How are tsunamis formed by undersea earthquakes, and why are they so destructive when they reach the coast?
Explain how tsunamis form and describe their impacts on coastal areas
A focused answer to the O-Level Geography outcome on tsunamis. How an undersea earthquake displaces water to generate a tsunami, why the wave grows and slows near the coast, the warning signs, and the impacts on people and places, with a worked walkthrough.
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What this dot point is asking
SEAB wants you to explain how tsunamis form from undersea earthquakes and to describe their impacts on coastal areas. The central insight is that a tsunami is not a single wave whipped up by wind; it is a series of waves generated when the sea floor suddenly moves and displaces a vast volume of water, and the wave that seems harmless in the deep ocean grows into a devastating surge as it reaches shallow coasts.
The answer
How a tsunami forms
A tsunami is usually generated by a powerful undersea earthquake, often at a subduction zone:
- The earthquake causes the sea floor to suddenly move up or down as the plates slip.
- This sudden movement displaces a huge volume of water above it, pushing it upward.
- The displaced water spreads out from the area as a series of fast-moving waves, a tsunami.
Tsunamis can also be triggered by undersea landslides or volcanic eruptions, but undersea earthquakes are the main cause.
Why the wave grows near the coast
A tsunami behaves very differently in deep and shallow water:
- In the open ocean it travels extremely fast (as fast as a jet aircraft) but has a low wave height, so it is barely noticeable to ships.
- As it nears the coast, the water becomes shallower, which slows the wave down. Because the front slows while the water behind keeps coming, the energy is squeezed into a smaller depth, so the water piles up and the wave height grows dramatically, surging far inland as a destructive wall of water.
Warning signs
A natural warning sometimes precedes a tsunami: the sea may suddenly draw back, exposing the sea floor, as the trough of the wave arrives first. This is a sign to move to high ground immediately.
The impacts on coastal areas
Tsunamis are highly destructive:
- Flooding and destruction: a powerful surge sweeps away or destroys homes, buildings, roads and boats, and drowns people and livestock.
- Contamination: seawater and debris contaminate farmland and freshwater, destroy crops and ruin soil.
- Infrastructure damage: power, water and communications are knocked out, hampering rescue.
- High death tolls: because tsunamis can strike with little warning, arrive fast and powerfully, and hit low-lying, densely populated coasts, they often cause very large losses of life.
Examples in context
Example 1. The 2004 Indian Ocean tsunami. A massive undersea earthquake off Sumatra in December 2004 displaced the sea floor and generated a tsunami that swept across the Indian Ocean, devastating coasts in Indonesia, Thailand, Sri Lanka and India and killing well over two hundred thousand people. With no regional warning system at the time, many had no notice. It is the defining example of how a subduction-zone earthquake can unleash a tsunami of catastrophic reach.
Example 2. The 2011 Japan (Tohoku) tsunami. The magnitude-9 Tohoku earthquake generated a tsunami that overtopped sea walls along Japan's northeastern coast, sweeping inland and causing the Fukushima nuclear accident. Even in well-prepared Japan, with warning systems and defences, the sheer scale of the surge caused immense destruction and loss of life, showing both the value of preparation and the limits of defences against the largest tsunamis.
Try this
Q1. Explain how an undersea earthquake generates a tsunami. [2 marks]
- Cue. The earthquake suddenly moves the sea floor up or down as the plates slip, displacing a huge volume of water above it, which then spreads out from the area as a series of fast-moving waves.
Q2. Explain why a tsunami wave grows much larger as it nears the coast. [3 marks]
- Cue. In shallow water near the coast the wave front slows down, but the water behind keeps moving, so the wave energy is squeezed into a smaller depth; this makes the water pile up and the wave height grow dramatically into a destructive surge.
Q3. State one natural warning sign of an approaching tsunami. [1 mark]
- Cue. The sea suddenly drawing back and exposing the sea floor, which is the trough of the wave arriving before the surge and a sign to move to high ground immediately.
Exam-style practice questions
Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Original6 marks(a) Explain how an undersea earthquake can generate a tsunami. (b) Explain why a tsunami wave is small in the open ocean but becomes very large and destructive near the coast.Show worked answer →
(a) An undersea earthquake at a subduction zone causes the sea floor to suddenly move up or down as the plates slip. This sudden movement of the sea floor displaces a huge volume of water above it, pushing it upward. The displaced water spreads out from the area as a series of fast-moving waves, a tsunami.
(b) In the deep open ocean the tsunami travels very fast but has a low wave height, so it is barely noticeable to ships. As it approaches the coast, the water becomes shallower, which slows the wave down. Because the front slows but the water behind keeps coming, the wave energy is squeezed into a smaller depth, so the water piles up and the wave height grows dramatically, surging far inland as a destructive wall of water.
Markers reward the generation (sea floor displaced by the earthquake, water pushed up and spreads out) and the growth near shore (shallowing slows the wave, energy compressed, height builds up).
Original5 marksDescribe the impacts of a tsunami on a coastal area and explain why tsunamis often cause very high death tolls.Show worked answer →
A tsunami floods coastal land with a powerful surge of water that sweeps away or destroys homes, buildings, roads and boats, and drowns people and livestock. It contaminates farmland and freshwater with salt and debris, destroys crops, and can damage infrastructure such as power and water supplies. The water carries dangerous debris and, when it retreats, drags people and wreckage back out to sea.
Tsunamis often cause very high death tolls because they can strike with little warning, especially where there is no warning system, giving people little time to reach high ground. The waves arrive fast and powerfully, low-lying densely populated coasts are highly exposed, and many people may be unaware of the danger signs.
Markers reward specific impacts (flooding, destruction of buildings and infrastructure, drowning, salt and debris contamination) and reasons for high death tolls (little warning, speed and power, exposed populations, lack of awareness).
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