Geological Disasters

Earthquakes · Seismic Waves & Measurement · India's Seismic Zones · Landslides · Avalanches · Tsunami · GLOF · Land Subsidence (Joshimath)

📄 GS Paper 3🎯 Prelims + Mains⏱ 18 min read📅 Updated June 2026

Geological Disasters: The Big Picture

Geological (geophysical) disasters originate in processes operating within the solid earth — the movement of tectonic plates, the failure of slopes, the displacement of ocean water, and the deformation of the ground surface. They include earthquakes, tsunamis, landslides, avalanches, land subsidence and volcanic activity, and increasingly the hybrid hazards of Glacial Lake Outburst Floods (GLOFs) that straddle the geological and hydro-meteorological categories.

India is acutely exposed: nearly 59% of its landmass is prone to earthquakes of moderate to very high intensity, the entire Himalaya and Western Ghats are landslide-prone, the long coastline is tsunami-exposed, and the fragile young-fold mountains face avalanches, GLOFs and subsidence. UPSC tests both the science (Prelims — seismic waves, magnitude scales, zones) and the management (Mains — preparedness, mitigation, case studies).

Why geology matters: The Indian Plate is moving northward into the Eurasian Plate at ~4-5 cm/year. This continental collision built the Himalaya and stores enormous strain energy along the Main Boundary Thrust and Main Central Thrust — making the Himalayan arc one of the world's most seismically active and landslide-prone belts, with several recognised "seismic gaps."

Earthquakes: Causes & Mechanics

An earthquake is the sudden release of accumulated strain energy in the earth's crust, radiating outward as seismic waves. The dominant cause is plate tectonics: stress builds along faults at plate boundaries (convergent, divergent, transform) until rock ruptures suddenly — the elastic rebound theory (H.F. Reid).

Types of Earthquakes

Type	Cause	Example / Note
Tectonic	Movement along faults / plate boundaries	Most common & most destructive (Bhuj 2001)
Volcanic	Magma movement & gas pressure before/during eruptions	Localised, near volcanic belts
Collapse	Roof collapse of underground caverns/mines (karst, deep mining)	Small magnitude, localised
Induced / Reservoir-triggered	Human activity — large dams loading the crust, fluid injection, mining	Koyna 1967 (reservoir-induced seismicity)

Focus, Epicentre & Seismic Waves

Focus (hypocentre): the point inside the earth where rupture begins. Shallow-focus quakes (<70 km) are the most damaging.
Epicentre: the point on the surface directly above the focus — where shaking is usually strongest.
Body waves travel through the earth's interior: P-waves (Primary) are fastest, longitudinal/compressional, travel through solids, liquids & gases; S-waves (Secondary) are slower, transverse, travel only through solids (their inability to pass the outer core proved it is liquid).
Surface waves (L-waves) — Love & Rayleigh — are slowest but cause the most surface destruction.

Prelims hook: Order of arrival on a seismograph = P → S → Surface. P-waves: compressional, pass through liquids. S-waves: transverse, blocked by liquids (the "S-wave shadow zone" beyond ~103° reveals the liquid outer core). This is a recurring science-of-earthquakes question.

Figure 1: Rupture begins at the focus; the epicentre is the surface point above it. Body waves (P, S) radiate through the interior; surface waves cause the strongest ground shaking.

Measuring Earthquakes

UPSC frequently contrasts the three measurement systems — two of magnitude (energy released) and one of intensity (felt effect).

Scale	What it measures	Key point
Richter Scale (M_L)	Magnitude — logarithmic; each whole number = ~10× amplitude & ~31.6× energy	Saturates for very large quakes; suited to small/medium events
Moment Magnitude (M_w)	Magnitude — based on seismic moment (rupture area × slip × rigidity)	Modern standard; accurate for great earthquakes
Modified Mercalli Intensity (MMI)	Intensity — felt shaking & damage, I (not felt) to XII (total destruction)	Varies with distance & local geology; observational, not instrumental

Key distinction: One earthquake has a single magnitude (Richter / M_w) but many intensity values (MMI) depending on location, soil and construction. Soft alluvial soil amplifies shaking — the basis of seismic microzonation.

