Tsunami

Also called: Seismic sea wave, Harbour wave · Topic: Natural Hazards and Disaster Management in India · NCERT: Class 6, Ch 2 "Oceans and Continents"; Class 7, Ch 2 "India and Her Neighbours"; Class 11, Ch 6 "Natural Hazards and Disasters: Causes, Consequences and Management"; Class 11, Ch 3 "Interior of the Earth"; Class 11, Ch 6 "Landforms and their Evolution"

Meaning

A tsunami is a series of long ocean waves. They form when the sea floor moves suddenly and pushes the whole column of water above it up or down. The cause is usually an undersea earthquake, but it can also be a volcanic eruption or an underwater landslide. The word is Japanese and means "harbour wave". It is also called a seismic sea wave.

Tsunamis matter because they can cross a whole ocean and flood crowded, low coasts far from where they start. The 2004 Indian Ocean tsunami killed more than 2,30,000 people in 14 countries [5]. After it, India set up its own tsunami warning system.

Explanation

How a tsunami forms (cause → process → result)

• Cause: the sea floor moves suddenly. This can happen because of:
• an undersea earthquake, which is the most common cause
• an undersea or coastal volcanic eruption

• a submarine landslide (a large slide of mud and rock down an underwater slope). NCERT does not list this cause.

• Process: the whole water column moves.

• The sea floor lifts or drops all the water above it, from the sea bed up to the surface.
• Gravity pulls the water back to its normal level.

• The energy spreads outward as waves.

• Result: a train of waves.

• The waves arrive many minutes apart. The first wave is often not the biggest danger.

• They flood low coasts far from the source.

• Subduction zone (a place where one tectonic plate sinks below another): these zones produce the largest earthquakes, so most tsunamis start there.

How it behaves: deep sea compared with the coast

• Wavelength is the distance from one wave crest to the next. Period is the time between two crests passing one point.
• A tsunami is a shallow-water wave, which means its wavelength is much longer than the depth of the water. This is true even in the deep ocean. So its speed depends on depth: the deeper the water, the faster the wave.

Feature	Deep ocean	Near the coast (shallow water)
Wavelength	Very long (hundreds of km)	Becomes shorter
Period	Stays the same	Stays the same
Height	Small. Ships rise only 1-2 m over several minutes and barely notice	Grows to 15 m or more
Speed	Fastest, about 700-800 km/h, as fast as a jet plane	Slows down sharply
Danger	Low for ships	Very high. Water floods far inland

• Correction to NCERT: NCERT says the waves move faster in shallow water. This is wrong. Tsunamis are fastest in the deep ocean and slow down near the coast.

Why the wave grows tall at the coast (shoaling)

• Shoaling is the change in a wave as it moves into shallower water.
• The front of the wave reaches shallow water first and slows down.
• The back of the wave is still in deep water and moving fast, so it catches up.

• The same energy is squeezed into a shorter, slower wave. The water piles up into a tall wall.

• Natural warning sign: sometimes the sea first pulls back a long way and leaves the sea floor bare.

• This happens when the trough (the low part of the wave) reaches the shore before the crest.
• People should move to high ground at once.

Detection and defence

• Detection instruments used by India's warning centre:

Instrument	What it does
Seismic stations	Record an undersea earthquake and find its location, depth and magnitude within minutes
Bottom pressure recorders (BPRs)	Sit on the deep sea floor. When a tsunami passes overhead, the weight of water above them changes, and they record the change in pressure
Tide gauges	Measure sea level at the coast and confirm whether a wave has really arrived

• Natural buffers are natural coastal features that absorb or break the wave's energy before it reaches people:
• Offshore bar (a ridge of sand or shingle in the sea, roughly parallel to the coast): waves break on it before they reach the shore.
• Barrier island (an offshore bar that has grown above sea level into a long island): it takes the first blow of the waves and shelters lagoons and the mainland.
• Beach (a gently sloping strip of sand at the shore): it spreads out the wave's energy.
• Dunes (hills of wind-blown sand behind the beach): they act as a natural wall against flooding.

