No one has ever seen the inside of the Earth. The deepest borehole ever drilled barely scratches 12 km into a planet whose radius is about 6,370 km. Yet we can say with confidence that the outer core is liquid and the inner core is solid. How? Earthquakes told us.

That is the first reason this topic matters for UPSC. Earthquakes are not just a disaster chapter. They are the key that unlocks the Earth's interior, plate tectonics, mountain building, tsunamis and the seismic zones of India. One concept, four syllabus doors.

The second reason is closer to home. A large share of India's land and people sit in zones where a strong quake is a matter of when, not if. In this guide, we at Legacy IAS Academy break the topic into the exact pieces UPSC tests: causes, seismic waves, shadow zones, measurement and India's zoning, including the most recent policy reversal that many aspirants have not yet updated in their notes.

What Is an Earthquake?

An earthquake is the shaking of the ground caused by a sudden release of energy stored in rocks. Rocks under stress bend slowly, much like a stretched ruler. When the stress exceeds the strength of the rock, it snaps along a fracture called a fault, and the stored energy radiates outward as waves.

Key Terms You Must Get Right

• Focus (Hypocentre): The point inside the Earth where the rupture begins and energy is first released.
• Epicentre: The point on the surface directly above the focus. It usually feels the waves first and suffers the most intense shaking.
• Isoseismal lines: Lines on a map joining places that experienced the same intensity of shaking.
• Seismograph: The instrument that records seismic waves; the record it produces is a seismogram.
• Foreshocks and aftershocks: Smaller tremors before and after the main shock along the same fault system.

Depth matters more than most aspirants realise. Shallow-focus earthquakes (less than about 70 km deep) do far more surface damage than deep-focus ones of the same magnitude, because the energy has less rock to travel through before it reaches buildings.

Causes of Earthquakes

Here's the thing. UPSC rarely asks you to list causes. It asks you to apply them, for instance, why a stable shield region like Latur or Koyna saw a deadly quake. So learn each cause with an Indian example attached.

1. Tectonic earthquakes: By far the most common and the most powerful. They occur along plate boundaries where plates collide, pull apart or slide past each other. The Himalayan quakes, from Kangra (1905) to Nepal (2015), belong here.
2. Volcanic earthquakes: Caused by the movement of magma and gas beneath a volcano. They are usually local and moderate. In India, the relevance is limited to Barren Island in the Andamans.
3. Reservoir-induced earthquakes: The weight of water in a large reservoir, and water seeping into faults, can trigger slip on stressed faults. The Koyna earthquake of 1967 in Maharashtra is the classic global example.
4. Collapse earthquakes: Small tremors from the collapse of underground cavities, especially in mining areas and limestone (karst) terrain.
5. Explosion earthquakes: Artificial tremors from nuclear tests or large blasts. Seismology is, in fact, how the world detects underground nuclear tests.

The Elastic Rebound Idea

Most tectonic earthquakes are explained by elastic rebound. Rocks on either side of a locked fault keep deforming as plates move. When friction finally gives way, the rocks snap back to a less strained position, and that sudden jerk is the earthquake. This is why a fault that has been "quiet" for a long time is not reassuring. It may simply be storing energy.

Seismic Waves: The Heart of the Earthquake Topic

When a fault ruptures, energy travels outward as seismic waves. They come in two families: body waves, which travel through the Earth's interior, and surface waves, which form when body waves interact with surface rocks and travel along the surface.

Body Waves

• P-waves (Primary): The fastest seismic waves and the first to arrive at a seismograph. They are longitudinal, meaning particles vibrate back and forth in the same direction the wave travels, much like sound. They pass through solids, liquids and gases.
• S-waves (Secondary): Slower, so they arrive after P-waves. They are transverse; particles move at right angles to the direction of travel, similar to a wave flicked along a rope. Crucially, S-waves cannot travel through liquids.

Surface Waves

• Love waves: Shake the ground side to side, horizontally. They are especially hard on building foundations.
• Rayleigh waves: Produce a rolling motion, like ocean waves moving across the ground.

Surface waves arrive last, but they are the most destructive. They carry large amplitudes and lose energy slowly near the surface, which is exactly where people and structures are.

