Earthquakes and Tsunamis: Terminology for UPSC Geography Optional

Earthquakes and tsunamis are the fastest geomorphic events on Earth: in seconds they raise or drown coasts, trigger landslides and liquefy plains, and within hours they send waves across an ocean. This post covers the vocabulary of earthquakes as events — their anatomy, measurement, geography and hazard — together with India’s seismic zonation and tsunami warning system.

Each entry opens with an exam-ready definition followed by mechanism, dated examples and a sketch line. The 2019 paper asked how magnitude and intensity are measured and how seismic zones are demarcated; a Paper II question asks candidates to demarcate India’s seismic zones and propose interventions for the most sensitive of them.

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Earthquake Anatomy and Mechanism

Earthquake

An earthquake is a sudden shaking of the ground caused by the abrupt release of elastic strain energy stored in the Earth’s lithosphere, usually by rapid slip on a fault, which radiates seismic waves outward from the rupture; most earthquakes are tectonic, but volcanic activity, cavity collapse and human activities also cause them.

• Types by cause: tectonic (at plate boundaries and within plates), volcanic, collapse (mines, caverns), induced (reservoirs, mining, fluid injection) and explosion-generated.
• Mechanism: stress from plate motion accumulates until it exceeds the frictional strength of a fault; the fault slips and energy travels as body and surface waves.
• Examples: plate-boundary — the Türkiye–Syria doublet of 6 February 2023 (Mw 7.8 and 7.5) on the East Anatolian Fault Zone, which killed more than 50,000 people; intraplate — the Latur (Killari) earthquake of 30 September 1993 (Mw 6.2) in the “stable” Deccan, which killed nearly 10,000.
• Geomorphic significance: earthquakes create fault scarps, trigger landslides, divert rivers, raise or drown coasts and liquefy alluvium.
• Don’t confuse with: the waves themselves — the earthquake is the rupture; seismic waves are its messengers.

Focus (Hypocentre) and Epicentre

The focus (hypocentre) is the point within the Earth where fault rupture begins and seismic waves are first released; the epicentre is the point on the ground surface vertically above the focus, where shaking is usually — though not always — strongest.

• Key features: focal depth is the distance between focus and epicentre; a great earthquake ruptures a whole fault plane, so the focus is only its starting point — in 2004 the rupture began off northern Sumatra and ran about 1,300 km north to the Andaman Islands.
• Location: the delay between P- and S-wave arrivals at a station gives its distance from the event; three or more stations fix the epicentre by triangulation.
• Examples: the 2025 Myanmar earthquake began about 10 km deep near the Sagaing–Mandalay border; the 2015 Gorkha earthquake’s epicentre lay some 80 km north-west of Kathmandu, yet the rupture ran east beneath the city, where damage was heaviest.
• Sketch: section with focus below, epicentre above, wave fronts and three recording stations.
• Don’t confuse with: the place that feels shaking first or worst — the epicentre is a geometric point, and intensity may peak elsewhere on soft ground.

Elastic Rebound Theory

The elastic rebound theory, proposed by Harry Fielding Reid in 1910 after the 1906 San Francisco earthquake, holds that rocks on either side of a locked fault bend elastically as plates move, storing strain until their strength is exceeded; the fault then slips suddenly and the rocks spring back towards an unstrained state, releasing energy as seismic waves.

• Evidence: survey lines across the San Andreas Fault had been bent before 1906 and were offset by up to about 6 m during the earthquake.
• Earthquake cycle: slow interseismic loading → sudden coseismic slip → postseismic relaxation, repeated as stick-slip behaviour.
• Examples: the Main Himalayan Thrust is locked beneath the Lesser Himalaya and loads at about 2 cm a year, released in great earthquakes such as 1934 Bihar–Nepal and 2015 Gorkha (Himalayan thrust system); GPS and satellite radar now measure such loading directly.
• Significance: the basis of seismic-gap and recurrence estimates, although real recurrence intervals are irregular.
• Sketch: three panels — a straight road across a fault, the road bent by strain, the road offset after rupture.