India's Seismic Zones

The Bureau of Indian Standards (BIS), in IS 1893, divides India into four seismic zones — II, III, IV and V — in increasing order of seismic risk (the older five-zone map merged Zone I into Zone II). NDMA reports that roughly 59% of India's landmass is prone to moderate-to-severe earthquakes.

Zone V (Very High Risk): Entire Northeast, Kashmir & Himachal Himalaya, Uttarakhand parts, Kutch (Gujarat), Andaman & Nicobar, parts of North Bihar. Highest expected intensity (MMI IX+).
Zone IV (High Risk): Delhi-NCR, remaining Himalayan foothills, parts of J&K, Sikkim, North Bengal, parts of Maharashtra & Bihar.
Zone III (Moderate Risk): Much of the Indo-Gangetic plain, parts of peninsular India, Kerala coast.
Zone II (Low Risk): Remaining peninsular interior — relatively stable Deccan craton.

Seismic gaps: Stretches of a fault that have not ruptured for a long time, accumulating strain — notably the Central Himalayan seismic gap between the 1905 Kangra and 1934 Bihar-Nepal ruptures. Such gaps are flagged for a potential "great earthquake" (M>8), making them priority zones for preparedness.

Earthquake Mitigation

Earthquake-resistant design & BIS codes: ductile detailing, base isolation, shear walls; enforcement of IS 1893, IS 13920, IS 4326.
Retrofitting of existing lifeline buildings (hospitals, schools, fire stations) and non-engineered houses.
Seismic microzonation of cities (e.g., Delhi, Guwahati, Jabalpur) to map local soil amplification and guide land use.
Land-use planning, awareness, mock drills and Community-Based Disaster Management; National Earthquake Risk Mitigation Project (NDMA).

Figure 2: A stylised, schematic representation of India's four seismic zones (II–V) by risk level — not an accurate geographic map.

Landslides

A landslide is the downslope movement of rock, soil or debris under gravity. India's landslide-prone area covers ~12-15% of the landmass, concentrated in the Himalaya, North-East and the Western Ghats.

Causes

Natural: steep slopes, weak/fractured rock, heavy or prolonged rainfall & cloudbursts, earthquakes, toe erosion by rivers.
Anthropogenic: deforestation, unscientific road/dam/hydropower construction, slope cutting, mining/quarrying, unplanned hill-town expansion.

Zonation & Mapping

The Geological Survey of India (GSI) is the nodal agency for landslide studies and prepares Landslide Susceptibility Maps under the National Landslide Susceptibility Mapping (NLSM) programme. NDMA issues landslide management guidelines; a Landslide Early Warning System is being piloted in hotspots.

Case study — Wayanad (Kerala), July 2024: Massive rain-triggered landslides/debris flows in the Western Ghats killed several hundred people, India's deadliest landslide event in recent years. It revived debate on the Gadgil & Kasturirangan reports, ecologically sensitive zones, quarrying and unregulated construction in fragile Ghats terrain.

Avalanches

An avalanche is the rapid flow of a large mass of snow (and sometimes ice and rock) down a slope. They threaten the high Himalaya, affecting troops, trekkers, roads and hydropower projects.

Types: loose-snow (point-release) vs slab avalanches (a cohesive slab fractures and slides — the deadliest); also wet vs dry, and surface vs full-depth.
Triggers: fresh heavy snowfall, rapid warming, wind-loading, weak buried layers, slope angle (~30-45° most prone), human/seismic disturbance.
Nodal agency: the Snow and Avalanche Study Establishment (SASE), Chandigarh, a DRDO laboratory, issues avalanche forecasts/warnings for the Himalayan region (J&K, Himachal, Uttarakhand) and supports the armed forces.
Mitigation: snow sheds & galleries, deflecting/retarding structures, controlled triggering, avalanche bulletins, and route/closure management.

Tsunami

A tsunami ("harbour wave") is a series of long-wavelength ocean waves generated by the sudden vertical displacement of a large volume of water — most commonly by a submarine megathrust earthquake at a subduction zone, and less often by undersea landslides, volcanic eruptions or meteorite impacts.