Shadow Zones: How Earthquakes Mapped the Earth's Interior

Scientists noticed something odd when they compared seismograms from stations around the world. Some stations, at certain distances from the epicentre, received no waves at all. These blind spots are called shadow zones, and they are a favourite Prelims trap.

• Stations within about 105° of the epicentre record both P and S-waves.
• Beyond about 105°, S-waves vanish completely. Since S-waves cannot cross liquid, this proved the outer core is liquid. The S-wave shadow therefore covers a very large part of the globe.
• P-waves are missing in a band between roughly 105° and 145°. They are not stopped; they bend (refract) sharply on entering the liquid core and re-emerge beyond 145°.

Aspirants memorise 105° and 145° and stop there. The examiner wants the reasoning: S-waves disappear because of a liquid layer, while P-waves only change direction because of a change in density. If you can explain why, you will never fall for a twisted statement in Prelims. — Legacy IAS Faculty

Changes in wave speed at depth also mark the major boundaries inside the Earth: the Mohorovičić discontinuity between crust and mantle, and the Gutenberg discontinuity between mantle and outer core. Test yourself on these with our static quiz on earthquakes and tsunamis.

Measuring Earthquakes: Magnitude vs Intensity

These two words are not interchangeable, and UPSC knows that many candidates treat them as synonyms.

• Magnitude measures the energy released at the source. It is a single number for a given earthquake. The Richter scale is logarithmic: each whole-number step means about ten times greater ground motion and roughly 32 times more energy. Seismologists today prefer the Moment Magnitude scale (Mw), which works better for very large quakes.
• Intensity measures the effects of shaking at a particular place: damage, human perception, ground cracking. It varies from place to place for the same earthquake. The Modified Mercalli scale runs from I to XII, and India's zoning has traditionally been described using the related MSK scale.

Simply put: one earthquake has one magnitude but many intensities.

Global Distribution of Earthquakes

Earthquakes are not scattered randomly. Plot them on a world map and the plate boundaries draw themselves.

• Circum-Pacific Belt ("Ring of Fire"): Accounts for the large majority of the world's earthquakes, including most of the biggest ones. Japan, Indonesia, Chile and Alaska sit along it.
• Alpine-Himalayan (Mid-Continental) Belt: Runs from the Mediterranean through Turkey and Iran to the Himalaya and into Myanmar and Indonesia. India's high-hazard zones belong to this belt.
• Mid-Oceanic Ridges: Frequent but generally shallow and moderate earthquakes along divergent boundaries, such as the Mid-Atlantic Ridge.

Why Is India So Earthquake-Prone?

The answer lies in one ongoing collision. The Indian Plate is still moving north into the Eurasian Plate at a few centimetres every year. The Himalaya is the result, and it is still rising.

• The Himalayan thrust system: Major faults like the Main Central Thrust, the Main Boundary Thrust and the Himalayan Frontal Thrust absorb this collision. Strain builds up along them and is released in large earthquakes.
• Seismic gaps: Some stretches of the Himalaya have not produced a great earthquake in centuries. Scientists worry these segments are storing energy, with the stretch between the 1905 Kangra and 1934 Bihar-Nepal ruptures often discussed as a key gap.
• The Northeast: Squeezed between the Himalayan and Burmese (Indo-Burman) plate boundaries, it has produced some of India's largest recorded quakes, including Shillong (1897) and Assam-Tibet (1950).
• Soft alluvial soils: The Indo-Gangetic plains can amplify shaking and are prone to liquefaction, where saturated soil temporarily behaves like a liquid.
• Intraplate stress: Ancient faults in Kutch, the Narmada-Son lineament and the Deccan respond to the plate's push, as seen at Bhuj and Latur.

For a broader view of how this links with landforms and resources, see our guide on important geography topics for UPSC Mains GS1.

Seismic Zones of India: The Official Classification

The seismic zones of India are defined by the Bureau of Indian Standards (BIS) in its earthquake design code, IS 1893 (Part 1). The code divides the country into zones based on expected ground shaking, and each zone carries a zone factor (Z) that engineers use in building design.

