Definition: An earthquake is the sudden shaking of the Earth caused by a rapid release of stored elastic energy along a fault or fracture in the lithosphere. The rupture begins at the focus (hypocenter) and radiates energy outward as seismic waves; the point directly above the focus on the surface is the epicenter.
Earthquakes: Causes, Seismic Waves, Scales and Seismic Zones of India
Earthquakes occur when rocks along a fault suddenly slip after long periods of stress build-up. This guide explains the basics clearly—faulting, seismic waves, and the difference between magnitude and intensity—then connects them to India’s seismic setting, seismic zoning, and practical risk reduction (from safer construction to preparedness).
1. Key terms
- Fault: A fracture in rock along which movement has occurred.
- Focus (Hypocenter): The point inside the Earth where rupture starts.
- Epicenter: The point on the surface vertically above the focus.
- Rupture: The breaking and sliding of rocks along a fault plane.
- Aftershocks: Smaller quakes as the crust adjusts after the mainshock.
- Foreshocks: Smaller quakes that may precede a larger event (not always present).
- Seismograph: Instrument that records ground motion; the record is a seismogram.
- Recurrence interval: Approximate time between major quakes on a fault (statistical, not a clock).
The rupture begins at the focus; the epicenter is the surface point directly above it.
2. Why Earthquakes Happen: Stress, Strain and Elastic Rebound
Rocks in the lithosphere deform slowly under tectonic forces (compression, tension, shear). Along faults, friction can “lock” movement while stress continues to accumulate. When the stress exceeds rock strength + friction, the fault slips suddenly—releasing stored elastic energy as seismic waves. This is the elastic rebound theory.
In short: slow stress build-up + sudden slip = earthquake.
3. Fault types and stress regimes
Earthquakes are closely tied to the type of stress acting on crustal blocks.
| Stress regime | Fault type | Movement | Typical setting |
|---|---|---|---|
| Tension (extension) | Normal fault | Hanging wall moves down | Rifts; divergent margins |
| Compression | Reverse/Thrust fault | Hanging wall moves up | Convergent margins; fold mountains |
| Shear | Strike-slip fault | Horizontal movement | Transform margins; lateral faults |
4. Types of Earthquakes (Concept + Examples)
- Tectonic earthquakes: Caused by fault movement due to plate tectonics (most common and most destructive).
- Volcanic earthquakes: Linked to magma movement; usually localized.
- Collapse earthquakes: Roof collapse in mines/caves; small magnitude but can be damaging locally.
- Induced earthquakes: Human-triggered (reservoir impoundment, deep fluid injection, mining); typically smaller but important for policy.
- Interplate vs intraplate:
- Interplate: Near plate boundaries (most global mega-quakes).
- Intraplate: Within a plate; often due to reactivation of old faults (important for “stable” peninsular India).
5. Seismic Waves: What Moves, What Damages
Seismic energy travels as body waves (through the Earth) and surface waves (along the surface). Surface waves usually cause the maximum damage.
| Wave | Type | Medium | Key feature | Damage potential |
|---|---|---|---|---|
| P-waves (Primary) | Body | Solid + liquid + gas | Fastest; compressional (push-pull) | Low–moderate |
| S-waves (Secondary) | Body | Only solid | Shear; does not travel through liquids | Moderate |
| Love waves | Surface | Surface layers | Horizontal shear, side-to-side | High |
| Rayleigh waves | Surface | Surface layers | Rolling motion (elliptical) | Very high |
Key idea: S-waves do not travel through liquids, which is why they do not pass through Earth’s outer core.
6. Magnitude vs intensity
| Feature | Magnitude | Intensity |
|---|---|---|
| Meaning | Energy released at the source | Observed effects/damage at a place |
| Varies with location? | No (single value for an event) | Yes (different places report different intensities) |
| Depends on | Source parameters (rupture size, slip) | Distance, local soil, building quality, depth |
| Common scales | Moment Magnitude (Mw), older Richter | Modified Mercalli Intensity (MMI) |
Modern seismology uses Moment Magnitude (Mw) because it captures large events better. Remember that the magnitude scale is logarithmic: a difference of 1 unit means a large increase in energy (roughly ~32 times).
