Earthquake Causes and Effects: Geography UPSC

• An earthquake is a sudden shaking of the ground caused by the abrupt release of strain energy stored in the rocks of the lithosphere; the energy spreads outward from the point of release as seismic waves.
  • It is a transient disturbance of the elastic or gravitational equilibrium of rocks, ranging from a faint tremor to motion violent enough to split the ground and bring buildings down.
  • The energy moves as wave motion through the surface layers. The usual triggers are faulting, folding, plate movement and volcanic eruption, along with human works such as dams and reservoirs.
• Earthquakes are the most visible expression of endogenic (tectonic) forces, and most are generated by movement along faults at plate boundaries; volcanic activity, isostatic adjustment and human activity add a smaller share.
• Minor tremors are recorded every few minutes somewhere on Earth, but the rare large events are among the most unpredictable and destructive natural hazards, especially in densely populated, poorly built areas.
• In India, one part of the country or another feels a shock almost every day, mostly too weak to notice. The worst-exposed tracts are the Himalaya and its foothills, the North-East, Kachchh (Kutch) and the Andaman and Nicobar Islands.

Earthquake: Basic Concepts and Measurement

Focus and Epicentre

• The focus or hypocentre is the point inside the Earth where rupture begins and energy is first released.
  • Focal depth ranges from near the surface to about 700 km; the deepest foci lie in slabs sinking into the mantle.
  • Most Himalayan earthquakes are shallow, with foci at about 10–30 km on the thrust faults beneath the range.
• The epicentre is the point on the ground surface vertically above the focus; it is reached first by the waves and usually shaken hardest.
  • Shaking generally decreases with distance from the epicentre; lines joining places of equal intensity are isoseismal lines.
• The seismograph records ground motion as a seismogram; the science of earthquakes and seismic waves is seismology.

Seismic Waves in Brief

• An earthquake sends out two groups of seismic waves: body waves (P and S), which travel through the Earth’s interior, and surface waves (Love and Rayleigh), which travel along the surface and do most of the damage.

Wave	Particle motion	Travels through	Speed and arrival	Damage
P (primary)	Push–pull along the path (longitudinal)	Solids, liquids, gases	Fastest; arrives first	Low
S (secondary)	At right angles to the path (transverse)	Solids only	About 60% of P speed; arrives second	Moderate
Love	Side to side, horizontal	Surface layers	Fastest surface wave	High
Rayleigh	Rolling, elliptical (up–down and forward)	Surface layers	Slowest; arrives last	Highest; most felt shaking

• Body waves: P waves compress and stretch the rock like sound in air; S waves shear it and cannot cross a liquid.

• Surface waves: Love waves, named after Augustus Edward Hough Love, shake the ground sideways; Rayleigh waves, named after Lord Rayleigh (John William Strutt), roll the ground like ocean swell and carry the largest amplitudes.

Measuring Earthquakes: Magnitude and Intensity

Magnitude: Richter and Moment Magnitude Scales

• Magnitude measures the energy released at the source; one earthquake has one magnitude, wherever it is recorded.
• The Richter scale was devised by Charles Francis Richter (1935), working with Beno Gutenberg, from the maximum wave amplitude on a standard seismograph.
  • It is logarithmic: each whole number means 10 times the ground-motion amplitude and about 32 times the energy.
    • A shock of 5.0 thus has ten times the amplitude of a 4.0 and releases about 31.6 times its energy.
  • It is open-ended, with no upper limit; the familiar “0 to 9” is only the range observed so far.
  • It was built for local Californian shocks and saturates above about magnitude 7, underestimating great earthquakes.
• The moment magnitude scale (Mw) of Thomas C. Hanks and Hiroo Kanamori (1979) is now the standard for large events.
  • It is based on the seismic moment: rock rigidity × area of the fault that slipped × average slip.
  • It does not saturate; the largest earthquake recorded is Chile 1960 (Mw 9.5).
  • Different scales explain why early figures for one event differ: the Bhuj earthquake of 2001 was first reported at 6.9 by one agency and 7.9–8.1 by others; its accepted size is Mw 7.7.
• Energy is concentrated in a few great earthquakes: small shocks are far more numerous, but each step of magnitude releases about 32 times more energy, so the few earthquakes above magnitude 7 release most of the world’s yearly seismic energy.
  • The Chile 1960 earthquake alone released roughly a quarter to a third of all seismic energy of the 20th century.

