Tsunami and its causes & Tsunami Warning Systems | UPSC

• A tsunami is a series of ocean waves of very long wavelength and long period set off when an impulsive disturbance suddenly displaces a large volume of water, most often a submarine earthquake.
  • The word is Japanese: “tsu” (harbour) + “nami” (wave), the “harbour wave” that wrecked ports though fishermen at sea noticed nothing.
  • It is not a tidal wave: tides play no part in its origin.
• A tsunami is a wave train, not a single wave; the waves arrive minutes to an hour apart, and the first is often not the largest.
• It is the main marine hazard of earthquakes and belongs chiefly to the convergent plate margins.
• Its ratio of height to length is so small in the open sea that it passes unnoticed under ships, yet it strikes coasts with enormous force.
• The hazard is real for India: of the coast long measured at 7,516 km, about 5,700 km is rated prone to tropical cyclones and tsunamis. The coastline is now reckoned at 11,098.81 km.

Causes of Tsunami

Conditions for a Tsunami

• An energy source must transfer energy to the water column, usually an undersea earthquake.
• The sea floor, or the water surface, must be displaced vertically.
  • Hence tsunamis rise mainly near oceanic trenches, where one plate is thrust beneath another along a megathrust fault.
  • Earthquakes on mid-oceanic ridges and transform faults (the Mid-Atlantic Ridge) are moderate, and their plates slide horizontally, so they rarely raise a tsunami.
• Tsunamigenic earthquakes are those capable of generating a tsunami:
  • magnitude generally above about 7;
  • shallow focus (within roughly the top 100 km, most within 50 km);
  • dip-slip (thrust or normal) motion that lifts or drops the sea floor.
• The most destructive events come from shocks above about magnitude 7.5, within about 50 km of the sea floor, which tilt and deform a stretch of sea floor from a few kilometres to over 1,000 km long.
  • Not every undersea shock raises one, because the outcome depends on how far the water column is displaced vertically. Purely horizontal movement seldom does it.
• The break-up of a seamount can also displace water vertically.

Types of Causes

Earthquakes

• Earthquakes cause roughly 80–90% of historical tsunamis.
  • At a subduction zone the overriding plate is dragged down and locked; when the fault slips it springs back upward, lifting the sea floor and the whole water column above it.

Landslides

• Submarine slumps and coastal landslides on continental slopes, trench walls or steep shores push water aside abruptly.
  • They can produce very high local waves from even moderate earthquakes, as in Papua New Guinea (1998), where an M 7.0 shock set off a submarine slump and waves of about 15 m.
  • A rockfall into Lituya Bay, Alaska (1958) sent water up to a record 524 m on the opposite slope.
  • Nice, France (1979): collapse of an airport embankment under construction, and a submarine slide offshore, sent a damaging wave along the coast.

Volcanic Eruptions

• Explosive volcanic eruptions, caldera collapse, pyroclastic flows entering the sea and volcano flank collapse all displace water.
  • Krakatau (Indonesia, 1883) produced waves of 30–40 m that killed about 36,000 people on Java and Sumatra.
  • That eruption reached its climax in late August 1883, when the volcano collapsed into its emptied magma chamber and the sea rushed into the crater.
  • The Santorini eruption in the Aegean is the classic ancient case, long linked with the decline of Minoan Crete.

Other Sources

• Meteorite or asteroid impacts in the ocean: the Chicxulub impact at the end of the Cretaceous raised a global mega-tsunami.
• Meteotsunamis: sudden jumps in atmospheric pressure (squall lines, or the pressure wave of a giant eruption) force sea-level oscillations.
• Anthropogenic sources: underwater nuclear or chemical explosions can raise small local waves.

Generation and Characteristics

Process of Generation

• Initial displacement: the sea floor rises or falls by metres over a zone that may be hundreds of kilometres long, and the sea surface above mirrors it.
• Collapse and spread: gravity pulls the bulge down and waves radiate outward in both directions from the rupture.
  • Where the coast faces the downthrown side, the trough arrives first and the sea withdraws before the crest; this “drawback” is a natural warning sign.