7. Depth Matters: Shallow vs Deep-Focus Earthquakes
- Shallow-focus (generally < ~70 km): most damaging because energy is released close to the surface.
- Intermediate (~70–300 km): common in subduction zones.
- Deep-focus (~300–700 km): occur in subducting slabs; felt over wide areas but usually less damaging at the epicenter compared to a similar shallow event.
Deep-focus earthquakes are typical of subduction zones (Wadati–Benioff zone).
8. Global Distribution: Why Some Belts Shake More
Earthquakes cluster along plate boundaries where stress accumulates and releases repeatedly.
- Circum-Pacific belt (“Ring of Fire”): Subduction margins; highest share of global large earthquakes.
- Alpine–Himalayan belt: Collision and complex faults from the Mediterranean to the Himalayas and SE Asia.
- Mid-ocean ridges: Frequent but mostly small-to-moderate quakes along spreading centers.
- Intraplate zones: Less frequent but can be destructive due to low preparedness (old faults reactivated).
9. India’s Seismicity: The Big Picture
India sits at the junction of major tectonic processes: collision in the north (Himalayas), subduction in the east (Andaman–Myanmar arc), and reactivated ancient faults in parts of the peninsular shield. So India has both boundary and intraplate seismicity.
- Himalayan arc: Compression + thrust faulting; potential for great earthquakes due to locked segments.
- North-East India: Highly complex plate interactions; frequent moderate-to-strong quakes.
- Andaman & Nicobar: Subduction earthquakes; also tsunami risk.
- Kachchh (Kutch) and western India: Intraplate quakes on old rifted structures and faults.
- Peninsular India: Generally stable, but intraplate quakes occur due to fault reactivation and local stresses.
10. Seismic zones of India (broad picture)
India is classified into Seismic Zones II to V (Zone V = very high hazard). The zoning supports design and planning, but damage also depends on local soil conditions and construction quality.
| Zone | Hazard level | Broad regions (illustrative) | Planning note |
|---|---|---|---|
| V | Very high | Himalayan belt pockets, North-East, Andaman & Nicobar, parts of Kutch | Plan critical infrastructure with highest safety; tsunami linkage for islands |
| IV | High | Foothills/adjacent plains in the north; parts near major active faults | High risk in densely populated plains near Himalayan front |
| III | Moderate | Large parts of Indo-Gangetic plains and peninsular cities | Moderate hazard + vulnerable buildings can still cause big losses |
| II | Low | Stable interiors in many regions | “Low” is not “no”; enforce basics and retrofit critical buildings |
Quick takeaway: The Himalayan belt, North-East India, Andaman–Nicobar and parts of Kachchh have the highest broad earthquake hazard.
11. Earthquake Hazards: Primary and Secondary
Primary hazards happen directly due to shaking/rupture. Secondary hazards are triggered effects.
| Hazard | What it looks like | Where it is common | One-line mitigation |
|---|---|---|---|
| Ground shaking | Vibrations; resonance in tall buildings | Everywhere | Ductile design + good detailing + code enforcement |
| Surface rupture | Fault breaks the ground surface | Near active faults | No-build zones along fault traces; careful siting |
| Liquefaction | Water-saturated sand behaves like liquid; buildings tilt | Alluvial plains, reclaimed land, coastal sediments | Soil improvement, deep foundations, microzonation |
| Landslides | Slope failure after shaking | Hills/mountains | Slope stabilization, drainage control, safer road cutting |
| Tsunami | Sea waves due to seafloor uplift/subsidence | Subduction margins | Early warning + evacuation routes + coastal zoning |
| Fires/industrial accidents | Gas line breaks, electrical short circuits | Cities | Flexible joints, shut-off valves, emergency planning |
12. Mitigation and preparedness
12.1 Structural safety (buildings and infrastructure)
- Ductile design: Buildings should bend without sudden collapse (especially in columns and joints).