Foreshock, Mainshock and Aftershock (Omori’s Law)

A mainshock is the largest earthquake in a sequence; foreshocks are smaller events that precede it in the same area, and aftershocks are the numerous smaller events that follow as the crust readjusts; Omori’s law states that aftershock frequency decays roughly in inverse proportion to the time elapsed since the mainshock.

• Omori’s law: Fusakichi Omori (1894) proposed n(t) = K / (c + t); Tokuji Utsu (1961) generalised it to n(t) = K / (c + t)^p, with p close to 1.
• Båth’s law: the largest aftershock is typically about 1.2 magnitude units smaller than the mainshock (Markus Båth, 1965).
• Examples: an M 7.2 foreshock struck two days before the Mw 9.0–9.1 Tōhoku mainshock of 11 March 2011; the Mw 7.3 shock of 12 May 2015 was a major aftershock of the Gorkha earthquake; the Türkiye sequence of 2023 was a doublet, its Mw 7.5 event following the Mw 7.8 mainshock by about nine hours.
• Significance: foreshocks are recognisable only in hindsight; aftershocks bring down weakened buildings and govern rescue and relief planning.

Shallow-, Intermediate- and Deep-Focus Earthquakes

Earthquakes are classed by focal depth as shallow-focus (0–70 km), intermediate-focus (70–300 km) and deep-focus (300–700 km); shallow events are by far the most numerous and damaging, while intermediate and deep events occur almost exclusively within slabs descending at subduction zones.

• Distribution: ridges, transforms and collision zones produce mainly shallow earthquakes; subduction zones produce all three along the Wadati–Benioff zone.
• Examples: shallow — Bhuj, 26 January 2001 (Mw 7.7) and Myanmar, 28 March 2025 (Mw 7.7, about 10 km); intermediate — Hindu Kush, 26 October 2015 (Mw 7.5, about 210 km deep), felt across north India; deep — Bolivia, 9 June 1994 (Mw 8.2, about 630 km).
• Mechanism puzzle: rock below about 70 km should flow rather than fracture; deep earthquakes are explained by dehydration of slab minerals or sudden mineral phase changes that allow faulting.
• Significance: deeper events are felt over wider areas but shake the surface less violently.

Measuring Earthquakes

Seismograph and Seismogram

A seismograph is an instrument that detects and records ground motion by means of a sensor, the seismometer — traditionally a damped pendulum or a mass on a spring, today an electronic broadband sensor; the time-series record it produces, showing ground motion against time, is a seismogram.

• History: Zhang Heng’s seismoscope (132 CE) indicated only the direction of shaking; John Milne‘s horizontal-pendulum seismograph (1880s, Japan) made global recording possible; the Wood–Anderson torsion seismometer was used to define the Richter scale.
• Reading a seismogram: P-waves arrive first, S-waves next and surface waves last; the S–P interval gives distance, amplitude gives magnitude, and first-motion directions reveal the type of faulting.
• India: the National Center for Seismology under the Ministry of Earth Sciences operates the national network and reports magnitudes.
• Sketch: a seismogram trace with P, S and surface-wave arrivals labelled and the S–P interval marked.

Magnitude — Richter Scale and Moment Magnitude

Magnitude is a single instrumental number expressing the size of an earthquake at its origin; the Richter (local) scale uses the logarithm of the maximum amplitude on a standard seismograph, while the moment magnitude (Mw) scale is calculated from the seismic moment — rock rigidity × fault area × average slip — and does not saturate for great earthquakes.

