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Earthquakes

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1

Learn the Concept

All

An earthquake is a sudden shaking of the ground caused by the abrupt release of accumulated energy in the Earth's crust, radiating outward as seismic waves. Fix the mechanism: along a locked fault, tectonic stress slowly builds strain in the rocks until they suddenly snap and rebound (the elastic rebound theory), releasing energy from a point called the focus (hypocentre); the point on the surface directly above is the epicentre. Earthquakes are overwhelmingly concentrated along plate boundaries, make seismic belts that mirror the plate map, and are among the most destructive and least predictable of natural hazards — so preparedness, not prediction, is the realistic defence.

In One Line
  • Mechanism: elastic rebound — strain builds, rocks snap along a fault.
  • Focus (hypocentre) = origin inside Earth; epicentre = point above it on the surface.
  • Waves: P (fastest), S (solids only), L/surface (most destructive).
  • Scales: Richter / moment magnitude (energy) vs Mercalli (intensity/damage).
Glossary (define-on-first-use)
  • Focus / Hypocentre: the point within the Earth where the earthquake originates.
  • Epicentre: the point on the surface vertically above the focus.
  • Magnitude vs Intensity: energy released (Richter/Mw) vs felt effects & damage (Mercalli).
  • Isoseismal line: a line joining places of equal earthquake intensity.
  • Liquefaction: loss of strength of saturated loose soil during shaking (behaves like liquid).

1. Causes

  • Tectonic (the vast majority) — movement along faults at plate boundaries; volcanic — magma movement before/during eruptions; collapse — cave-ins in mines or caverns; reservoir-induced seismicity — water loading behind large dams (e.g. Koyna, 1967); and other human-induced causes (deep mining, fluid injection, large explosions).

2. Elastic rebound theory

  • Proposed by H.F. Reid after the 1906 San Francisco earthquake: rocks on either side of a fault are slowly deformed and store elastic strain as plates move; when the stress exceeds the rock's strength, the fault ruptures and the rocks snap back (rebound) to a less-strained shape, releasing the stored energy as seismic waves.
Focus, epicentre & seismic waves ground surface focus (hypocentre) epicentre seismic waves radiate →
Fig 1 — Energy releases at the focus; the epicentre lies directly above; waves radiate outward.

3. Focus depth & seismic waves

  • By focal depth: shallow (0–70 km, the most common & damaging), intermediate (70–300 km), and deep (300–700 km, at subduction zones along the Benioff zone).
  • Waves: P-waves (fastest, arrive first), S-waves (slower, through solids only), surface/L-waves (slowest but most destructive). The seismograph records them; the P–S arrival gap gives the distance to the epicentre, and three stations locate it (triangulation).

4. Measuring earthquakes

  • Richter scale (1935, C.F. Richter): a logarithmic measure of magnitude (energy); each whole number = ~32× more energy.
  • Moment Magnitude Scale (Mw): the modern standard for large quakes (based on the energy of fault rupture; no saturation at high values).
  • Mercalli scale (Modified Mercalli): measures intensity — the felt effects and damage at a place (I–XII, Roman numerals), which decreases with distance from the epicentre.
  • Isoseismal lines map zones of equal intensity.
ScaleMeasuresNature
RichterMagnitude (energy)Logarithmic, instrument
Moment (Mw)Magnitude (rupture energy)Modern standard
MercalliIntensity (damage)Observed effects (I–XII)

5. Global distribution (seismic belts)

  • Circum-Pacific belt (the Ring of Fire) — ~68% of earthquakes, subduction zones; Mediterranean–Himalayan (Alpide) belt — ~21%, collision zones (includes the Himalaya & India); and the mid-oceanic ridge belt — shallow quakes along divergent boundaries. The belts coincide with plate boundaries.
World seismic belts Circum-Pacific~68% Mediterranean–Himalayan (~21%) mid-ocean ridge belt
Fig 2 — The circum-Pacific, Mediterranean–Himalayan and mid-ocean-ridge belts trace the plate boundaries.

6. Effects, prediction & preparedness

  • Effects: ground shaking & building collapse, ground rupture, liquefaction (saturated soil loses strength), triggered landslides and tsunamis, fires and floods (dam failure).
  • Prediction remains unreliable (precursors like foreshocks, radon, animal behaviour are inconsistent); hence the emphasis on preparedness — earthquake-resistant (ductile) building design, seismic micro-zonation, building codes, early-warning systems, land-use planning and public awareness. In India the Himalaya, NE India and Kutch fall in the highest seismic zones (IV–V).
Key Facts to Remember
  • Elastic rebound (Reid, 1906 SF); focus/hypocentre within, epicentre above.
  • Focal depth: shallow (0–70 km, most damaging) / intermediate / deep (to 700 km).
  • Richter/Mw = magnitude (log); Mercalli = intensity (I–XII); isoseismals = equal intensity.
  • Belts: circum-Pacific (~68%), Mediterranean–Himalayan (~21%), mid-ocean ridges.
  • India: highest hazard in Zones IV–V (Himalaya, NE, Kutch); Koyna = reservoir-induced.
Connect the Dots
  • Interior of the Earth: seismic waves probe the interior (earlier leaf).
  • Plate tectonics: quakes trace boundaries; Benioff zones (earlier leaf).
  • Tsunamis: submarine quakes generate them (next leaf).
  • Disaster management (Paper II): seismic zones, codes, preparedness.
2

Prepare for the Exam — Prelims

Prelims

Tested on elastic rebound, focus vs epicentre, Richter vs Mercalli, the seismic belts, and India's zones & reservoir-induced quakes.