Propagation in the Open Ocean

• A tsunami is a shallow-water wave even in the deep ocean, because its wavelength (100–500 km) is far greater than the ocean depth.
  • Its speed therefore depends only on depth: speed = √(g × d), where g is gravitational acceleration and d the water depth.
  • The fall in speed with depth is steep, as this worked example for a wave of 18-minute period shows:

• Over a 4,000–5,000 m deep ocean this gives 700–800 km/h, a jet-liner’s speed, and up to about 950 km/h over the deepest basins.
  • Older accounts gave 650–900 km/h for the open ocean.
• In deep water the wave height is usually under 1 m and the slope is gentle, so it passes unnoticed beneath ships.
  • Sanriku, Japan, 15 June 1896: fishermen at sea noticed nothing, as the wave was only about 40 cm high in deep water, yet it ravaged some 275 km of the Honshu coast and killed about 22,000 people.
• The whole water column moves, and energy loss is small, so a tsunami can cross an entire ocean: the 1960 Chile tsunami killed people in Japan about 22 hours later.

Shoaling and Run-up at the Coast

• As the wave enters shallow water, speed falls with depth, the wavelength shortens and the rear of the wave catches up with the front.
  • Over an outer shelf 80–180 m deep the speed is already down to about 100–150 km/h.
• The period stays constant and the energy flux is nearly conserved, so the energy is squeezed into less water and wave height rises sharply; this is the shoaling effect.
  • A wave of well under a metre at sea can rise to 10–30 m, and locally more, at the shore.
  • In confined coastal waters close to the source, heights above 30 m have been recorded.
• The wave rarely breaks like a surf wave; it arrives as a fast-rising flood or turbulent bore that runs far inland.
  • Run-up is the maximum height above sea level reached on land; inundation distance is how far inland it reaches.
• Coastal shape amplifies the wave: funnel-shaped bays, harbours and estuaries concentrate the energy, while offshore reefs, wide shelves and mangroves dampen it.
• Effects on the coast: the flood runs hundreds of metres inland past the normal high-water mark, flattens houses, tosses boats ashore and carries people out to sea on the backwash.
  • Rocks of several hundred tonnes can be shifted inland, and the wave can travel up rivers that open to the sea.

Tsunami, Wind Waves and Storm Surge

Distribution

• The Pacific Ocean records most tsunamis because it is girdled by subduction zones, the Ring of Fire, and averages about two damaging tsunamis a year.
• The Indian Ocean has two main source zones:
  • the Andaman–Sumatra (Sunda) subduction zone in the east;
  • the Makran subduction zone off Pakistan and Iran in the north-west.
• The Mediterranean and Caribbean carry lower but real risk, as shown by the Lisbon earthquake (1755), whose tsunami struck Portugal and Morocco and crossed the Atlantic to the West Indies; tens of thousands died.

Major Tsunamis

• Other Pacific-wide tsunamis of the mid-20th century:
  • Kamchatka (4 November 1952, Mw 9.0): waves of 15–18 m destroyed Severo-Kurilsk in the Kuril Islands and damaged Hawaii.
  • Andreanof Islands, Aleutians (9 March 1957, Mw 8.6): run-up reached about 16 m on Kauai, Hawaii.

Indian Ocean Tsunami, 2004

• On 26 December 2004 an earthquake of M 9.1–9.3 ruptured about 1,300 km of the boundary where the Indian Plate subducts beneath the Burma Plate, off north-west Sumatra. The epicentre lay near 3.3° N, 95.9° E.
  • The sea floor rose by several metres, displacing tens of cubic kilometres of water.
• Run-up reached about 51 m in Aceh; about 2,28,000 people died in 14 countries, the worst tsunami disaster on record. Malaysia was largely shielded by Sumatra.
• India: official estimates give 10,749 dead and 5,640 missing, mostly on the Tamil Nadu coast (Nagapattinam) and in the Andaman and Nicobar Islands, with Andhra Pradesh, Keralam (Kerala) and Puducherry also hit.
  • The Nicobars lay closest to the source. Car Nicobar, Katchal, Nancowry, Campbell Bay, Champion Island, Chowra and Teresa were badly struck.
  • The waves reached the Nicobars within minutes and the Tamil Nadu coast in about two to three hours, yet no Indian Ocean warning system existed to use that time.
  • Coasts backed by mangroves (Pichavaram, Muthupet) suffered less damage than open shores.
• Early estimate of financial loss in India (₹ crore):

• Changes to the land: beaches at Campbell Bay disappeared, Trinket Island split in two, Car Nicobar subsided, and Indira Point lost its shoreline. The mouth of the Adyar at Chennai was also altered.
  • Sea water entered the Kalpakkam nuclear plant, which shut down for a number of days.
• Effects on the Earth: NASA scientists calculated (January 2005) that the shift of mass shortened the day by about 2.68 microseconds, moved the mean North Pole by about 2.5 cm and reduced the Earth’s oblateness by about one part in 10 billion.
• Farmland: salt water soaked to about 90 cm in Cuddalore and Nagapattinam, which lost the root zone of paddy, with over 9,500 hectares of Nagapattinam left unfit for cultivation and soil pH reaching about 9.
  • Reclamation meant flushing with Cauvery water and about two tonnes of gypsum per hectare, over 3–4 years. Salt-tolerant cashew was advised.
• Marine life: the fringing reefs of the islands suffered breakage and burial under debris. Mangrove nurseries, hatcheries and aquaculture ponds from Keralam to Odisha were damaged, which hit fishing and marine exports.