• Richter scale: devised by Charles Francis Richter (1935) with Beno Gutenberg for southern California, using a Wood–Anderson instrument and a reference distance of 100 km. It is logarithmic: each whole number is a tenfold increase in amplitude and roughly 32 times more energy, so a magnitude 7 releases about 1,000 times the energy of a magnitude 5.
• Limitations: the local scale saturates above about magnitude 6.5–7 and the surface-wave scale near 8, so both understate great earthquakes.
• Moment magnitude: Hiroo Kanamori (1977) related earthquake energy to seismic moment, and Thomas C. Hanks and Hiroo Kanamori (1979) defined Mw = (2/3) log10 M0 − 6.07, with M0 in newton-metres. It is now the standard magnitude reported worldwide, though the media still say “Richter”.
• Key features: one earthquake has one magnitude; the scale is open-ended, but fault size limits real events to about 9.5.
• Examples: Valdivia, Chile, 22 May 1960 — Mw 9.5, the largest recorded; Sumatra–Andaman 2004 — Mw 9.1; Türkiye 2023 — Mw 7.8; Myanmar 2025 — Mw 7.7.
• Sketch: a ladder of magnitudes 5 to 9 with “×32 energy” marked between steps.

Intensity — Modified Mercalli and MSK-64 Scales

Intensity measures how strongly an earthquake is felt and how much damage it causes at a particular place, graded from observed effects on people, buildings and the ground rather than from instruments; the Modified Mercalli (MM) and MSK-64 scales both use twelve degrees, from I (not felt) to XII (total destruction).

• Modified Mercalli: Giuseppe Mercalli (1902) proposed a ten-degree scale, later expanded to twelve; Harry Oscar Wood and Frank Neumann (1931) adapted it to American building types.
• MSK-64: devised by Sergei Medvedev, Wilhelm Sponheuer and Vít Kárník (1964); it grades damage by building type and proportion damaged, was used for India’s zonation, and was succeeded in Europe by the European Macroseismic Scale of 1998.
• Controls on intensity: magnitude, focal depth, distance, local geology (soft sediment amplifies shaking), duration and construction quality.
• Examples: the 1905 Kangra earthquake reached intensity X around Kangra and Dharamshala but also VIII at distant Dehradun; the 2025 Myanmar earthquake brought down a 33-storey tower under construction in Bangkok, about 1,000 km away, on soft clay.

Isoseismal Line

An isoseismal line is a line on a map joining places that experienced the same intensity of shaking in a given earthquake; isoseismals enclose the zone of maximum damage, the meizoseismal area, and usually form concentric but elongated loops around the epicentre.

• Construction: field damage surveys and questionnaires; today supplemented by online “felt” reports collected by seismological agencies.
• Key features: loops elongate along the strike of the causative fault; bulges and islands of high intensity mark soft alluvium or ridge-top amplification; close spacing means rapid attenuation.
• Examples: Charles Stewart Middlemiss‘s isoseismal map of the 1905 Kangra earthquake, with an outlying high-intensity patch around Dehradun; the meizoseismal “slump belt” of north Bihar in the 1934 Bihar–Nepal earthquake.
• Significance: estimates the epicentre and size of pre-instrumental earthquakes and feeds historical data into zonation maps.
• Sketch: elongated concentric loops labelled X, IX, VIII, VII around an epicentre, with a detached VIII patch over alluvium.

Where and Why Earthquakes Strike

World Earthquake Belts

The world earthquake belts are the narrow zones in which most of the Earth’s earthquakes are concentrated, all following plate boundaries: the circum-Pacific belt, the Alpine–Himalayan (Alpide) belt and the mid-oceanic ridge belt, with a small share of earthquakes scattered through plate interiors.

• Circum-Pacific belt: subduction zones from Chile to Alaska, Japan and Indonesia, producing about 81 per cent of the world’s largest earthquakes; it coincides with the volcanic Ring of Fire.
• Alpide belt: from the Mediterranean through Türkiye, Iran, the Himalaya and Myanmar to Sumatra, producing about 17 per cent of the largest earthquakes through collision and associated faulting.
• Mid-oceanic ridges: frequent shallow, moderate earthquakes, mostly far offshore, as along the Mid-Atlantic and Carlsberg ridges.
• Intraplate events: Latur (1993), Jabalpur (22 May 1997, Mw 5.8) and New Madrid, USA (1811–12) show that plate interiors are not immune.
• Link: the tectonic logic of each belt is set out under plate margins.