Intensity: Mercalli and MSK Scales

• Intensity measures the effects of shaking at a given place: what people felt, and what happened to buildings and the ground.
  • One earthquake has many intensities, falling with distance and varying with local geology and construction.
• The Mercalli scale of Giuseppe Mercalli (1902) was expanded to twelve grades and revised as the Modified Mercalli Intensity (MMI) scale by Harry O. Wood and Frank Neumann (1931).
  • It runs from I (not felt) to XII (total destruction) and is written in Roman numerals.
• India’s zoning uses the comparable Medvedev–Sponheuer–Karnik (MSK-64) scale, also of twelve grades.

Basis	Magnitude	Intensity
Measures	Energy released at the focus	Effects of shaking at a place
Values per earthquake	One	Many (maps as isoseismals)
Scales	Richter (ML), moment magnitude (Mw)	Modified Mercalli, MSK-64
Numbers	Arabic, logarithmic, open-ended	Roman, I–XII, descriptive
Depends on	Fault area and slip	Distance, depth, ground, buildings

• Indian example: the Bihar–Nepal earthquake of 1988 was of moderate magnitude, yet soft, water-saturated Gangetic alluvium amplified the shaking and raised intensities across north Bihar, damaging tens of thousands of houses.

Magnitude	Class	Typical effect
Below 4.0	Minor	Felt, rarely damaging
4.0–4.9	Light	Rattling, slight damage
5.0–5.9	Moderate	Damage to weak buildings
6.0–6.9	Strong	Destructive over tens of km
7.0–7.9	Major	Serious damage over large areas
8.0 and above	Great	Destruction over hundreds of km

Causes of Earthquakes

• An earthquake occurs whenever stress disturbs the equilibrium of crustal rocks faster than they can adjust; the disturbance may come from faulting, plate motion, magma, isostatic readjustment or human loading.
• The old idea that stable, isostatically balanced shields are immune was disproved by Koyna (1967) and Latur (1993) in the Deccan: no region is entirely free of seismic risk.
• Older explanations also invoked gaseous expansion and contraction inside the Earth; modern seismology explains almost all large earthquakes by fault slip.
• The causes usually listed fall into two sets:
  • Natural: faulting and folding, upwarping and downwarping, plate movement, volcanic eruption, gaseous expansion and contraction, and large landslides.
  • Human: deep mining, blasting of rock with dynamite for construction, deep tunnels, nuclear explosions, and the load and hydrostatic pressure of reservoirs and lakes.

Faulting and the Elastic Rebound Theory

• Proponent: Harry Fielding Reid (1910), one of the investigators of the San Francisco earthquake of 18 April 1906, published the elastic rebound theory.
• Assumptions:
  • Rocks are elastic: under stress they bend and store energy, like a stretched rubber band.
  • Two blocks on either side of a fault are driven in opposite directions by slow, continuous tectonic movement, but friction locks the fault.
• Mechanism (the earthquake cycle):
  1. Strain accumulates for decades to centuries as the locked blocks deform.
  2. When stress exceeds the frictional strength of the fault, it ruptures suddenly.
  3. The deformed blocks snap back (rebound) towards an unstrained shape in seconds, offsetting features that cross the fault.
  4. The stored energy radiates as seismic waves; aftershocks follow as the surrounding crust readjusts, and strain begins to build again.
• Evidence: survey lines across the San Andreas Fault showed bending over the decades before 1906; fences and roads were then offset by several metres along a rupture more than 400 km long.
• Indian examples: the Bihar–Nepal (1934), Assam (1950) and Bhuj (2001) earthquakes were produced by slip on faults; in the Deccan, buried rift faults beneath the basalts are linked to the Koyna and Bhatsa seismicity.
• Evaluation:
  • GPS and satellite radar (InSAR) now measure strain building across faults, which strongly supports Reid’s model.
  • Recurrence is irregular, so the model explains how but not when; stress transferred from neighbouring ruptures can bring a fault forward or delay it.
  • It does not explain deep-focus earthquakes (below about 300 km), where rocks should flow rather than break; these are attributed to mineral phase changes and dehydration in sinking slabs.

Plate Movements

• Plate tectonics is the most complete explanation of where and why earthquakes occur: most epicentres lie on plate boundaries, where plates separate, collide or slide past each other.

Boundary	Motion and stress	Focal depth	Size	Examples
Divergent (constructive)	Plates pull apart; tension, normal faults, rising magma	Shallow (mostly under 30 km)	Moderate	Mid-Atlantic Ridge, Carlsberg Ridge, East African Rift
Convergent (destructive)	Collision and subduction; compression, thrusts	Shallow to deep (to about 700 km)	Largest	Chile 1960, Sumatra 2004, Japan 2011, Nepal 2015
Transform (conservative)	Plates slide past; shear, strike-slip faults	Shallow	Strong, destructive on land	San Andreas, North Anatolian, Türkiye 2023, Myanmar 2025

• At subduction zones, foci deepen towards the overriding plate along an inclined belt, the Wadati–Benioff zone, which traces the sinking slab into the mantle.
  • The world’s great megathrust earthquakes (Mw 9 and above) all occur here.
• In the Himalaya, the Indian plate is thrust beneath the Eurasian plate along the Main Himalayan Thrust; the convergence of roughly 4–5 cm a year, partly absorbed across the range, loads the faults that produce great Himalayan earthquakes.