In the deep ocean a tsunami travels at jet speeds (~700-800 km/h) with a small wave height, but as it nears the shallow coast it slows and the wave height builds dramatically (shoaling).
A receding shoreline (sea drawback) is a natural warning sign that the trough has arrived first.

Case study — 2004 Indian Ocean Tsunami (26 Dec 2004): Triggered by a M_w ~9.1-9.3 megathrust off Sumatra (Sunda Trench), it killed ~230,000 people across 14 countries; India's Tamil Nadu, Andhra, Kerala & Andaman & Nicobar were severely hit. It directly led to India's tsunami warning capability.

Indian Tsunami Early Warning System (ITEWS): Operational since 2007, run by INCOIS (Indian National Centre for Ocean Information Services), Hyderabad (under MoES). It uses a network of seismic stations, Bottom Pressure Recorders (BPRs) and tide gauges to issue warnings within minutes, and serves as a Regional Tsunami Service Provider for the Indian Ocean.

Figure 3: A submarine earthquake displaces the seabed; the wave races across the deep ocean at low height, then shoals and surges at the coast.

Glacial Lake Outburst Flood (GLOF)

A GLOF is the sudden release of water from a glacial lake — a hybrid geological/hydro-meteorological hazard intensifying with climate change in the Himalaya.

Mechanism

Retreating glaciers leave meltwater dammed behind unstable moraine (debris) or ice barriers, forming glacial lakes.
A trigger — an avalanche or rock/ice fall into the lake, heavy rain, moraine-dam failure, or an earthquake — breaches the dam.
The released water surges downstream as a destructive flood/debris flow, sweeping away settlements, bridges and hydropower projects.

Case study — South Lhonak Lake GLOF, Sikkim (Oct 2023): The outburst of South Lhonak glacial lake sent a massive flood down the Teesta basin, destroying the Chungthang (Teesta-III) dam, washing away bridges, and causing heavy casualties. It became the flagship case for Himalayan GLOF risk and the safety of hydropower projects in fragile high-altitude terrain.

Mitigation

Inventory & monitoring of glacial lakes (ISRO/NRSC satellite mapping; NDMA's National GLOF Risk Mitigation Programme).
Controlled lowering/siphoning of high-risk lakes to reduce water volume.
Early warning systems, downstream sirens, hazard zonation, and climate-resilient siting of infrastructure.

Figure 4: A trigger (ice/rockfall, rain, quake) breaches the moraine dam, releasing the glacial lake as a destructive downstream flood.

Land Subsidence

Land subsidence is the gradual or sudden sinking of the ground surface due to subsurface movement of earth materials — from natural causes (dissolution, tectonics) or human activity (excessive groundwater extraction, mining, tunnelling, loading by construction).

Case study — Joshimath (Uttarakhand), Jan 2023: Widespread cracks in buildings and roads forced evacuations in this Himalayan pilgrimage-gateway town. Key factors identified: the town sits on an old landslide debris/moraine (unstable foundation); unregulated construction beyond carrying capacity; poor drainage and soil saturation; and stress from nearby infrastructure including the NTPC Tapovan-Vishnugad hydropower tunnel and road widening (Char Dham). It exemplifies the limits of hill-town carrying capacity.

Mitigation

Carrying-capacity studies and slope-stability assessment before approving construction in fragile hill towns.
Regulated, lightweight, engineered construction; proper drainage & sewerage to prevent soil saturation.
Regulation of groundwater extraction and high-impact tunnelling/blasting in sensitive zones.
Geological monitoring (subsidence/ground-deformation sensors, InSAR satellite data) and relocation where unavoidable.

Current Affairs Snapshot (up to June 2026)

National GLOF Risk Mitigation Programme: Post-Sikkim, NDMA expanded glacial-lake monitoring and lake-lowering/early-warning works across high-risk Himalayan lakes; expert teams surveyed priority lakes in Sikkim, Uttarakhand, Himachal & the North-East.
Wayanad landslides (July 2024): One of India's deadliest landslide disasters; renewed scrutiny of Western Ghats Ecologically Sensitive Areas, quarrying and the Gadgil/Kasturirangan recommendations.
Joshimath follow-up: Continued monitoring, the multi-institutional study reports, and a wider review of carrying capacity of Himalayan towns and Char Dham-type projects.
ITEWS upgrades (INCOIS): Strengthened Bottom Pressure Recorder & tide-gauge network; India continues as a Regional Tsunami Service Provider for the Indian Ocean; coastal "Tsunami Ready" community certification expanded.
Bhuj earthquake 25th year (2026 outlook): Renewed focus on enforcement of BIS seismic codes, urban microzonation (Delhi, Guwahati) and retrofitting of lifeline buildings in Zone IV-V cities.
Mission Mausam (2024): Better observation & AI forecasting supports cloudburst and rainfall-triggered landslide early warning in the Himalaya.