Megathrust Earthquake

A megathrust earthquake is a very large earthquake on the gently dipping interface between a subducting plate and the overriding plate — the megathrust — where rupture over hundreds of kilometres can reach magnitude 9 or more and suddenly lift the sea floor, generating ocean-wide tsunamis.

• Mechanism: the locked interface drags the edge of the overriding plate down and compresses it; at rupture the edge rebounds upward and seaward, displacing the water column.
• Examples: Valdivia, Chile, 1960 (Mw 9.5); Alaska, 27 March 1964 (Mw 9.2); Sumatra–Andaman, 26 December 2004 (Mw 9.1, rupture about 1,300 km long); Tōhoku, Japan, 11 March 2011 (Mw 9.0–9.1); Cascadia, 26 January 1700, dated from the tsunami it sent to Japan.
• Continental analogue: the Main Himalayan Thrust generates great thrust earthquakes such as 1934 Bihar–Nepal, but on land and without tsunamis.
• Indian exposure: the Andaman–Sunda subduction zone in the Bay of Bengal and the Makran subduction zone off Pakistan and Iran, whose 1945 earthquake sent a tsunami to the Gujarat and Mumbai coasts.

Seismic Gap

A seismic gap is a segment of an active fault or plate boundary that has not produced a large earthquake for an unusually long time compared with neighbouring segments, and is therefore thought to have accumulated enough strain to be a likely site of a future great earthquake.

• Basis: elastic rebound — if slip on neighbouring segments has relieved stress, the unbroken segment is the one still loaded.
• Examples: the central Himalayan gap between the ruptures of the 1905 Kangra and 1934 Bihar–Nepal earthquakes, covering Garhwal–Kumaun and far-western Nepal, where Roger Bilham has argued enough slip deficit exists for an earthquake of magnitude 8 or more; the 2015 Gorkha earthquake released only part of the strain stored beneath Nepal.
• Limitations: forecasting from gaps has a mixed record — the 2004 rupture broke a segment not generally regarded as a gap — so gaps indicate hazard, not timing.
• Significance: guides preparedness priorities in Uttarakhand and Himachal Pradesh.

Reservoir-Induced Seismicity (Koyna)

Reservoir-induced (reservoir-triggered) seismicity is earthquake activity caused or triggered by the filling of large artificial reservoirs, when the weight of impounded water and the diffusion of raised pore-water pressure into faults beneath reduce the friction holding already-stressed faults, allowing them to slip.

• Mechanism: an immediate effect of loading and a delayed effect of pore-pressure diffusion; seismicity rises and falls with the annual filling and drawdown of the reservoir; only faults already close to failure are triggered.
• Koyna: after the Koyna dam’s Shivajisagar reservoir in Maharashtra was impounded in 1962, tremors began; the M 6.3 earthquake of 10 December 1967 killed about 180 people and remains the largest known reservoir-triggered earthquake, and seismicity continues with the loading cycle.
• Other examples: Kariba (Zambia–Zimbabwe), Kremasta (Greece) and Xinfengjiang (China); scientists still debate whether the Zipingpu reservoir contributed to the 2008 Wenchuan earthquake.
• Related: induced seismicity from fluid injection (Oklahoma in the 2010s) and rockbursts in deep mines such as the Kolar Gold Fields.
• Significance: siting and monitoring of large dams such as Tehri in the Himalaya.

Earthquake Swarm

An earthquake swarm is a sequence of many earthquakes clustered in space and time without a single, clearly dominant mainshock; activity waxes and wanes over days to years and is usually driven by migrating fluids, magma movement or slow fault slip rather than by one large rupture.