- Site selection: Avoid soft soils, reclaimed land, steep unstable slopes when possible.
- Retrofitting: Strengthen old/weak buildings (schools, hospitals, bridges, heritage structures).
- Quality control: Good materials, correct reinforcement detailing, supervision at construction stage.
12.2 Non-structural safety (fastest wins)
- Anchor cupboards and water heaters; secure false ceilings and lighting.
- Keep heavy items low; create clear evacuation paths.
- Emergency kits, first aid, and drills in schools and offices.
- Community training: “Drop, Cover, Hold” and safe evacuation.
12.3 Planning and governance
- Microzonation: City-level maps that capture local soil amplification and liquefaction risk.
- Land-use regulation: Avoid critical facilities on high-risk sites; protect open spaces for evacuation.
- Early warning and communication: Even a few seconds can stop trains, open fire station doors, and trigger alerts.
- Resilient lifelines: Water, electricity, telecom, hospitals need redundancy.
13. Key takeaways
- Earthquakes are sudden fault slips after slow stress accumulation (elastic rebound).
- Surface waves usually cause the most damage; local soil can amplify shaking significantly.
- Magnitude measures energy released; intensity describes effects at a location and varies place to place.
- Shallow earthquakes tend to be more damaging than deeper ones of similar magnitude.
- India’s highest broad hazard zones include the Himalayan belt, North-East India, Andaman–Nicobar and parts of Kachchh.
- Secondary hazards (liquefaction, landslides, fires, tsunamis) can dominate overall losses.
- Risk reduction works best through safer construction, retrofitting, microzonation and preparedness drills.
14. Quick check questions
Q1. Which of the following statements about seismic waves is/are correct?
A) P-waves can travel through solids and liquids.
B) S-waves can travel through liquids but not through gases.
C) Surface waves generally cause more damage than body waves.
D) Love waves are body waves that travel through the Earth’s core.
Q2. “Intensity” of an earthquake differs from its “magnitude” because intensity primarily depends on:
A) Amount of energy released at the source
B) Distance from epicenter and local ground conditions
C) Depth of the Earth’s outer core
D) Rate of sea-floor spreading
Q3. Liquefaction is most likely to occur in:
A) Dry granite hills
B) Saturated loose sandy alluvium
C) Solid basalt plateaus with low groundwater
D) Desert dunes with no groundwater
Q4. Deep-focus earthquakes (300–700 km) are typically associated with:
A) Mid-ocean ridges
B) Subduction zones
C) Hotspot volcanism only
D) River deltas
Q5. Which of the following regions is generally categorized as very high seismic hazard in India?
A) Parts of the Himalayan belt and North-East India
B) Central Indian plateau only
C) Coastal plains of western India only
D) Thar desert only
Answers: Q1-A and C, Q2-B, Q3-B, Q4-B, Q5-A
15. FAQs
What is the difference between focus and epicenter?
The focus is the point inside the Earth where rupture begins; the epicenter is the surface point directly above it.
Why are shallow earthquakes usually more destructive?
Because the energy is released close to the surface, so less energy is lost before it reaches buildings and people.
Can earthquakes be predicted accurately?
Exact prediction (time, place and magnitude) is not reliably possible. Risk reduction focuses on preparedness, codes, retrofit and early warning.
What causes liquefaction during earthquakes?
Strong shaking increases pore-water pressure in saturated loose sands, reducing friction between grains so the soil behaves like a fluid.
Is peninsular India completely safe from earthquakes?
No. Peninsular India is relatively stable, but intraplate earthquakes can occur due to reactivation of old faults and rift zones.
How does an undersea earthquake generate a tsunami?
A tsunami forms when the seafloor is suddenly uplifted or subsides, displacing a large volume of water; this is common in subduction-zone megathrust earthquakes.