Intraplate Earthquakes

• Some damaging earthquakes occur far from plate boundaries, inside plates: New Madrid, USA (1811–12), Charleston, USA (1886), and in India Kachchh (1819), Koyna (1967), Latur (1993), Jabalpur (1997) and Bhuj (2001).
• Plate tectonics explains them only indirectly: compressive stress transmitted through the plate reactivates old zones of weakness, such as ancient rifts.
  • Latur (1993) and Jabalpur (1997) are attributed to this mechanism: India’s northward drift against the Eurasian plate builds stress in the interior of the plate until an old weakness gives way.
  • The Bhuj earthquake occurred on a reverse fault in the old Kachchh rift, some 300–400 km from the plate boundary.
• Rift-associated shocks of the Indian shield: Bhadrachalam (1969) in the Godavari graben and Broach (Bharuch, 1970) in the Narmada rift.
  • Beneath the Deccan basalts, buried rift structures and lineaments, such as the Kurduwadi lineament of Maharashtra, are held responsible for recurring tremors in the Deccan.

Volcanic Activity

• Vulcanicity and earthquakes act as cause and effect of each other: rising magma and expanding gases fracture the crust, and large earthquakes can open paths for magma.
• Swarms of small tremors are a leading precursor of eruptions and are monitored for warning.
• Volcanic earthquakes are generally local and of modest magnitude, their size depending on the violence of the eruption; the catastrophic exception is Krakatau (Krakatoa) (1883), whose explosion generated tsunamis that killed about 36,000 people on Java and Sumatra.

Isostatic Adjustment

• When load is added to or removed from the crust, it readjusts to regain isostatic balance, and the readjustment can trigger earthquakes.
  • Post-glacial rebound in Fennoscandia and northern Canada, where land is still rising after ice-sheet melting, is accompanied by earthquakes.
  • Rapid uplift and erosion of young fold mountains such as the Himalaya keep them isostatically unstable.

Human-Induced Earthquakes

• Human activity can trigger or induce earthquakes by changing stress or fluid pressure on faults that are already close to failure.

Reservoir-Induced Seismicity (RIS)

• Reservoir-induced seismicity is earthquake activity triggered by the impounding of large reservoirs behind dams.
• Mechanism:
  • The weight of water adds load on the rocks beneath.
  • More importantly, water seeps down fractures and raises pore-water pressure, which reduces the friction clamping a fault, so it slips more easily.
  • Seismicity often tracks the filling and draining cycle of the reservoir.
• Koyna, Maharashtra: the reservoir, Shivaji Sagar, was impounded in 1962; the M 6.3 earthquake of 11 December 1967 killed about 180 people and is the largest known reservoir-triggered earthquake.
  • Hundreds of small shocks still occur each year beneath the Koyna and Warna reservoirs; a scientific borehole about 3 km deep has been drilled there to study the fault zone.
• World examples: Marathon Dam, Greece (tremors from 1931); Lake Mead behind Hoover Dam, USA (from 1936); Kariba (Zambia–Zimbabwe), Kremasta (Greece) and Nurek (Tajikistan).
• Indian concern: seismicity near Bhatsa (Maharashtra) and the safety of high dams such as Tehri in Zone V–VI terrain.

Other Human Causes

• Deep mining: rock bursts, as in the old Kolar Gold Fields, Karnataka.
• Fluid injection and extraction: wastewater injection in Oklahoma, USA; gas extraction at Groningen, Netherlands; an enhanced geothermal project linked to the Pohang, South Korea earthquake (2017, Mw 5.5); pumping of groundwater and oil.
• Explosions: underground nuclear tests and large-scale blasting for dams, roads and tunnels.
• Induced earthquakes are usually small, but they occur in places not designed for shaking.