Tohoku Tsunami, Japan, 2011

• On 11 March 2011 an M 9.0–9.1 megathrust earthquake struck about 130 km east of Sendai, where the Pacific Plate subducts beneath north-east Honshu.
• Waves overtopped sea walls built for smaller events; run-up reached about 40 m near Miyako, and nearly 20,000 people were dead or missing.
• The quake shifted parts of Honshu about 2.4 m east, moved the Earth’s figure axis by 10–25 cm and shortened the day by about 1.8 microseconds.
• Flooding disabled the Fukushima Daiichi nuclear plant, forcing a 20 km evacuation zone; it showed that engineered defences alone cannot stop an extreme tsunami.

Non-seismic Tsunamis: Recent Lessons

• Palu (2018): a strike-slip fault, normally a weak tsunami source, triggered coastal and submarine landslides inside a narrow bay; waves arrived within minutes.
• Anak Krakatau (2018): a flank collapse of the volcano at night sent waves onto Java and Sumatra with no earthquake to trigger a warning.
• Hunga Tonga (2022): the eruption’s atmospheric pressure wave drove sea-level changes across all oceans, arriving faster than an ordinary tsunami.

Tsunami Risk to India

• East coast and islands: the Andaman–Sumatra subduction zone threatens the Andaman and Nicobar Islands (near-field, minutes) and the east coast (far-field, hours).
  • Major coastal projects in the Nicobars lie within the 2004 rupture zone.
• West coast: the Makran subduction zone produced the 1945 Makran tsunami (M 8.1, 28 November 1945), which killed hundreds to a few thousand people on the Makran coast and reached Gujarat and Mumbai.
  • Older accounts put the waves at 12–15 m at Pasni and Ormara and about 2 m at Mumbai. A 2021 field study estimated run-up of 2.3–13.7 m on that coast and argued for a far lower toll, at most 150. Thirteen tsunami deaths are confirmed at Bombay (now Mumbai).
• Kachchh (Kutch), 16 June 1819: the Rann of Kachchh earthquake raised the Allah Bund ridge (about 80 km long, ~6 m high), sank the Sindri area, and a local tsunami flooded part of the Great Rann. A further sea movement was reported there in the shock of 1845.
• Bay of Bengal, 31 December 1881: the Car Nicobar earthquake, put at about Mw 7.9, raised a tsunami of up to 0.8 m recorded on tide gauges around the Bay. No casualties were reported.

Tsunami Warning and Mitigation

Tsunami Early Warning System

• Earthquakes cannot be predicted, so the time of a tsunami cannot be foretold; historical records and numerical models only show where tsunamis are likely.
• The most effective defence is an end-to-end early warning system, built on two equally important parts:
  • Detection: a network of sensors to identify and confirm a tsunami;
  • Dissemination: a communication chain that reaches the coast in time for evacuation.
• Sensors:
  • Seismic networks locate the earthquake and estimate its size within minutes.
  • Bottom pressure recorders (tsunami buoys) on the deep sea floor sense a passing tsunami as a tiny change in water pressure; the DART (Deep-ocean Assessment and Reporting of Tsunamis) buoys of the US are the best-known design.
  • Tide gauges and coastal radars confirm arrival and size at the shore.
• Warnings reach the public by SMS, radio, television, sirens on dedicated platforms, mosque loudspeakers and police vehicles with loudspeakers.
• Natural warning signs: strong shaking on a coast, or a quick rise or fall of the water, should be treated as an alert in itself, since a wave can arrive within minutes.
  • Move off the beach to high ground, keep away from rivers opening to the sea, and wait for the all-clear, as several waves follow.
  • Land under 8 m above the sea is most at risk.
• Reports of unusual animal behaviour before the 2004 wave, such as elephants in Thailand and deer at Point Calimere running for higher ground, remain anecdotal.