• Key features: similar magnitudes throughout; no Omori-type decay; often shallow.
• Examples: Palghar (Dahanu), Maharashtra — a long-lived swarm of small tremors, the largest around magnitude 4, that began in November 2018 in the stable Deccan region; the Reykjanes Peninsula of Iceland, where swarms tracked magma intrusion before the Grindavík-area eruptions from December 2023.
• Significance: most swarms end quietly, but some precede larger events — the L’Aquila earthquake of 6 April 2009 (Mw 6.3) followed months of swarm activity — which makes public communication difficult.
• Don’t confuse with: an aftershock sequence, which follows a clear mainshock and decays predictably.

Coseismic Uplift and Subsidence (Allah Bund)

Coseismic uplift and subsidence are permanent vertical movements of the ground surface produced at the moment of a large earthquake, when the crust above a slipping fault is raised or dropped by up to several metres, creating new scarps, raised shorelines, dammed channels and drowned land.

• Examples: the Rann of Kachchh earthquake of 16 June 1819 raised the Allah Bund, a low ridge tens of kilometres long across a former Indus distributary, while nearby Sindri fort subsided beneath a new lake; in 2004 parts of the northern Andaman coast rose, exposing coral reefs, while Car Nicobar and Indira Point on Great Nicobar subsided; in 1964 Alaska’s Montague Island rose by about 11 m.
• Mechanism: elastic rebound on dipping faults lifts the hanging-wall side and lowers areas farther away, following the pattern of slip at depth.
• Significance: repeated coseismic uplift builds raised beaches and marine terraces; dated uplifted shorelines and dead corals reconstruct prehistoric earthquakes.

Liquefaction

Liquefaction is the temporary loss of strength of loose, water-saturated sand or silt during strong shaking, when cyclic loading raises pore-water pressure until grains lose contact with one another and the soil behaves like a liquid; buildings tilt or sink, slopes spread laterally and sand boils erupt at the surface.

• Conditions: young, loose granular sediment; a shallow water table; strong, prolonged shaking, generally from earthquakes above about magnitude 5.5.
• Landforms: sand boils (sand blows), craters, ground fissures and lateral spreads.
• Examples: Niigata, Japan, 1964, where apartment blocks tilted over intact; Bhuj, 2001, with sand boils across the Rann and Banni and damage at Kandla port; the 1934 Bihar–Nepal earthquake, which fissured and slumped the north Bihar plains; the 2025 Myanmar earthquake, which produced sand blows along the Sagaing Fault.
• Significance: delta cities, the Indo-Gangetic plain and reclaimed land are highly susceptible; ancient liquefaction features help date prehistoric earthquakes.

Zonation, Hazard and Tsunamis

Seismic Zonation of India (BIS IS 1893)

Seismic zonation of India is the division of the country into zones of expected earthquake shaking by the Bureau of Indian Standards in IS 1893 (Part 1), the earthquake-resistant design code; the map in force, from the 2016 edition, has four zones — II, III, IV and V — each carrying a zone factor that engineers must use in design.