Classification of Earthquakes

Basis	Classes	Notes and examples
Cause: natural	Tectonic, volcanic, isostatic, plutonic	Tectonic are the most frequent and destructive (San Francisco 1906, Bhuj 2001)
Cause: human	Anthropogenic, artificial or induced	Koyna 1967, Pohang 2017
Focal depth (classical, Beno Gutenberg)	Moderate 0–50 km; intermediate 50–250 km; deep 250–700 km	Plutonic = deep-focus
Focal depth (current usage)	Shallow 0–70 km; intermediate 70–300 km; deep 300–700 km	Shallow ones cause most damage
Human casualties	Moderately hazardous (under 50,000 deaths); highly hazardous (50,000–1,00,000); most hazardous (over 1,00,000)	See table below

• Tectonic earthquakes result from slip on faults and are by far the most common and damaging.
• Volcanic earthquakes are confined to volcanic regions.
• Isostatic earthquakes follow regional imbalance, especially in active mountain belts.
• Plutonic earthquakes are the deep-focus events, with foci roughly 250–700 km down in subducting slabs.
• Shallow-focus earthquakes concentrate energy near the surface, so a shallow M 6 can do more damage than a deep M 7.

Casualty class	Deaths	Examples
Moderately hazardous	Under 50,000	Latur 1993 (about 9,700), Nepal 2015 (about 9,000), Bhuj 2001 (about 20,000)
Highly hazardous	50,000–1,00,000	Kashmir 2005 (about 86,000), Türkiye–Syria 2023 (over 59,000)
Most hazardous	Over 1,00,000	Shaanxi, China 1556 (about 8,30,000), Tangshan, China 1976 (officially about 2,42,000), Sumatra–Andaman 2004 (about 2,28,000, mostly by tsunami)

Distribution of Earthquakes

World Distribution of Earthquakes

• Earthquakes are concentrated in narrow belts, closely matching the distribution of volcanoes, in:
  • zones of young fold mountains;
  • zones of faulting and fracturing;
  • junctions of continental and oceanic crust;
  • zones of active volcanoes;
  • above all, along plate boundaries.

Belt	Share of largest earthquakes	Tectonic setting	Examples
Circum-Pacific (“Ring of Fire”)	About 81%	Subduction of Pacific-floor plates beneath the Americas, Asia and island arcs	Chile 1960, Alaska 1964, Mexico City 1985, Japan 2011, Kamchatka 2025
Alpine–Himalayan (Alpide, Mid-Continental)	About 17%	Collision of African, Arabian and Indian plates with Eurasia	Kashmir 2005, Nepal 2015, Türkiye–Syria 2023, Myanmar 2025
Mid-Atlantic and other ridges	Small share	Divergence, transform faults, fissure eruptions	Iceland, Azores
Intraplate and rift zones	Small share	Old weak zones, rifts	East African Rift, New Madrid, Latur

• The Circum-Pacific belt combines all the conditions for earthquakes: continent–ocean junctions, young fold mountains (Rockies, Andes), active volcanoes and subduction zones.
  • The Mexico City earthquake of 1985 showed how soft lake-bed sediments amplify distant shaking, killing about 10,000 people.
• The Alpine–Himalayan belt runs from the Alps and the Mediterranean through Türkiye and Iran to the Himalaya and the Myanmar arc; its earthquakes are shallow and destructive because they occur beneath populated continental land.
• Deep earthquakes (below about 100 km) occur almost only at convergent margins, where they mark the subducted plates.
• Counted by number and not by size, about 70% of all earthquakes fall in the Circum-Pacific belt and another 20% in the Mediterranean–Himalayan belt (Asia Minor, the Himalaya, parts of north-west China). The remainder lie in plate interiors and on spreading ridges.

Earthquakes in India

Seismic Regions of India

• Strong shocks concentrate along plate boundaries, above all the convergence of the Indian and Eurasian plates. India can be read as three broad belts plus the island arc:
• Himalayan region: the zone of highest seismic activity, also called the zone of maximum intensity, where the collision of the Indian and Eurasian plates continues along the Main Himalayan Thrust and its branches (Main Central Thrust, Main Boundary Thrust, Himalayan Frontal Thrust).
  • The most exposed ground lies north-east of an arc through Mussoorie, Shimla, Kangra, Dalhousie and Gulmarg, and across north Bihar, Assam, Sikkim, Arunachal Pradesh, western Nagaland and Manipur, with Bhutan and south-east Nepal beyond.
  • The range has not reached isostatic equilibrium and is still rising.
  • The belt continues through the Sulaiman–Kirthar ranges in the west and the Myanmar (Burmese) arc in the east, with the Shillong plateau and north-east India among the most active parts.
  • A central seismic gap between Uttarakhand and western Nepal has had no great earthquake for about two centuries, so strain is believed to be accumulating there.
  • The stretch between Everest and Badrinath was once read as stable because no great shock was on record there. The same silence is now read as a gap where strain is stored, and the Nepal earthquake of 2015 broke only part of it.
• Indo-Gangetic plains: the zone of comparative intensity, with moderate activity, mostly around magnitude 6–6.5, and stronger shocks along the foothills.
  • Thick soft alluvium amplifies shaking from Himalayan earthquakes, and the plains are densely populated, so moderate shocks do outsized harm.
• Peninsular shield: the zone of minimum intensity, long considered a stable block, but it carries intraplate earthquakes on old rifts and faults (Koyna, Latur, Jabalpur, Kachchh); the Narmada–Son and Kachchh rifts are the chief weak zones.
• Andaman–Nicobar arc: a subduction zone where the Indian plate sinks beneath the Burma microplate; source of the 2004 earthquake.
• A finer division gives seven seismic regions. Counts of shocks above magnitude 5 from 1897 to 2014 show how uneven the hazard is, with the 5.0–5.9 column approximate:

Seismic region	5.0–5.9	6.0–6.9	7.0–7.9	8.0+	Average return period
Kashmir and Western Himalaya	25	8	2	2	2.5–3 years
Central Himalaya	68	28	4	1	1 year
North-East India	200	128	15	4	Under 4 months
Indo-Gangetic basin and Rajasthan	14	6	–	–	5 years
Khambhat and Rann of Kachchh	4	4	1	1	12–20 years
Peninsular India, with Lakshadweep	31	11	–	–	2.5–3 years
Andaman and Nicobar Islands	90	80	1	1	Under 8 months

Year	Earthquake	Magnitude	Significance
1819	Rann of Kachchh	About Mw 7.7–8.2	Raised the Allah Bund scarp; Sindri fort sank
1897	Shillong (Great Assam)	Mw 8.0	Surface faulting, liquefaction across Assam
1905	Kangra	About 7.8	Over 20,000 deaths
1934	Bihar–Nepal	Mw 8.0	About 10,000 deaths; widespread liquefaction
1950	Assam–Tibet	Mw 8.6	Largest in India; landslide-dammed rivers
1967	Koyna	M 6.3	Reservoir-triggered, about 180 deaths
1991	Uttarkashi	Mw 6.8	About 770 deaths
1993	Latur	Mw 6.2	About 9,700 deaths in a “stable” shield
1999	Chamoli	Mw 6.6–6.8	About 100 deaths
2001	Bhuj	Mw 7.7	About 20,000 deaths; intraplate
2004	Sumatra–Andaman	Mw 9.1–9.3	Tsunami on Tamil Nadu and Andaman coasts
2005	Kashmir	Mw 7.6	About 86,000 deaths, mostly in Pakistan-occupied Kashmir and Pakistan
2011	Sikkim	Mw 6.9	Landslides, damage to Teesta hydro projects
2016	Imphal, Manipur	Mw 6.7	North-east arc

• The Calcutta disaster of 11 October 1737, long listed as India’s deadliest earthquake with 3,00,000 dead, is now regarded as a severe cyclone and storm surge.
  • Contemporary records give about 3,000 deaths in a town of roughly 20,000, and there is no reliable evidence of a major earthquake (Roger Bilham, 1994).
• The national record: a Rajya Sabha reply of 7 August 2025 gave 80,029 earthquakes from 1 January 2020 to 30 June 2025 in the grid 0–40° N, 60–100° E, of which 3,032 lay within the country, including the Bay of Bengal and the Andaman Sea.
  • The National Seismological Network grew from about 150 stations in 2020 to 168 by that reply and 177 by August 2026. In that month it located 424 earthquakes, 268 of them in India and its neighbourhood.
• Recent shocks of magnitude 5 or more in India, from the monthly reports of the National Center for Seismology:
  • Udalguri, Assam: M 5.8, 14 September 2025.
  • Morigaon, Assam: M 5.1, 5 January 2026.
  • Kamjong, Manipur: M 5.2, 21 April 2026.
  • Chamba, Himachal Pradesh: M 5.0, 5 June 2026.
  • Upper Siang, Arunachal Pradesh: M 5.4, 5 August 2026.
• Beyond the border in 2026: Myanmar (M 6.0, 3 February), Bhutan (M 5.8, 7 June) and Afghanistan (M 6.2, 27 June).
  • A magnitude 7.0 shock struck Baqen County in Nagqu, Tibet, on 22 October 2026. Local authorities reported 16 deaths on 23 October.

