Tsunami – Mechanism and Properties
Geography › Mountains, Volcanoes, Earthquakes and Tsunami · Topic 4.11
A tsunami is a series of long sea waves caused by a sudden displacement of a large volume of water, most often by an undersea earthquake at a subduction zone, and also by landslides, volcanic eruptions or meteorite impacts. In the deep ocean a tsunami travels at jet speed with a small height, but on reaching shallow coastal water it slows down, rises steeply and floods the land.
6 min read · UPSC Prelims · Geography · Mountains, Volcanoes, Earthquakes and Tsunami
At a Glance
- Meaning Japanese for harbour wave
- Main cause Undersea earthquakes at subduction zones
- Speed in deep ocean Up to about 700 to 800 km an hour
- Wavelength Often over 100 km
- Most affected ocean Pacific, along the Ring of Fire
- Exam link UPSC Prelims, Geography: Mountains, Volcanoes, Earthquakes and Tsunami
Where Does It Fit?
This is the eleventh topic of the category on mountains, volcanoes, earthquakes and tsunami. The previous topic described the effects of earthquakes, including tsunamis as one of them, and this one explains how a tsunami works. The next topic examines the 2004 Indian Ocean tsunami and warning systems. Previous topic: Effects of Earthquakes. Next topic: The 2004 Indian Ocean Tsunami and Warning Systems.
What Is a Tsunami?
In short: a tsunami is not a single wave but a train of very long waves caused by a sudden movement of the sea floor or a large mass of water.
The word tsunami comes from Japanese and means harbour wave, because the waves seemed to appear suddenly in harbours, and it is also called a seismic sea wave. Tsunamis should not be called tidal waves, since they have no connection with tides, which are caused by the gravitational pull of the Moon and the Sun, as in Tides and Their Types. A tsunami is generated when a large volume of water is suddenly displaced. The water surface, pushed up or down, then spreads out as waves in all directions, as ripples spread when a stone is dropped in a pond, but on an ocean scale.
What Causes a Tsunami?
- Undersea earthquakes: the most common cause, producing about 80 per cent of tsunamis; it occurs where a plate suddenly slips upward on a thrust fault at a subduction zone, lifting the sea floor and the water above it. Earthquakes of magnitude of about 7 or more, with shallow focus and vertical movement, are the usual source.
- Submarine and coastal landslides: slumping of sediments on the continental slope or collapse of cliffs and glaciers into the sea.
- Volcanic eruptions: collapse of a volcano flank or caldera, or explosive eruptions under water, as at Krakatoa in 1883 and Hunga Tonga in 2022.
- Meteorite impacts: rare, but a large impact in the ocean could cause a huge tsunami.
Since most large earthquakes occur at subduction zones around the Pacific, as in Distribution of Volcanoes and Earthquakes – Ring of Fire, the Pacific Ocean has by far the most tsunamis. Japan, Chile, Indonesia, Alaska and the Philippines are most exposed.
What Are the Properties of Tsunami Waves?
| Property | Deep ocean | Near the coast |
|---|---|---|
| Speed | Very fast, about 700 to 800 km an hour, depending on depth | Slows down sharply as the water gets shallow |
| Wave height | Small, often less than 1 m, hardly noticed by ships | Rises to several metres or even tens of metres |
| Wavelength | Very long, often 100 to 200 km | Shortens as the wave slows |
| Wave period | 10 minutes to an hour between crests | Same period; the waves arrive for hours |
| Energy | Spread over the entire depth of the water | Concentrated into a smaller volume, giving destructive force |
The speed of a tsunami in the open ocean depends on the depth of the water: it is roughly the square root of the product of the acceleration due to gravity and the depth. In an ocean 4,000 m deep this gives about 200 m per second, around 720 km an hour. Tsunamis travel across entire oceans with little loss of energy, so a tsunami from Chile in 1960 caused damage in Japan the next day, after crossing the Pacific in about 22 hours.
How Does a Tsunami Behave When It Reaches the Coast?
As the wave moves from deep to shallow water, the front slows while the back is still moving fast, so the wave is compressed and grows taller in a process called shoaling. The first sign at the coast is sometimes a sudden recession of the sea far beyond the usual low-tide mark, called drawback, which is a natural warning to move inland immediately, as many people at the beach in 2004 did not know. The waves arrive as a surge or a rapidly rising tide that can carry water far inland, depending on the slope of the land. Often the first wave is not the largest, and later waves, coming minutes to hours apart, can be more destructive. Bays, estuaries and funnel-shaped coasts amplify the run-up, while coral reefs, mangroves and coastal forests can reduce the energy. Tsunamis are described among the effects of earthquakes in Effects of Earthquakes.