High-probability areas
  • Elastic rebound theory = Reid (1906 San Francisco).
  • Focus = origin inside; epicentre = point above.
  • Richter/Mw = magnitude (log); Mercalli = intensity (I–XII).
  • Belts: circum-Pacific (~68%), Mediterranean–Himalayan (~21%), ridges.
  • Koyna = reservoir-induced seismicity.
Common traps
  • Magnitude (energy) ≠ intensity (damage).
  • Richter is logarithmic (one step = ~32× energy).
  • Deep quakes occur at subduction (Benioff) zones.
  • Liquefaction affects saturated loose soils.
Revision tip: Causes → elastic rebound → focus/epicentre → P/S/L waves → Richter/Mercalli → belts → preparedness.
2

Prepare for the Exam — Mains

Mains

Optional Paper I / GS Paper I & disaster management. The examiner rewards the mechanism, distribution, and the prediction-vs-preparedness argument.

Likely question types
  • Explain the causes and mechanism of earthquakes. (15)
  • Discuss the global distribution of earthquakes. (10)
  • “Earthquakes cannot be predicted, only prepared for.” Examine. (15)
  • Distinguish between magnitude and intensity. (10)
Dimensions to cover
  • Causes & elastic rebound.
  • Focus/epicentre, waves, depth.
  • Scales & distribution.
  • Effects & preparedness.
Value-additions
  • Reid; Koyna; Bhuj 2001.
  • Zones IV–V (India).
  • Micro-zonation; building codes.
3

How to Write the Answer

Mains
Worked example — decoding the question

“Earthquakes cannot be predicted, only prepared for.” Critically examine. (15)
Explain why prediction fails, then detail the preparedness toolkit.

Introduction · ~20%
Define earthquakes; note their sudden, boundary-concentrated nature.
Body · ~60%
Why prediction fails: inconsistent precursors, complex rupture. Preparedness: earthquake-resistant design, seismic micro-zonation, building codes, early-warning, land-use planning, awareness & drills. Add India's zones (IV–V) and lessons from Bhuj (2001). Use the focus/epicentre diagram.
Conclusion · ~20%
Resilience through preparedness is the only reliable defence.
10-mark: ~150 words · ~7 min
15-mark: ~250 words · ~12 min
Common mistakes
  • Confusing magnitude & intensity.
  • Saying quakes are reliably predictable.
  • Mixing focus & epicentre.
Model opening line: “An earthquake is a few seconds of stored centuries — released too suddenly to foresee, too powerfully to ignore.”
4

Prelims Questions

Prelims

30 interactive MCQs — previous-year questions are included and tagged with their year. Select an answer for instant feedback and an explanation; your score is tracked and you can review wrong answers at the end.

Practice set
Question 1 of 30
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Score
5

Mains Questions

Mains

15 mains questions — previous-year questions are included and tagged. Write a full answer, then click Evaluate My Answer for paragraph-level feedback. Reveal the model structure after you attempt.

15 marks · ~250 words · ~11 min
Explain the causes and mechanism of earthquakes.
Evaluate My Answer
Model answer structure

Intro: define the key term and give a one-line context hook.
Body 1: conceptual / historical basis with a named fact, date or example.
Body 2: evidence and analysis — sites, inscriptions, coins, archaeologists.
Body 3: limitations or the counter-view (balanced).
Conclusion: a specific, reasoned judgment / way forward.

10 marks · ~150 words · ~7 min
Discuss the global distribution of earthquakes.
Evaluate My Answer
Model answer structure

Intro: define the key term and give a one-line context hook.
Body 1: conceptual / historical basis with a named fact, date or example.
Body 2: evidence and analysis — sites, inscriptions, coins, archaeologists.
Body 3: limitations or the counter-view (balanced).
Conclusion: a specific, reasoned judgment / way forward.

15 marks · ~250 words · ~11 min
'Earthquakes cannot be predicted, only prepared for.' Critically examine.
Evaluate My Answer
Model answer structure

Intro: define the key term and give a one-line context hook.
Body 1: conceptual / historical basis with a named fact, date or example.
Body 2: evidence and analysis — sites, inscriptions, coins, archaeologists.
Body 3: limitations or the counter-view (balanced).
Conclusion: a specific, reasoned judgment / way forward.