Seismic Zoning of India

• The Bureau of Indian Standards (BIS) zones the country in its earthquake design code, IS 1893 (Part 1), first issued in 1962, which sets how strongly buildings must resist shaking in each zone.
• Older five-zone map: based on Modified Mercalli intensity, it ran from Zone I (V or below) through Zone II (VI), Zone III (VII) and Zone IV (VIII) to Zone V (IX and above).
• Older seismic coefficient map: horizontal seismic coefficient isolines ran from 0.01 to 0.08.
  • Low (0.01–0.02): extensive parts of the Peninsula, Madhya Pradesh and Rajasthan.
  • Medium (0.04–0.05): the Satluj–Ganga plain, the Godavari basin, Ratnagiri and Raigad, and parts of Gujarat.
  • High: Jammu and Kashmir, Himachal Pradesh, Uttarakhand, north Bihar, the North-East, Kachchh and the Andaman and Nicobar Islands.
• 2016 code (in force): four zones, II, III, IV and V; the old Zone I was merged into Zone II in 2002.
  • About 59% of the land area was classed as prone to moderate or higher shaking.
• 2025 revision and its withdrawal: BIS notified IS 1893 (Part 1): 2025 on 6 November 2025. It was based on Probabilistic Seismic Hazard Assessment (PSHA), which models active faults and their rupture potential instead of relying mainly on past epicentres.
  • It added a new Zone VI (highest hazard) for the entire Himalayan arc and the Andaman–Nicobar Islands.
  • BIS withdrew the notification by a gazette notice of 3 March 2026, so the 2016 map with Zones II–V remains in force. The central Himalayan seismic gap, which drove much of the revision, is a scientific finding that stands apart from the code.
  • The case against the code: the Ministry of Housing and Urban Affairs raised technical and financial concerns in February 2026. Developers said changing standards in mid-project was unworkable and put the extra cost at about 20%, with 30–50% cited for metro and public infrastructure in the big cities. Metro corporations were reported as not consulted.
  • The case for it: seismologist C. P. Rajendran argued in March 2026 that the code was a real scientific advance and that fragile regions are left on weaker standards. He put about 61% of the land and 75% of the population in tracts prone to moderate or strong tremors.
  • The government directed that stakeholder consultations be completed before any further step.

Zone	Hazard (MSK intensity)	Zone factor Z	Typical areas
II	Low (VI or less)	0.10	Much of the peninsular interior
III	Moderate (VII)	0.16	Keralam (Kerala), Goa, parts of peninsular and central India
IV	High (VIII)	0.24	Delhi, Indo-Gangetic plains near the Himalaya, parts of Himachal and Uttarakhand
V	Very high (IX and above)	0.36	North-east, Kashmir, parts of Himachal and Uttarakhand, Rann of Kachchh, north Bihar, Andaman–Nicobar

• By State: the whole Himalaya, the North-East, western and northern Punjab, Haryana, Uttar Pradesh, Delhi and parts of Gujarat fall in Zones IV and V. The rest of the northern plains and the western coastal tract are at moderate risk, and much of the Peninsula at low risk.
• Damage to buildings is considerable in Zones III to V. Zone II may see minor damage to rural buildings, but collapse is not expected.

Effects and Hazards of Earthquakes

• An earthquake becomes a disaster only where it strikes people and structures; the toll depends less on magnitude than on population density, construction quality, ground conditions and time of day.
  • The Kangra earthquake of 1905 (about 7.8) killed some 20,000 in the sparsely peopled hills; the Tangshan earthquake of 1976 (about 7.6) killed around 2,42,000 in a crowded industrial city.
  • Most deaths come from collapsing buildings, not from the shaking itself.
• Seismic risk to a structure is often written as Hazard × Exposure × Vulnerability × Location:
  • Hazard: a shock strong enough to harm the weakest structures.
  • Exposure: what stands in its path, such as buildings, bridges, dams, power plants and lifelines.
  • Vulnerability: how easily that stock is damaged, with weak construction at most risk.
  • Location: how far the stock lies from the source.

Primary effects	Secondary effects
Ground shaking, surface faulting	Landslides, avalanches
Uplift and subsidence	Liquefaction, ground failure
Collapse of buildings and infrastructure	Floods from dammed rivers or failed dams
Loss of life and injury	Tsunamis, fires, disease, economic loss