How Is a Tsunami Different from Normal Waves and Tides?
| Feature | Wind waves | Tides | Tsunami |
|---|---|---|---|
| Cause | Wind on the water surface | Gravity of Moon and Sun | Sudden displacement of water |
| Wavelength | Tens to hundreds of metres | Very long, thousands of km | 100 to 200 km |
| Period | Seconds | About 12 or 24 hours | 10 minutes to an hour |
| Predictable | By forecasts | Very predictable | Warned after the trigger, not forecast ahead |
As an analytical point, the biggest danger is not the wave in the open sea but the lack of time: a coast near the source may have only minutes, while a distant coast has hours, so warning systems and public education decide the death toll more than the size of the wave. The major tsunamis, including the Indian Ocean event, are in The 2004 Indian Ocean Tsunami and Warning Systems.
Why Is This Topic Important?
- Concept Frequent Prelims theme
- Hazard Disaster management link
- Distinction Not tides or ordinary waves
What Came Next?
The next topic describes the 2004 Indian Ocean tsunami and warning systems. Next topic: The 2004 Indian Ocean Tsunami and Warning Systems.
Key People and Terms
- Tsunami Japanese for harbour wave
- Shoaling Wave growth in shallow water
- Drawback Sea retreating before the wave
- Run-up Height reached on land
- Subduction zone Common tsunami source
- Wavelength Distance between crests
Exam Corner
Points to Remember
- About 80 per cent of tsunamis occur in the Pacific.
- Deep ocean speed is about 700 to 800 km an hour.
- Wavelength is over 100 km.
- The first sign may be drawback of the sea.
- A tsunami is not a tidal wave.
Do Not Confuse With
- Tsunami and tidal wave Displacement of water versus tides
- Tsunami and storm surge Earthquake wave versus cyclone surge
- Wave height at sea and at the coast Small in deep water, large near shore
Memory Hook
Fast and low at sea, slow and tall ashore; if the sea runs away, you run away.
Mains Angle
Questions ask for causes, features and mitigation. These points add depth.
- Cause Subduction-zone earthquakes
- Behaviour Shoaling and run-up
- Defence Warning, mangroves and planning
A Question You May Face
An original practice question, not a past paper question.
“What is a tsunami? Explain its causes and why it is most destructive near the coast.”
How to Answer
- Define Meaning and mechanism.
- Causes Earthquakes and landslides.
- Properties Speed and wavelength.
- Coast Shoaling and run-up.
GS Relevance
Frequently Asked Questions
What is a tsunami?
A tsunami is a series of very long sea waves caused by a sudden displacement of a large volume of water, usually by an undersea earthquake. The word is Japanese for harbour wave. Tsunamis are not related to tides and should not be called tidal waves.
What causes a tsunami?
Most tsunamis, about 80 per cent, are caused by undersea earthquakes at subduction zones, where the sea floor is suddenly lifted. Other causes are submarine and coastal landslides, volcanic eruptions such as Krakatoa and Hunga Tonga, and, very rarely, meteorite impacts.
How fast does a tsunami travel?
In the deep ocean a tsunami can travel at about 700 to 800 km an hour, similar to a jet aircraft, with a small height of often less than a metre and a wavelength over 100 km. It slows down in shallow water but gains height.
Why is a tsunami more destructive near the coast?
As a tsunami enters shallow water, the front slows down while the back is still moving fast, so the wave is compressed and grows much taller, a process called shoaling. The energy is concentrated in a smaller volume of water, causing violent flooding.
What is drawback in a tsunami?
Drawback is the sudden retreat of the sea far beyond the normal low-tide line, which can occur before a tsunami crest arrives. It is a natural warning that a tsunami is coming, and people should move quickly to high ground.
PYQ Practice — Statement Analysis
1 Most tsunamis are caused by undersea earthquakes.
At subduction zones.
2 The Pacific Ocean has more tsunamis than any other ocean.
It is bordered by the Ring of Fire.
3 A tsunami in the deep ocean is a very tall wave.
It is often less than a metre high.
4 Tsunamis are caused by the gravitational pull of the Moon.
That describes tides.
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