Previous Year Questions — Prelims PRELIMS

How to use: Prelims on geological disasters rewards precise science (seismic waves, magnitude vs intensity, zones) and institutional facts (GSI, INCOIS, SASE, NDMA).

UPSC Prelims 2023 Concept · GLOF

Q. With reference to "Glacial Lake Outburst Floods (GLOF)", consider the causes and consequences. (Prelims has tested GLOF mechanism & Himalayan disaster themes; the Sikkim 2023 event is highly relevant.)

Key Points to Remember

GLOF = sudden release of water from a glacial lake when a moraine/ice dam breaches.
Triggers: avalanche/rockfall into the lake, heavy rain, dam failure, earthquakes; worsened by glacier retreat (climate change).
Nodal monitoring: ISRO/NRSC mapping + NDMA's GLOF risk programme; mitigation by lake-lowering & EWS.
Sikkim South Lhonak GLOF (Oct 2023) destroyed the Teesta-III (Chungthang) dam.

UPSC Prelims 2018 Science · Seismic waves

Q. Regarding seismic waves and the earth's interior — which waves cannot pass through the liquid outer core, and what does this prove? (Recurring Prelims theme on P/S waves and shadow zones.)

Key Points to Remember

P-waves: fastest, longitudinal, pass through solids, liquids & gases.
S-waves: transverse, travel only through solids — blocked by the liquid outer core (S-wave shadow zone).
Surface waves (L): slowest, cause the most surface damage.
Focus = source within earth; epicentre = point on surface above it.

UPSC Prelims 2017 Institutions · INCOIS / Tsunami

Q. Which Indian agency operates the Tsunami Early Warning System and what technology does it rely on? (Prelims tests nodal agencies — INCOIS, GSI, SASE, NDMA.)

Key Points to Remember

INCOIS, Hyderabad (under MoES) runs the Indian Tsunami Early Warning System (since 2007).
Relies on seismic stations + Bottom Pressure Recorders (BPRs) + tide gauges.
GSI = nodal for landslides & susceptibility mapping; SASE (DRDO) = avalanches.
BIS (IS 1893) defines India's four seismic zones (II–V).

Previous Year Questions — Mains with Model Answer Structures MAINS

How to use: Each model answer is a structured outline. Flesh out each point into 2–3 sentences in the exam. PYQs are covered up to UPSC Mains 2025.

UPSC GS3 2016 12.5 marks · 200 words

Q. "The frequency of earthquakes appears to have increased in the Indian subcontinent. However, India's preparedness for mitigation of their impact has significant gaps. Discuss various aspects."

Model Answer Structure

Intro: India's high seismic exposure — ~59% of land prone, Zones III–V; the Indian Plate's northward push into Eurasia builds Himalayan strain.
Why impact is rising: Hazard largely constant but vulnerability & exposure growing — unplanned urbanisation, non-engineered buildings, dense hill towns, "seismic gaps."
Preparedness gaps: Weak enforcement of BIS codes (IS 1893/13920), little retrofitting, low awareness, no operational EQ early warning, patchy urban DM & microzonation.
Institutional response: NDMA guidelines, National Earthquake Risk Mitigation Project, NDRF, microzonation of select cities.
Way forward: Enforce & retrofit codes, microzonation, mock drills, CBDM (Aapda Mitra), "Build Back Better."
Conclusion: Shift from response to prevention/mitigation ("left of the bang") to close the gaps.

UPSC GS1 2021 10 marks · 150 words

Q. "Differentiate the causes of landslides in the Himalayan region and the Western Ghats." (Closely related to disaster-management treatment of landslide hazard.)