Damage to Buildings, Towns and Cities

• Ground shaking collapses buildings, bridges, dams, pipelines and power lines, cutting water, power, transport and communication.
  • Bhuj, 26 January 2001: the towns of Bhuj, Bhachau, Anjar and Gandhidham, and Ratnal village, were laid waste.
• Why buildings fail: the foundation moves with the ground while inertia holds the upper floors back for an instant.
  • That lag sets up differential stress, so walls crack and tear apart and roofs part from their supports. Near the epicentre vertical movement adds to it.
  • The toll also turns on the intensity, distance from the epicentre, soil, design and quality of construction.
• Material and shape: reinforced concrete suffers least, while adobe and unreinforced brick suffer most, and projecting cornices, balconies, towers and arcades add to the risk.
  • Square or rectangular plans perform better than L, T, U or H shapes, and lighter roofs are safer in prone areas.
• Soft ground amplifies shaking: alluvium, lake beds, reclaimed swamps and filled land shake longer and harder than bedrock.
  • Mexico City (1985) and north Bihar (1934, 1988) are classic cases.
• Codes and their enforcement decide the toll: the collapse of a new hospital in the San Fernando earthquake (1971) led to stricter codes in California.
• Case study: Nepal (Gorkha), 25 April 2015: Mw 7.8, focus about 8.2 km deep on the Main Himalayan Thrust.
  • It killed nearly 9,000 people and destroyed over half a million houses. Early counts gave about 8,000 deaths.
  • Damage and losses were about US$7 billion, against an early estimate of US$10 billion; Kathmandu’s unplanned, poorly built urban fringe and heritage monuments suffered most.
• Case study: Türkiye–Syria, 6 February 2023: an Mw 7.8 shock on the East Anatolian Fault, followed nine hours later by a second shock of about Mw 7.5–7.7.
  • Over 59,000 people died, largely because of poor enforcement of building codes.
• Case study: Myanmar, 28 March 2025: an Mw 7.7 strike-slip rupture of about 500 km along the Sagaing Fault, one of the longest continental ruptures recorded.
  • Part of it moved at supershear speed, faster than S waves travel.
  • About 5,400 people died, mostly around Mandalay and Nay Pyi Taw, and an unfinished tower collapsed in Bangkok, about 1,000 km away.
• Transport and services: surface structures such as bridges respond to the vibration, while buried lines such as oil pipelines respond to strain in the surrounding ground.
  • Highways, railways, airports, water, sewage, fuel, power and telecommunications can all fail together.
  • Jammed cars, fires, fallen poles and debris block the roads that relief needs.

Ground Deformation and Liquefaction

• Uplift, subsidence and surface faulting permanently change the land.
  • Kachchh 1819: the Allah Bund ridge rose while nearby land sank under the sea.
  • Assam 1897: fault scarps several metres high broke the surface.
  • Alaska 1964: coasts were raised by up to about 11 m.
  • Andaman 2004: North Andaman rose and parts of the Nicobars subsided.
• Liquefaction: shaking makes water-saturated sand behave like a liquid, so buildings tilt or sink and sand boils erupt.
  • It was widespread in the Bihar plains (1934) and the Rann of Kachchh (2001).
  • 1934: a belt of slumping about 320 km long, from Bettiah to Purnea, surrounded the epicentral tract. Buildings tilted or sank, and fissures threw up sand and water that left the land unfit for cultivation.
  • The soft, high water-table plains of Uttar Pradesh and Bihar favour it, and it matters most for dams, bridges, buried pipelines and buildings near rivers, coasts and lakes.
• New lakes and marshes: subsidence of the Mississippi flood plain in the New Madrid earthquakes (1811–12) created lakes and swamps, the largest being Reelfoot Lake, Tennessee.

Landslides and Avalanches

• In mountains, shaking destabilises slopes and triggers landslides and debris flows, blocking roads and burying settlements.
  • Dry rock and earth slide as earth avalanches over long distances, while wet soil slumps. Cracks and fissures left by a shock prepare later failures.
  • Uttarkashi 1991: the shock cracked Varunavat Parvat, and those cracks fed the slides that hit Uttarkashi town in 2003.
  • Peru 1970: ice and rock from Huascarán fell as a debris avalanche that buried the town of Yungay.
  • Nepal 2015: avalanches struck Everest base camp and destroyed the Langtang valley village.
  • Sikkim 2011: landslides blocked highways and damaged the Teesta hydro projects, including works at Singik.

Floods

• Landslides and displaced rock can dam rivers, flooding the valley above, and the natural dams later burst as flash floods; dams and embankments may also crack and fail.
  • Assam 1950: landslides blocked the Dihang (Siang) and Subansiri; their bursting caused devastating floods downstream and shifted channels.

Tsunamis

• Large, shallow, undersea earthquakes (generally above about magnitude 7) with vertical displacement of the sea floor generate tsunamis; these are called tsunamigenic earthquakes.
  • Sumatra–Andaman, 26 December 2004: an Mw 9.1–9.3 megathrust rupture where the Indian plate sinks beneath the Burma microplate killed about 2,28,000 people across the Indian Ocean, against early counts of about 2.4 lakh, including over 10,000 in India (Tamil Nadu, Andaman and Nicobar).
  • Tohoku, Japan, 11 March 2011: an Mw 9.0–9.1 earthquake raised waves with run-up of up to about 40 m, left nearly 20,000 dead or missing, and disabled cooling at the Fukushima Daiichi nuclear plant, causing meltdowns and radioactive release.
    • The wave at the plant, put at 10 m in early accounts, was about 14–15 m. It destroyed the emergency generators.
  • Lisbon (1755) is the classic historical case.