Under IS 1893 (Part 1):2016, the code currently in force, India has four seismic zones: II, III, IV and V. The older Zone I was merged into Zone II during the 2002 revision, after the 1993 Latur earthquake struck an area that had been considered almost safe.

Taken together, Zones III, IV and V cover close to 59% of India's landmass. That figure alone is a strong opening line for any Mains answer on earthquake preparedness.

The 2025 Zone VI Proposal and Its Rollback

This is the current-affairs layer UPSC loves to add on top of static geography.

• In November 2025, BIS notified a revised code, IS 1893 (Part 1):2025. It used Probabilistic Seismic Hazard Assessment (PSHA) and created a new highest-risk Zone VI, placing the entire Himalayan arc in it.
• Under that map, around 61% of the country fell in moderate-to-high hazard categories.
• In March 2026, BIS withdrew the 2025 code through a gazette notification after the Ministry of Housing and Urban Affairs flagged sharp increases in construction and infrastructure costs and insufficient consultation.
• As a result, IS 1893:2016 is back in force, and India officially has four zones (II to V).

Major Earthquakes in India: A Quick Timeline

Earthquake Risk Management in India

Hazard is natural. Disaster is not. An earthquake becomes a disaster only when it meets vulnerable buildings and unprepared people. That is the framing UPSC expects in GS3.

• Institutions: The National Center for Seismology under the Ministry of Earth Sciences monitors earthquakes; NDMA has issued dedicated earthquake management guidelines under the Disaster Management Act, 2005.
• Building codes: IS 1893 (design forces), IS 4326 (earthquake-resistant construction) and the National Building Code. The real gap in India is enforcement, especially for non-engineered houses.
• Structural measures: Base isolation, shear walls, ductile detailing and retrofitting of hospitals, schools and lifeline infrastructure.
• Non-structural measures: Microzonation of cities, land-use planning, school safety drills, community volunteers and insurance.
• Early warning: Systems that detect the fast P-wave and send alerts seconds before the damaging waves arrive. Seconds are enough to stop trains, shut gas lines and get people under tables.

To build answer frameworks around this, use our GS3 Disaster Management complete notes and practise with the Disaster Management PYQs (2013–2025) with trends. For ongoing developments, follow our disaster management current affairs section.

How UPSC Tests This Topic

Let's be honest. Most aspirants read this chapter once in NCERT and assume they are done. Then Prelims throws a statement about shadow zones or tsunamis and strike-slip faults, and confidence collapses.

• Prelims: Properties of P and S-waves, shadow zones, magnitude vs intensity, seismic zones of specific states or cities, and the earthquake-tsunami link. Revise with our solved GS1 Geography Prelims PYQs.
• Mains GS1: Distribution of earthquakes, Himalayan seismicity and plate tectonics. Study the question pattern in our Geography PYQs (2013–2025) trend analysis.
• Mains GS3: Preparedness, building codes, urban vulnerability and case studies like Bhuj and Nepal.
• Geography Optional: Seismology and the Earth's interior at greater depth. Pick the right references from our Geography Optional booklist.

Keep feeding your notes with fresh examples from our GS1 Geography current affairs page. Every major quake in the news is a ready-made case study.

Frequently Asked Questions

How many seismic zones does India have right now?

Four: Zones II, III, IV and V, under IS 1893 (Part 1):2016. The 2025 code that added Zone VI was withdrawn in March 2026, so the 2016 classification applies again.

Which seismic wave is the most destructive?

Surface waves, particularly Love and Rayleigh waves. They arrive last but have the largest amplitude near the surface, where buildings stand.

Why do S-waves not pass through the outer core?

S-waves are transverse and need a rigid medium to travel. The outer core is liquid, so S-waves cannot pass, which creates the large S-wave shadow zone beyond about 105° from the epicentre.

Which seismic zone is Bengaluru in?

Bengaluru lies in Zone II, the low-risk category. Low risk is not zero risk, though; Latur showed that stable regions can still experience damaging intraplate earthquakes.

Is every undersea earthquake capable of causing a tsunami?

No. A tsunami needs a strong, shallow quake that vertically displaces the seafloor, typically along thrust faults at subduction zones. Strike-slip faults, which move sideways, rarely generate large tsunamis.