• Mangroves (salt-tolerant trees in the tidal zone, with thick stilt roots): their roots and trunks slow the water and trap debris.

• Built defences such as sea walls, embankments and coastal shelterbelts of casuarina add to natural buffers. They do not replace them.

Where it occurs

• World: tsunamis are frequent around the Pacific Ring of Fire (the belt of volcanoes and earthquake zones around the edges of the Pacific Ocean) and in Southeast and South Asia.
• India's two tsunami source zones:

Source zone	Sea	Region threatened
Andaman-Sumatra subduction zone	Bay of Bengal / Andaman Sea	Andaman and Nicobar Islands; east coast (Tamil Nadu, Andhra Pradesh, Odisha, West Bengal)
Makran subduction zone (off Pakistan and Iran)	Arabian Sea	Gujarat, Maharashtra and the west coast

• 26 December 2004 Indian Ocean tsunami:
• Trigger: a magnitude 9.1 earthquake off the north-west coast of Sumatra, Indonesia. Estimates range from Mw 9.1 to 9.3. Mw (moment magnitude) is the scale that measures the total energy a large earthquake releases.
• Ranking: it was the third-largest earthquake in the world since 1900, and the largest since the 1964 Prince William Sound (Alaska) earthquake [5].
• Fault: the Sunda megathrust. A megathrust is a huge, gently sloping fault where one plate slides under another. Here the Indo-Australian Plate slides under the Burma (Sunda) micro-plate.
• Rupture: the break ran northward for at least 1,200 km, almost to Myanmar, in about 8 minutes. The crust moved by an average of 15 m [6].
• Waves: they reached up to 30 m high and travelled more than 4 km inland in Aceh Province, Sumatra [6].
• Deaths:
  • NOAA lists 2,27,898 dead or missing, of whom 1,67,540 were in Indonesia [6].
  • More than half of all victims were in Aceh, and Banda Aceh was the worst-hit city [6].
  • NCERT gives a figure of over 3,00,000.

• India:

  • Over 10,000 dead (verify the current official figure).
  • Tamil Nadu, especially Nagapattinam district, had the most deaths on the mainland.
  • The rupture zone ran under the Andaman and Nicobar Islands, so they were hit within minutes. Car Nicobar was badly damaged.
  • Land sank at Indira Point, the southern tip of Great Nicobar, and part of it went under the sea.

• Indian Tsunami Early Warning Centre (ITEWC):

• Where and who: it is at INCOIS (Indian National Centre for Ocean Information Services), Hyderabad, under the Ministry of Earth Sciences. India set it up in response to the 2004 tsunami [2].
• Since when: it has been working since October 2007.
• Speed: it can issue an advisory within about 10 minutes of an earthquake.

• Regional role: since 2011 it has been a UNESCO-IOC Regional Tsunami Service Provider (RTSP). UNESCO-IOC is UNESCO's Intergovernmental Oceanographic Commission. ITEWC works under the Indian Ocean Tsunami Warning and Mitigation System (IOTWMS) and serves 25 Indian Ocean countries [2][3].

• Tsunami Ready (UNESCO-IOC programme):

• It recognises coastal communities that are ready for a tsunami. A community must meet 11 indicators, including a risk reduction plan, mapped inundation zones (areas the water would flood) and public display of tsunami information [2].
• The Odisha State Disaster Management Authority (OSDMA) ran a pilot in 6 villages [2]: Boxipalli-Venkatraipur (Ganjam), Noliasahi (Jagatsinghpur), Keutajanga (Puri), Jayadevkasaba Pahi (Balasore), Podhuan (Bhadrak) and Tantiapal Sasan (Kendrapara).
• Venkatraipur (Ganjam) was recognised on 7 August 2020 [4], and Noliasahi (Jagatsinghpur) in 2020 [2].

• With these two villages, India became the first country in the Indian Ocean region, and Odisha the first state, to get this recognition [2].