10 marks · ~150 words · ~7 min
Distinguish between the magnitude and intensity of an earthquake.
Evaluate My Answer
Model answer structure

Intro: define the key term and give a one-line context hook.
Body 1: conceptual / historical basis with a named fact, date or example.
Body 2: evidence and analysis — sites, inscriptions, coins, archaeologists.
Body 3: limitations or the counter-view (balanced).
Conclusion: a specific, reasoned judgment / way forward.

15 marks · ~250 words · ~11 min
Explain the elastic rebound theory and the terms focus and epicentre.
Evaluate My Answer
Model answer structure

Intro: define the key term and give a one-line context hook.
Body 1: conceptual / historical basis with a named fact, date or example.
Body 2: evidence and analysis — sites, inscriptions, coins, archaeologists.
Body 3: limitations or the counter-view (balanced).
Conclusion: a specific, reasoned judgment / way forward.

10 marks · ~150 words · ~7 min
Describe the seismic belts of the world.
Evaluate My Answer
Model answer structure

Intro: define the key term and give a one-line context hook.
Body 1: conceptual / historical basis with a named fact, date or example.
Body 2: evidence and analysis — sites, inscriptions, coins, archaeologists.
Body 3: limitations or the counter-view (balanced).
Conclusion: a specific, reasoned judgment / way forward.

10 marks · ~150 words · ~7 min
Discuss reservoir-induced and human-induced seismicity.
Evaluate My Answer
Model answer structure

Intro: define the key term and give a one-line context hook.
Body 1: conceptual / historical basis with a named fact, date or example.
Body 2: evidence and analysis — sites, inscriptions, coins, archaeologists.
Body 3: limitations or the counter-view (balanced).
Conclusion: a specific, reasoned judgment / way forward.

15 marks · ~250 words · ~11 min
Examine the effects of earthquakes and measures to reduce their impact.
Evaluate My Answer
Model answer structure

Intro: define the key term and give a one-line context hook.
Body 1: conceptual / historical basis with a named fact, date or example.
Body 2: evidence and analysis — sites, inscriptions, coins, archaeologists.
Body 3: limitations or the counter-view (balanced).
Conclusion: a specific, reasoned judgment / way forward.

10 marks · ~150 words · ~7 min
Explain the scales used to measure earthquakes.
Evaluate My Answer
Model answer structure

Intro: define the key term and give a one-line context hook.
Body 1: conceptual / historical basis with a named fact, date or example.
Body 2: evidence and analysis — sites, inscriptions, coins, archaeologists.
Body 3: limitations or the counter-view (balanced).
Conclusion: a specific, reasoned judgment / way forward.

10 marks · ~150 words · ~7 min
Describe the seismic zonation of India.
Evaluate My Answer
Model answer structure

Intro: define the key term and give a one-line context hook.
Body 1: conceptual / historical basis with a named fact, date or example.
Body 2: evidence and analysis — sites, inscriptions, coins, archaeologists.
Body 3: limitations or the counter-view (balanced).
Conclusion: a specific, reasoned judgment / way forward.

15 marks · ~250 words · ~11 min
Why is the Himalayan region highly earthquake-prone?
Evaluate My Answer
Model answer structure

Intro: define the key term and give a one-line context hook.
Body 1: conceptual / historical basis with a named fact, date or example.
Body 2: evidence and analysis — sites, inscriptions, coins, archaeologists.
Body 3: limitations or the counter-view (balanced).
Conclusion: a specific, reasoned judgment / way forward.

10 marks · ~150 words · ~7 min
Explain liquefaction and its significance.
Evaluate My Answer
Model answer structure

Intro: define the key term and give a one-line context hook.
Body 1: conceptual / historical basis with a named fact, date or example.
Body 2: evidence and analysis — sites, inscriptions, coins, archaeologists.
Body 3: limitations or the counter-view (balanced).
Conclusion: a specific, reasoned judgment / way forward.

10 marks · ~150 words · ~7 min
Discuss earthquake-resistant building design.
Evaluate My Answer
Model answer structure

Intro: define the key term and give a one-line context hook.
Body 1: conceptual / historical basis with a named fact, date or example.
Body 2: evidence and analysis — sites, inscriptions, coins, archaeologists.
Body 3: limitations or the counter-view (balanced).
Conclusion: a specific, reasoned judgment / way forward.

15 marks · ~250 words · ~11 min
Analyse earthquakes as a natural hazard in India.
Evaluate My Answer
Model answer structure

Intro: define the key term and give a one-line context hook.
Body 1: conceptual / historical basis with a named fact, date or example.
Body 2: evidence and analysis — sites, inscriptions, coins, archaeologists.
Body 3: limitations or the counter-view (balanced).
Conclusion: a specific, reasoned judgment / way forward.

10 marks · ~150 words · ~7 min
Describe the causes of earthquakes.
Evaluate My Answer
Model answer structure

Intro: define the key term and give a one-line context hook.
Body 1: conceptual / historical basis with a named fact, date or example.
Body 2: evidence and analysis — sites, inscriptions, coins, archaeologists.
Body 3: limitations or the counter-view (balanced).
Conclusion: a specific, reasoned judgment / way forward.

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