Model Answer Structure

Intro: Both are India's principal landslide belts but differ in geology, climate and human pressure.
Himalaya: young, tectonically active fold mountains; steep slopes, fragile rock, seismicity, frost action, glacial melt, road/hydropower cutting.
Western Ghats: older, stable but heavy orographic monsoon rainfall, lateritic/weathered soils, deforestation, quarrying, slope construction.
Common drivers: deforestation, unscientific construction, cloudbursts intensifying with climate change.
Case links: Wayanad 2024 (Ghats); Himalayan slides along Char Dham/NH routes.
Way forward & conclusion: GSI susceptibility mapping, EWS, ESA regulation, slope stabilisation; tailor mitigation to each terrain.

Representative (not asked verbatim) 15 marks · 250 words

Q. "Recent Himalayan disasters such as the Sikkim GLOF (2023) and Joshimath subsidence (2023) highlight the limits of carrying capacity in fragile mountain ecosystems. Examine the causes and suggest measures." (Representative framing — no exact-year PYQ on these specific events; built from GS3 disaster themes.)

Model Answer Structure

Intro: The young, tectonically active Himalaya is fragile; warming + unregulated development is pushing towns past their carrying capacity.
Sikkim GLOF causes: glacier retreat enlarging South Lhonak lake, moraine-dam instability, trigger event; Teesta-III dam loss showed hydropower vulnerability.
Joshimath subsidence causes: town on old landslide debris, over-construction, poor drainage, tunnelling/NTPC project & road widening stress.
Common thread: ignoring geology & carrying capacity; climate change as a threat multiplier.
Measures — structural: lake-lowering, EWS & sirens, drainage, slope stabilisation, climate-resilient siting of dams/roads.
Measures — governance: carrying-capacity & cumulative-impact assessments, regulated construction, GSI/ISRO monitoring (InSAR), relocation where needed.
Conclusion: Development in the Himalaya must be risk-informed and ecologically calibrated — resilience over expediency.

UPSC GS3 2011 10 marks · 150 words

Q. "Bring out the causes for the formation of heat islands... " (GS3 has repeatedly tested geophysical/environmental hazards; here adapted to a representative tsunami-preparedness question.) Representative: "Discuss the generation of tsunamis and assess the effectiveness of India's tsunami early-warning system since 2004."

Model Answer Structure

Intro: Define tsunami; note the 2004 Indian Ocean event (M_w ~9.1) as the turning point.
Generation: submarine megathrust earthquakes at subduction zones; also undersea landslides, volcanic eruptions; deep-ocean speed vs coastal shoaling.
India's system: ITEWS run by INCOIS, Hyderabad (2007) — seismic + BPR + tide gauges; Regional Tsunami Service Provider.
Effectiveness: warnings within minutes, "Tsunami Ready" community certification, coastal awareness.
Gaps: last-mile dissemination, local landslide-triggered tsunamis, coastal encroachment.
Conclusion: Strong technical backbone; needs community-level preparedness and coastal-zone regulation to be fully effective.

⚡ Quick Revision

Focus = source inside earth; Epicentre = surface point above
Waves: P (fast, all media) → S (solids only) → Surface (most damage)
Magnitude = Richter / M_w; Intensity = MMI
Zones: II–V (BIS IS 1893), V = highest
GLOF: moraine-dam breach → downstream flood

📌 Must-Know Facts

~59% of India earthquake-prone
2004 Tsunami: M_w ~9.1, Sumatra megathrust
Sikkim GLOF: South Lhonak, Oct 2023
Joshimath subsidence: Jan 2023
Wayanad landslides: July 2024
Koyna: reservoir-induced quake, 1967

🏛️ Key Agencies / Bodies

GSI — landslides & susceptibility maps
INCOIS (Hyderabad) — tsunami warning (ITEWS)
SASE (DRDO) — avalanche forecasting
BIS — IS 1893 seismic zoning/codes
NDMA / NRSC-ISRO — GLOF & risk mapping

✍️ Answer Keywords

Elastic rebound · seismic gap
Microzonation & retrofitting
Carrying capacity (hill towns)
Megathrust / subduction zone
Moraine-dam breach (GLOF)
Risk-informed development