• Mangrove shields:

• In 2004, villages behind the Pichavaram mangroves (Cuddalore, Tamil Nadu) and the Muthupet mangroves (Tiruvarur, Tamil Nadu) suffered less damage than open coasts nearby.
• Bhitarkanika (Kendrapara, Odisha) is a similar mangrove shield on the east coast.

Don't confuse with

• Ordinary wind waves: these are driven by wind and move only the surface water. A tsunami moves the whole water column from the sea bed to the surface, so it has a very long wavelength and floods far inland.
• Storm surge: this is a rise in sea level pushed ashore by a cyclone's winds and low air pressure. A tsunami is caused by the sea floor moving (earthquake, volcano or landslide), not by the weather.
• Tide: this is the regular rise and fall of the sea caused by the pull of the Moon and Sun. A tsunami is sudden and has nothing to do with tides, even though it is called a "harbour wave".
• Deep-water tsunami compared with coastal tsunami: in the deep ocean the wave is fast (about 700-800 km/h) but low. At the coast it is slow but tall (15 m or more). Speed and height change in opposite directions.

Prelims Hooks

• Trap: a statement that tsunamis are "faster in shallow water" is wrong. They are fastest in the deep ocean (about 700-800 km/h) and slow down near the coast.
• From deep to shallow water: the period stays the same, the wavelength shortens and the height increases.
• 2004 tsunami: M 9.1, off north-west Sumatra, on the Sunda megathrust. The rupture ran about 1,200 km north towards Myanmar [6]. It was the third-largest earthquake since 1900 [5].
• ITEWC: at INCOIS, Hyderabad, under the Ministry of Earth Sciences. Working since October 2007, a UNESCO-IOC RTSP since 2011, and serving 25 countries [2][3].
• Instrument and role: a bottom pressure recorder detects the tsunami from pressure changes on the deep sea floor. A tide gauge confirms that the wave has reached the coast.
• First Tsunami Ready villages in the Indian Ocean region: Venkatraipur (Ganjam) and Noliasahi (Jagatsinghpur), both in Odisha (2020) [2][4]. Mangrove–state pairs: Pichavaram and Muthupet (Tamil Nadu), Bhitarkanika (Odisha).

Mains Points

• Why tsunamis cause heavy losses in India (GS-III)
• Two subduction source zones lie close to India: Andaman-Sumatra in the Bay of Bengal and Makran in the Arabian Sea.
• Coasts such as Nagapattinam and the Andaman and Nicobar Islands are low-lying and crowded with fishing villages, ports, beaches and farmland.

• In 2004 the Indian Ocean had no warning system, unlike the Pacific, so people had no time to leave.

• Policy chain after 2004, from detection to the village

• Detection: ITEWC at INCOIS (2007) uses seismic stations, BPRs and tide gauges and issues advisories in about 10 minutes.
• Regional role: as a UNESCO-IOC RTSP (2011), it serves 25 Indian Ocean countries [3]. A tsunami crosses whole oceans, so no country can manage the risk alone.
• Community level: Tsunami Ready villages in Odisha (2020) have evacuation maps, route signs, drills and emergency plans [2][4].

• This matches the focus on early warning in the Hyogo (2005) and Sendai (2015-2030) frameworks.

• Ecosystem-based disaster risk reduction and its limits

• Mangroves (Pichavaram, Muthupet, Bhitarkanika), dunes and barrier islands absorb wave energy. Their record in 2004 supports protecting them under the CRZ Notification 2019. CRZ (Coastal Regulation Zone) is the strip of coast from the High Tide Line inland where building is controlled by law under the Environment (Protection) Act, 1986. This is a better approach than relying only on sea walls.
• Near-field tsunamis start close to the coast, so they leave very little warning time. In 2004 the Andaman and Nicobar Islands were hit within minutes. Warnings must therefore be backed by drills, evacuation maps, land-use control along the shore and natural buffers.

Related concepts

• 2004 Indian Ocean tsunami
• Indian Tsunami Early Warning Centre
• Coastal defence by natural buffers

Read more

• Tsunamis and coastal defences