Tsunami: Mechanism, Properties and Effects

At a Glance

• Main cause Undersea earthquake at a subduction zone
• Speed in deep water 700 to 800 km per hour
• Wave length Up to hundreds of km
• 2004 event Sunda trench, magnitude about 9.1
• India’s centre INCOIS, Hyderabad
• Exam link UPSC Prelims, Geography: Geomorphic Processes and Landform Evolution

Where Does It Fit?

This topic closes the first part of the macro, which treats earthquakes and volcanoes, with a hazard that is created by both. It leads into the second part on the forces and processes that shape the surface of the Earth. Previous topic: Distribution of Volcanoes: Pacific Ring of Fire, Mediterranean Volcanism. Next topic: Exogenic and Endogenic Forces: Gradation, Degradation and Aggradation.

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: Tidal Currents, Types and Importance. 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: Pacific Ring of Fire, Mediterranean Volcanism, 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?

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 epicentre above it Main cause Sudden slip along faults at plate boundaries Scales Magnitude…">Earthquakes: Types, Causes, Effects and Frequency.

How Is a Tsunami Different from Normal Waves and Tides?

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 described below.

What Happened on 26 December 2004?

In short: a huge subduction earthquake off Sumatra sent a tsunami across the Indian Ocean that struck coasts within minutes to hours.

On the morning of 26 December 2004, a magnitude 9.1 earthquake struck off the west coast of northern Sumatra in Indonesia, one of the largest ever recorded. It happened on the Sunda trench, where the Indian plate subducts beneath the Burma plate, as in Plate Tectonics: Major and Minor Lithospheric Plates. A segment of the fault about 1,200 to 1,300 km long slipped by several metres, lifting the sea floor and displacing a vast volume of water. The tsunami hit the nearby Aceh coast within 20 to 30 minutes, reached Thailand in about two hours, Sri Lanka and the east coast of India in about two to three hours, and then travelled on to the Maldives and the east coast of Africa. The Sumatra earthquake and the tsunami it caused killed about 230,000 people in 14 countries, with Indonesia, Sri Lanka, India and Thailand worst hit, and was among the deadliest natural disasters in recorded history.

How Was India Affected?

More than 10,000 people were killed in India, and several lakh were affected. The earthquake also shifted the position of land in the Andaman and Nicobar Islands, with the southern group subsiding and the northern group rising. The disaster showed that India’s coast was vulnerable, though the country had not been affected by a major tsunami for a long time, and it led to the Disaster Management Act of 2005 and the creation of the National Disaster Management Authority.

How Do Tsunami Warning Systems Work?

1. Detection of the earthquake: a network of seismographs finds the location, depth and magnitude within minutes.
2. Sea-level sensors: bottom pressure recorders on the ocean floor, linked to surface buoys, called DART (Deep-ocean Assessment and Reporting of Tsunamis) buoys, detect the passing wave, and tide gauges on the coast confirm it.
3. Modelling: computers compare the event with a database of pre-run scenarios to predict arrival times and wave heights along the coasts.
4. Warning dissemination: alerts are sent through the disaster management network, SMS, radio, television, sirens and the media to authorities and the public.
5. Response: evacuation to higher ground along marked routes.

In the Pacific, the system dates from the 1940s and the Pacific Tsunami Warning Center in Hawaii, and after the 2004 tragedy the Intergovernmental Oceanographic Commission of UNESCO helped create an Indian Ocean Tsunami Warning and Mitigation System. The July 2025 magnitude 8.8 earthquake off Kamchatka in Russia triggered warnings across the Pacific, and its tsunami caused little loss of life, which showed the value of such 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 begins the study of the forces that shape the land, namely exogenic and endogenic forces. Next topic: Exogenic and Endogenic Forces: Gradation, Degradation and Aggradation.

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

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