collision zones like the Himalayas produce large but mostly shallow quakes.
• Transform boundaries: Shear stress along strike‑slip faults produces horizontal displacement, as seen along the San Andreas fault.
• Intraplate regions: Within stable plate interiors, ancient rifts and faults can be reactivated, producing rare but damaging earthquakes (e.g., Latur 1993, Bhuj 2001).

2. Fault Types and Stress Regimes

Three principal stress regimes generate characteristic fault types:

3. Classification of Earthquakes

• By origin:
  • Tectonic earthquakes: Caused by faulting related to plate movements; constitute the vast majority.
  • Volcanic earthquakes: Associated with volcanic eruptions and magma intrusion.
  • Collapse (subsidence) earthquakes: Due to roof collapse in mines or caverns.
  • Induced earthquakes: Triggered by anthropogenic activities like reservoir impoundment, mining, fluid injection or extraction.
• By depth: Shallow (<70 km), intermediate (70–300 km) and deep (300–700 km). Most destructive quakes are shallow because energy is released near the surface. Deep quakes occur in subducting slabs and are less damaging at the epicenter but can be felt over wide areas.
• By magnitude: Micro (<2), minor (2–4), light (4–5), moderate (5–6), strong (6–7), major (7–8), great (>8). Each unit increase implies about 32 times more energy release.
• By location: Interplate earthquakes occur along plate boundaries; intraplate earthquakes occur within a plate due to reactivation of ancient structures.

4. Seismic Waves

When an earthquake occurs, energy propagates as seismic waves. Understanding these waves helps determine the Earth's interior structure and informs engineering design.

Seismic shadow zones: Because S‑waves cannot pass through liquids and P‑waves are refracted at the core–mantle boundary, certain areas on Earth’s surface receive no direct waves from a given earthquake. Study of these zones helps infer the structure of Earth’s interior.

5. Measurement and Scales

Seismographs record ground motion as a seismogram. From these records, scientists derive magnitude and intensity:

• Magnitude: Measures the total energy released at the source. The Moment Magnitude Scale (Mw) is widely used today and is logarithmic. The older Richter Scale is suited for local, moderate events.
• Intensity: Describes the severity of shaking and damage at a specific location. The Modified Mercalli Intensity (MMI) scale (I–XII) and the Medvedev–Sponheuer–Karnik (MSK) scale are commonly used in India. Intensity varies with distance from the epicenter, local geology and construction practices.

6. Global Distribution and Earthquake Belts

Earthquakes are not randomly distributed; they cluster along tectonic plate boundaries. Major belts include:

• Circum‑Pacific Belt (Ring of Fire): Encircles the Pacific Ocean; associated with subduction zones and volcanism. Approximately 80 % of the world’s largest earthquakes occur here (e.g., Japan, Chile, Alaska).
• Alpine–Himalayan Belt: Extends from the Mediterranean region through Turkey, Iran, the Himalayas and into Southeast Asia. Created by the collision of the African, Arabian and Indian plates with the Eurasian plate.
• Mid‑Oceanic Ridges: Divergent boundaries where new oceanic crust is formed; generally produce low‑magnitude, shallow earthquakes (e.g., Mid‑Atlantic Ridge).
• Intraplate regions: Stable continental interiors occasionally experience earthquakes due to reactivation of ancient faults, crustal flexure or isostatic adjustments.

7. Earthquake Activity in India

India lies at the intersection of several active tectonic regimes. The Indian plate is colliding with the Eurasian plate at ~47 mm/year, causing uplift of the Himalayas and stress accumulation along several thrusts. Key seismotectonic regions include:

• Himalayan Arc: The Main Central Thrust, Main Boundary Thrust and Main Frontal Thrust accommodate convergence. The region is prone to great earthquakes (e.g., 1897 Assam, 1905 Kangra, 1950 Assam–Tibet) and numerous moderate events (e.g., 1991 Uttarkashi, 2015 Nepal–Gorkha).
• North‑East India: Complex interactions among the Indian, Eurasian, Burmese and Sunda plates. Frequent earthquakes of magnitude 5–7 occur across Assam, Meghalaya, Mizoram, Nagaland and Tripura.
• Andaman & Nicobar Islands: Part of the Sunda–Andaman subduction system. Subduction earthquakes (e.g., 2004 Sumatra–Andaman) can generate tsunamis.
• Western India and Kachchh: Ancient rift systems and transform faults (e.g., Allah Bund fault) have produced damaging intraplate earthquakes, most notably the 1819 Rann of Kachchh quake and the 2001 Bhuj quake.
• Peninsular Shield: Generally stable but not completely aseismic. The 1967 Koyna (reservoir‑induced) and 1993 Latur earthquakes highlight the vulnerability of the Deccan plateau.

According to the Ministry of Earth Sciences, around 59 % of India’s landmass is vulnerable to moderate to severe seismic hazard. The Bureau of Indian Standards (IS 1893) divides the country into four seismic zones (II, III, IV and V) with increasing hazard.

8. Seismic Zones of India

The latest seismic zoning map (IS 1893: Part 1) groups India into four zones. The zone factor indicates the effective horizontal peak ground acceleration expected under the Maximum Considered Earthquake (MCE). A higher zone factor implies a greater hazard and stricter design requirements.

9. Major Earthquakes in India (Illustrative)

The following table summarizes selected significant earthquakes in the Indian subcontinent. These events demonstrate the range of tectonic settings and underscore the need for preparedness.