• Basis of demarcation: the historical earthquake catalogue; the maximum intensity observed or expected on the MSK-64 scale (Zone V: IX and above; IV: VIII; III: VII; II: VI or less); mapped tectonic features and active faults; and regional geology.
• Evolution: the first map appeared in 1962; after Koyna (1967) the country was divided into five zones, I to V; after Latur (1993) and Jabalpur (1997), the 2002 revision merged Zone I into Zone II, leaving four zones, retained in 2016 with zone factors of 0.10, 0.16, 0.24 and 0.36 for Zones II to V.
• Present zones: Zone V (about 11 per cent of the land) — all of North-East India, parts of Jammu & Kashmir, Ladakh, Himachal Pradesh and Uttarakhand, the Rann of Kachchh, north Bihar and the Andaman and Nicobar Islands; Zone IV (about 18 per cent) — Delhi, the rest of the Himalayan states, parts of the Indo-Gangetic plain and the Koyna region; Zone III (about 30 per cent) — including Mumbai, Chennai and Kolkata; Zone II (about 41 per cent) — the stable interior.
• The 2025 revision and its withdrawal: IS 1893 (Part 1):2025, issued by BIS in November 2025, used probabilistic seismic hazard assessment, created a sixth and highest zone (Zone VI) covering the entire Himalayan arc from Jammu & Kashmir and Ladakh to Arunachal Pradesh, placed about 61 per cent of India in moderate-to-high hazard, and assigned towns on zone boundaries to the higher zone. After the Ministry of Housing and Urban Affairs objected to cost increases and inadequate consultation, it was withdrawn in early March 2026, and the four-zone 2016 map again applies.
• Sketch: outline map of India with Zones II–V shaded, Himalaya, North-East, Kachchh and the Andamans darkest.

Seismic Microzonation

Seismic microzonation is the mapping of earthquake hazard within a city or district at a scale of roughly 1:10,000 to 1:25,000, subdividing a regional zone according to local ground conditions — soil type and thickness, water table, basin shape, slope, and susceptibility to liquefaction or landslides — that amplify or reduce shaking.

• Why it is needed: shaking varies sharply within one zone — the lake-bed clays of Mexico City amplified the 19 September 1985 earthquake, and Bangkok’s soft clays magnified distant shaking in 2025.
• Method: geological and geotechnical mapping, boreholes, shear-wave velocity surveys, ambient-noise measurements and site-response modelling combined with probabilistic hazard inputs.
• Indian examples: microzonation maps have been prepared with Ministry of Earth Sciences support for cities including Delhi, Guwahati, Kolkata and Bengaluru.
• Significance: it translates the national map into building bylaws and land-use decisions — the core of the interventions asked for in the Paper II question.

Seismic Hazard vs Seismic Risk

Seismic hazard is the probability that a given level of ground shaking, or a related effect such as liquefaction, will occur at a place within a stated period — a property of nature; seismic risk is the expected loss of lives, property and livelihoods, which combines that hazard with the exposure and vulnerability of people and buildings.

• Relationship: risk = hazard × exposure × vulnerability; hazard cannot be reduced, but exposure and vulnerability can.
• Measures: probabilistic hazard maps express, for example, the ground acceleration with a 2 per cent chance of being exceeded in 50 years.
• Examples: sparsely peopled parts of Arunachal Pradesh face very high hazard but lower absolute risk, while Delhi, in Zone IV, faces moderate hazard but very high risk from dense, largely non-engineered building stock on Yamuna alluvium; Japan’s strict codes keep risk low despite extreme hazard, whereas weak enforcement magnified losses in Türkiye in 2023.
• Link: the wider concept is treated under geomorphic hazard.

Tsunami (Run-Up, Drawdown)

A tsunami is a series of long-wavelength ocean waves generated by the sudden displacement of a large volume of water — most often by vertical sea-floor movement in a submarine earthquake, but also by landslides, volcanic collapse or explosions — that crosses oceans at jet-aircraft speed and grows dramatically in height as it enters shallow coastal water.

• Mechanism: the whole water column moves; wavelengths are 100–500 km; speed equals the square root of gravitational acceleration times depth — about 700 km an hour over 4,000 m of water — falling to a few tens of kilometres an hour near shore as height builds up (shoaling). The word is Japanese for “harbour wave”.
• Run-up and drawdown: run-up is the maximum height above sea level reached by water on land — more than 40 m on the Iwate coast in 2011; drawdown is the withdrawal of the sea when a trough arrives first, as on Thai beaches in 2004 — a natural warning to flee.
• Examples: Indian Ocean, 26 December 2004 — about 228,000 deaths in 14 countries; in India 10,749 confirmed dead and 5,640 missing, worst in Nagapattinam (Tamil Nadu) and the Andaman and Nicobar Islands, with waves reaching the Tamil Nadu coast about two hours after the earthquake; Krakatau, 1883, about 36,000 deaths; Anak Krakatau flank collapse, 22 December 2018, a non-seismic tsunami; Lituya Bay, Alaska, 1958, a landslide wave with a run-up of 524 m.
• Coastal link: mangroves, coral reefs and dunes reduced damage in 2004 (coastal landforms).
• Don’t confuse with: storm surges or “tidal waves” — tsunamis are unrelated to tides or weather.
• Sketch: a wave growing taller as the sea shallows, with run-up height and inundation limit marked.