Global and Regional Systems

• Pacific Tsunami Warning Center (PTWC): set up by the US in 1949 at Ewa Beach, Hawaii, after the 1946 Aleutian tsunami, and the earliest network to alert Pacific coasts.
  • After the 1960 Chile tsunami, UNESCO’s Intergovernmental Oceanographic Commission (IOC) built an international Pacific Tsunami Warning System in the 1960s.
• Indian Ocean Tsunami Warning and Mitigation System (IOTWMS): coordinated by the IOC after 2004 and fully operational since 2011.
  • It runs on three Regional Tsunami Service Providers (RTSPs): India, Australia and Indonesia.
• Similar IOC systems cover the Caribbean and the North-east Atlantic and Mediterranean.

India’s Tsunami Early Warning Centre

• The Indian Tsunami Early Warning Centre (ITEWC) at the Indian National Centre for Ocean Information Services (INCOIS), Hyderabad, under the Ministry of Earth Sciences, was inaugurated on 15 October 2007.
  • India volunteered for the international system after 2004 and built its own centre at a cost of ₹125 crore. The 8.4-magnitude Indonesian shock of 12 September 2007 served as its first live test.
  • It has been rated among the most modern systems in the world, and INCOIS received the Subhash Chandra Bose Aapda Prabandhan Puraskar for 2025.
• Network:
  • a national network of broadband seismic stations, 17 in its first form, plus real-time data from global networks;
  • bottom pressure recorders in the Bay of Bengal and Arabian Sea (six at the start, four and two), deployed with the National Institute of Ocean Technology;
  • about 50 real-time tide gauges and coastal radar stations.
• Performance: it detects Indian Ocean earthquakes above M 6 in under about 12 minutes and uses a database of pre-run model scenarios to issue location-specific warnings, alerts and watches by coastal zone.
  • At the start it took under about 20 minutes to detect such a shock and could alert agencies 13 minutes after an undersea quake.
  • An early count gave 339 earthquakes of M 6.5 or more monitored after October 2007.
  • Advisories reach the Home Ministry’s control room and State emergency centres by satellite link, telephone, fax, SMS and e-mail.
• It serves as an RTSP for the whole Indian Ocean, advising more than two dozen rim countries, and works round the clock.

Mitigation and Preparedness

• Hazard mapping: inundation and coastal multi-hazard vulnerability maps guide land use and evacuation routes.
• Land-use control: Coastal Regulation Zone setbacks, and siting of critical facilities away from low shores.
  • The no-development zone once ran to 500 m. The 2019 notification sets 200 m on sparsely populated rural coast and 50 m on densely populated rural coast.
  • Building inside these limits, and mining of beach sand as at Kanniyakumari, removes the coast’s natural buffer.
• Bioshields: mangroves, casuarina belts and coastal dunes absorb wave energy, as the 2004 experience on the Tamil Nadu coast showed.
  • Vegetation protects well only where it covers over 70% of the coastal strip. Older figures gave Tamil Nadu sufficient cover on only 110 km of its 1,076 km coast.
• Engineering: sea walls, breakwaters, tsunami-resistant buildings and vertical evacuation shelters on raised ground.
  • A sea wall along the whole Tamil Nadu coast was proposed after 2004, with financial and ecological limits.
• Community preparedness:
  • Tsunami Ready is an IOC recognition for communities that meet set indicators of mapping, signage, plans, drills and awareness.
  • Venkatraipur (Ganjam) and Noliasahi (Jagatsinghpur), Odisha, were the first Indian Ocean communities recognised, in 2020, and 24 more Odisha villages followed in 2024; the programme is being extended to the Andaman and Nicobar Islands.
  • A December 2025 report said INCOIS was coordinating extension of the programme to all coastal States and Union Territories on a pilot scale. The IOWave25 exercise drew in Andaman and Nicobar communities.
  • Regional IOWave exercises test the whole warning chain, and 5 November is World Tsunami Awareness Day (UN General Assembly, 2015).
• Institutions: the National Disaster Management Authority issued national guidelines on the management of tsunamis in 2010.

Evaluation of Warning Systems

• Near-field tsunamis leave little time: the Andaman and Nicobar Islands or the Makran coast may get only minutes, so self-evacuation on strong shaking or sea withdrawal matters more than any bulletin.
• Non-seismic tsunamis from landslides and volcanoes escape seismic-based warnings, as Palu, Anak Krakatau and Tonga showed.
  • Warning centres are adding volcano-tsunami procedures, and INCOIS is mapping submarine landslide zones.
• The “last mile” is the weakest link: sirens, power and phone networks often fail in the very quake that sets off the tsunami.
• Over-warning and complacency are risks, since many alerts end in small waves; yet the 2025 Kamchatka event, where millions were warned within minutes and about 1.9 million people evacuated in Japan, shows how far systems have come since 2004.