10. Earthquake Hazards and Effects

Earthquake impacts are not limited to ground shaking; secondary hazards often cause greater damage than the tremor itself. Key hazards include:

11. Mitigation, Preparedness and Policy Framework

Structural measures

• Adopt earthquake‑resistant design codes (e.g., IS 1893 for design, IS 4326 for construction practices). Incorporate features like base isolation, shear walls and energy dissipating devices.
• Retrofitting of vulnerable structures, especially schools, hospitals, heritage buildings and bridges, to enhance ductility and prevent collapse.
• Site selection: avoid building critical infrastructure on soft soils, reclaimed land or known fault lines. Conduct geotechnical investigations and microzonation studies.

Non‑structural measures

• Secure heavy furnishings, appliances and shelves; use latches for cabinets; anchor water heaters and gas cylinders.
• Prepare emergency kits (water, food, flashlight, first aid, batteries); maintain clear escape routes.
• Regularly conduct drills in schools, offices and communities to practise "Drop, Cover and Hold" and evacuation protocols.

Planning and governance

• Implement seismic microzonation and hazard mapping to guide land‑use planning and zoning regulations.
• Strengthen early warning and monitoring networks (e.g., National Center for Seismology; Indian Tsunami Early Warning Centre). Integrate warnings with public alert systems.
• Enforce compliance with building codes through rigorous inspection and capacity building of engineers and masons.
• Integrate earthquake risk reduction into disaster management plans at national, state and local levels. Encourage community‑based disaster preparedness.

Safety tips for individuals

• Before: Learn about local seismic hazards; ensure your home meets seismic standards; know safe spots (under sturdy furniture or against interior walls).
• During: When indoors, drop to the floor, cover your head and neck under a sturdy table, and hold on. Stay away from windows, mirrors and heavy furniture. If outdoors, move to an open area away from buildings, trees and power lines.
• After: Check for injuries and hazards (gas leaks, electrical damage); use battery‑powered radios for information; expect aftershocks; assist others and follow official instructions.

12. Key Points to Remember

• Earthquakes occur when accumulated strain along faults is released; the focus is the point of rupture and the epicenter is the surface point above it.
• The Indian plate is moving northwards and colliding with Eurasia, making the Himalayas one of the most seismically active regions in the world.
• P‑waves travel through solids, liquids and gases and are faster than S‑waves; S‑waves travel only through solids.
• Magnitude measures energy release; intensity measures effects on the surface and varies with distance and local conditions.
• The Bureau of Indian Standards divides India into four seismic zones (II–V); about 59 % of India’s area is prone to moderate to severe earthquakes.
• Historical earthquakes like the 1897 Assam, 1905 Kangra, 1950 Assam, 1993 Latur, 2001 Bhuj and 2004 Sumatra–Andaman illustrate the variety of Indian seismicity.
• Hazards include ground shaking, surface rupture, liquefaction, landslides, tsunamis and fires; mitigation requires structural measures, community preparedness and strict enforcement of building codes.

13. Quick Check Questions

Q1. Which seismic waves cannot travel through liquids?

A) P‑waves B) S‑waves C) Love waves D) Rayleigh waves

Q2. The moment magnitude scale differs from the Richter scale because it:

A) Measures earthquake intensity B) Uses logarithms of amplitude only C) Accounts for fault area, slip and rock rigidity D) Is useful only for small earthquakes

Q3. Which of the following pairs is correctly matched?

A) Normal fault – Compression B) Reverse fault – Tension C) Strike–slip fault – Shear D) Thrust fault – Divergent boundary

Q4. The 2001 Bhuj earthquake occurred due to:

A) Subduction of the Indian plate beneath the Sunda plate B) Transform fault movement along the San Andreas fault C) Reactivation of an intraplate rift system in Kachchh D) Volcanic eruption in the Deccan Traps

Q5. Zone V on India’s seismic zoning map corresponds to:

A) Low damage risk (MSK VI) B) Moderate damage risk (MSK VII) C) High damage risk (MSK VIII) D) Very high damage risk (MSK IX or above)

Answers: Q1 – B; Q2 – C; Q3 – C; Q4 – C; Q5 – D

14. FAQs

What is the difference between epicenter and hypocenter (focus)?

The hypocenter (focus) is the point within the Earth where rupture starts; the epicenter is the point on the surface directly above the focus. The epicenter is often used in media reports because it helps locate the affected region.

Why are shallow earthquakes usually more destructive?

Shallow earthquakes (depth <70 km) release energy close to the surface, so seismic waves lose little energy before reaching buildings and people. Deeper earthquakes spread their energy over a larger area and often cause less damage at any given point.

Can earthquakes be predicted?

Despite significant advances in seismology, precise short‑term prediction of earthquakes (exact time, place and magnitude) is not possible. Scientists can estimate long‑term probabilities and identify high‑risk zones. Therefore, emphasis is placed on preparedness and mitigation.

What is liquefaction and why is it dangerous?

Liquefaction occurs when water‑saturated, unconsolidated sediments lose strength and behave like a fluid during intense shaking. Buildings and infrastructure may settle or tilt, pipelines may float, and foundations can fail, leading to severe damage even if ground shaking is moderate.

Is peninsular India completely safe from earthquakes?

No. Although the Indian peninsula is relatively stable compared to the Himalayas, intraplate earthquakes have occurred (e.g., Koyna 1967, Latur 1993). Hence basic seismic design principles should be followed even in low‑hazard zones.

How do undersea earthquakes generate tsunamis?

Subduction earthquakes can cause sudden uplift or subsidence of the sea floor, displacing huge volumes of water. This disturbance propagates as long‑wavelength tsunami waves that travel across ocean basins. In shallow coastal waters, the waves increase in height, inundating coastal areas.

15. Related Topics

• Plate Tectonics and Plate Boundaries
• Tropical Cyclones and Disaster Management
• Ocean Currents: Types and Climatic Effects
• Indian Monsoon and Climate Variability
• Major Soil Types of India and Liquefaction Risk