Tsunami Early Warning (INCOIS)

Tsunami early warning is the rapid detection of tsunami-generating earthquakes and sea-level changes, followed by graded alerts to coastal populations; India’s system is run by the Indian Tsunami Early Warning Centre at the Indian National Centre for Ocean Information Services (INCOIS), Hyderabad, under the Ministry of Earth Sciences, operational since October 2007.

• Components: a seismic network that detects and locates earthquakes within minutes; bottom-pressure recorders in the Bay of Bengal and Arabian Sea that sense waves in the deep ocean; coastal tide gauges; and a pre-computed database of tsunami scenarios that turns an earthquake’s location and magnitude into forecasts. Advisories are graded as warning, alert and watch, with a first bulletin in about 10–20 minutes.
• International role: UNESCO’s Intergovernmental Oceanographic Commission recognised it in 2011 as a Tsunami Service Provider for the Indian Ocean, alongside Australia and Indonesia.
• Community level: Venkatraipur (Ganjam) and Noliasahi (Jagatsinghpur) in Odisha were the first Indian communities certified “Tsunami Ready”.
• Limitations: a tsunami from the Andaman trench can reach the islands within minutes, so strong shaking and drawdown must themselves be treated as warnings.

PYQs Built on These Terms

• Discuss the methods of measuring the intensity and magnitude of earthquakes. How are seismic zones demarcated? (2019)
• Demarcating the seismic zones of India, suggest suitable interventions required in most sensitive seismic zones for sustainable human settlements. (Paper II, Contemporary Issues)

Frequently Asked Questions

What is the difference between magnitude and intensity of an earthquake?

Magnitude measures the energy released at an earthquake’s focus and is a single instrumental number, while intensity describes the shaking and damage at a particular place and varies across the affected area. The 2025 Myanmar earthquake had one magnitude, Mw 7.7, but its intensity ranged from severe near Mandalay to damaging in distant Bangkok.

How many seismic zones are there in India now?

India currently has four seismic zones — II, III, IV and V — under IS 1893 (Part 1):2016. A 2025 revision introduced a sixth, highest zone (Zone VI) covering the Himalayan arc, but the Bureau of Indian Standards withdrew it in early March 2026, so the four-zone map remains the legal basis for design.

Why is the moment magnitude scale preferred to the Richter scale?

Moment magnitude is preferred because it is calculated from the physical size of the rupture — fault area, slip and rock rigidity — so it measures great earthquakes accurately. The Richter scale, built on wave amplitude for California, saturates around magnitude 7 and understates events such as the 2004 Sumatra–Andaman earthquake, now rated Mw 9.1.

Why do some undersea earthquakes cause tsunamis and others do not?

A tsunami forms only when an earthquake moves the sea floor vertically over a large area, as on subduction megathrusts. Strike-slip earthquakes, which move the floor sideways, and deep or small earthquakes rarely displace enough water. Magnitude usually has to exceed about 7, and a shallow focus beneath the sea is essential.

Can earthquakes be predicted?

No method yet predicts the exact time, place and size of an earthquake. Seismologists can forecast probabilities from fault slip rates, seismic gaps and past records, and early-warning systems can give seconds of notice once rupture has begun. Preparedness therefore rests on zonation, building codes and microzonation rather than on prediction.