Fires and Other Secondary Effects

• Fires start from broken gas lines, stoves, short-circuits, live wires and damaged furnaces, and spread because broken mains leave no water.
  • San Francisco 1906 and Tokyo–Yokohama (Kanto) 1923 lost more to fire than to shaking; the Kanto earthquake killed about 1,05,000 people.
  • The İzmit earthquake of 1999 in Türkiye set a major oil refinery ablaze.
• Other effects:
  • injury and disability, with deaths from heart attack, and epidemics from polluted water and broken sewers;
  • changes in springs and groundwater, with sudden bursts of water where springs normally appear;
  • psychological trauma;
  • long-term economic loss across farming, industry, trade, transport and services, which can feed civic unrest.

Earthquake Management

• Earthquake management organises resources and responsibilities for the humanitarian side of an emergency, from pre-disaster risk reduction to post-disaster recovery, with the aim of limiting loss of life and property.

Risk Recognition and Hazard Mapping

• The first step is to identify where and how strongly the ground is likely to shake: national seismic zoning and city-level microzonation, which maps soil amplification and liquefaction risk for planning.
  • Reports for Chennai, Coimbatore, Bhubaneswar and Mangalore were released in July 2025, and Patna, Varanasi, Lucknow, Kanpur, Meerut, Agra, Dhanbad and Amritsar were in an advanced stage at that date.
• Land-use planning keeps critical facilities off active faults, soft fills and unstable slopes.

Monitoring, Prediction and Early Warning

• India’s seismic network is run by the National Center for Seismology (NCS) under the Ministry of Earth Sciences, which reports every significant earthquake in near-real time.
• Prediction (stating time, place and magnitude) is not yet possible.
  • Suggested precursors: foreshocks, changes in ground tilt, well levels, radon emission, the P- to S-wave velocity ratio, and unusual animal behaviour.
  • Haicheng, China (1975) was evacuated after foreshocks, but Tangshan (1976) struck with no warning.
  • A small shock near Bhuj in December 2000 went unnoticed before the 2001 earthquake.
• Forecasting instead gives probabilities over decades from fault slip rates, seismic gaps and past recurrence; this underpins probabilistic hazard maps.

• Early warning systems detect the fast, weak P waves and send alerts before the slower, destructive S and surface waves arrive.
  • The warning time is a few seconds to about a minute: enough to stop trains, shut gas valves and take cover.
  • Japan: a nationwide system operates, using electronic signals that travel faster than the seismic waves.
  • Uttarakhand: the Bhookamp Alert app (2021) with IIT Roorkee.
  • All India: smartphone-based alerts are issued in partnership with NDMA and NCS.

Structural and Non-Structural Measures

• Earthquake-resistant construction is the most effective safeguard. Past events suggest that over 95% of lives lost came from the collapse of buildings that were not earthquake-resistant.
  • Codes: IS 1893 (design forces), IS 4326 (earthquake-resistant construction) and IS 13920 (ductile detailing of reinforced concrete).
  • Measures: retrofitting of schools, hospitals and lifeline buildings, and base isolation, used in the district hospital rebuilt at Bhuj after 2001.
  • Cost remains the chief obstacle in India, so structures are prioritised by importance and zone, using a hazard map for each zone.
• Traditional Himalayan building styles show proven resilience: Kashmir’s timber-laced taq and dhajji dewari houses and the light Assam-type houses of the north-east.
• Non-structural measures:
  • public awareness and drills;
  • NDMA guidelines on earthquake management, under the Disaster Management Act, 2005, amended with effect from 9 April 2025;
  • the National Disaster Response Force (NDRF) for search and rescue;
  • insurance;
  • the Sendai Framework (2015–2030), which shifts the emphasis from relief to reducing risk.

Safety Rules

Phase	At home and at work	In the open or on the road
Before	Fix gas appliances and heavy shelves; store water, food, a torch and first aid; learn to shut the mains; vacate dilapidated buildings; hold drills	Know the nearest first-aid post and fire station; train in first aid and rescue
During	Stay calm; take cover under a desk or table or against an inner wall, away from glass; use no flame	Move clear of buildings and wires and stay in the open; in a car, stop and stay inside
After	Expect aftershocks; shut water, gas and power; use a torch, not matches; stay out of damaged buildings; check water before drinking	Keep off beaches and low river banks; keep streets clear for rescue vehicles