Tsunami generation and propagation: WACE Year 12 Earth and Environmental Science
“Explain how tsunamis are generated and how they propagate and amplify at the shore”
A tsunami forms when a large volume of water is suddenly displaced, most often by a subduction-zone earthquake that thrusts the sea floor up or down, but also by underwater landslides or volcanic collapse. In deep water the waves travel very fast with low height and long wavelength, so they pass unnoticed. As they reach shallow water near the coast they slow and pile up, growing into a destructive surge.
What this dot point is asking
SCSA wants you to explain the physics of tsunami generation and why a barely noticeable deep-ocean wave becomes a coastal disaster. The key contrast is between deep-water and shallow-water behaviour, driven by conservation of energy.
Generation
A tsunami needs a sudden vertical displacement of a large volume of water.
- Subduction-zone earthquakes are the main cause. When the overriding plate snaps upward in a large undersea thrust earthquake, it lifts the water column above it, setting the whole ocean depth into motion.
- Underwater landslides and volcanic collapse or eruption can also displace enough water to generate tsunamis.
Because most great undersea thrust earthquakes occur at subduction zones, tsunamis cluster around the Ring of Fire, including the Sunda Trench north of Australia.
A tsunami is not a surface wave like a wind wave; it sets the entire depth of the ocean in motion. That is why it carries enormous energy and why its speed depends on water depth, travelling fastest in the deep ocean and slowing as it reaches the shallow coast.
Propagation in deep water
In the open ocean a tsunami behaves very differently from a normal wave.
- It travels extremely fast, comparable to a jet aircraft, because speed increases with water depth.
- Its height is small, often under a metre, and its wavelength is very long, so ships at sea barely feel it.
- It can cross an entire ocean while losing little energy.
Amplification at the shore
As the tsunami enters shallow water near the coast, conservation of energy transforms it.
- The front slows as depth decreases, so the back of the wave catches up and the water piles up, a process called shoaling.
- Wavelength shortens and height grows dramatically, producing a fast-rising surge or wall of water.
- Often the sea draws back first, exposing the sea floor, a warning sign that water is being pulled into the building wave (drawback).
- The surge can flood far inland, and several waves usually arrive, with later ones sometimes larger.
Tsunami: a series of waves from sudden displacement of water. Subduction-zone earthquake: an undersea thrust quake, the main tsunami cause. Shoaling: slowing and growth of a wave entering shallow water. Wavelength: the distance between successive wave crests. Drawback: the retreat of the sea before a tsunami surge.
Why Western Australia is exposed
WA's northwest coast faces the Sunda subduction zone, so a great earthquake there can send a tsunami across the eastern Indian Ocean to the WA coast within hours. The 2004 Indian Ocean tsunami, generated by a giant subduction earthquake, killed huge numbers of people around the ocean and reached WA, prompting the development of regional warning systems.
A question may give deep-ocean speed and height and ask why the coast is at risk. Explain that the wave travels fast and low in deep water, carrying huge energy with little visible sign; on reaching shallow water it slows, so the energy compresses into a shorter, much higher wave that surges inland. Link the change to conservation of energy.
Why tsunamis behave as shallow-water waves everywhere
A subtle but powerful idea SCSA rewards is that a tsunami behaves as a shallow-water wave even in the deep ocean. A wave is classed as shallow-water when its wavelength is much greater than the water depth, and a tsunami's wavelength can be over 100 kilometres while the ocean is only a few kilometres deep, so the condition is always met. This is why its speed depends only on water depth and not on its period: the deeper the water, the faster it goes. The practical consequence is that a tsunami refracts and slows over shallow continental shelves and can be focused by undersea ridges and bays, so two stretches of coast the same distance from the source can experience very different wave heights. Understanding the wave as a shallow-water wave therefore explains both its trans-ocean speed and the uneven, sometimes locally amplified, run-up at the coast.
- Calling a tsunami a giant wind wave
- It involves the whole water depth, not just the surface.
- Saying height stays constant
- Height is small in deep water and grows greatly at the shore.
- Forgetting drawback
- The sea often retreats first, a key natural warning.
- Assuming one wave
- Tsunamis arrive as a series, and later waves can be larger.
Exam-style questions
Questions in the style of SCSA exam questions on this dot point, each with a worked answer. They are written by ExamExplained unless tagged "Past paper"; the year shows the paper a question is modelled on.
WACE 2021-style6 marksIn the deep open ocean a tsunami was recorded travelling at about 720 km/h with a height of only 0.6 m, yet it reached over 8 m at the coast. Explain, using the physics of wave behaviour, why the wave was barely noticeable at sea but devastating at the shore.Show worked answer →
A 6 mark answer rewards the deep-versus-shallow contrast grounded in conservation of energy.
- At sea
- A tsunami sets the whole ocean depth in motion. Its speed depends on water depth, so in deep water it travels extremely fast (720 km/h, like a jet) with a very long wavelength and small height (0.6 m). Spread over a long wavelength and the full depth, the energy produces little surface change, so ships barely feel it.
- At the shore (shoaling)
- As the wave enters shallow water the front slows (speed falls with depth) while the back, still in deeper water, catches up. The same energy is forced into a shorter wavelength and shallower water, so the height grows dramatically (to over 8 m), producing a destructive surge.
- Conservation of energy
- The wave's energy is roughly conserved; as speed and wavelength fall near shore, height must rise to compensate.
Markers reward speed-depends-on-depth, the shoaling pile-up, and the energy-conservation link explaining the height increase.
WACE 2023-style7 marksExplain how a subduction-zone earthquake generates a tsunami and why the northwest coast of Western Australia is exposed to tsunami hazard.Show worked answer →
A 7 mark answer needs the generation mechanism plus the WA exposure reasoning.
Generation. At a subduction zone the overriding plate is dragged down and locked against the descending plate, accumulating strain. In a large undersea thrust earthquake the locked zone suddenly ruptures and the seafloor snaps upward (or drops), displacing the entire water column above it. This sudden vertical displacement of a huge volume of water sets off the tsunami, which radiates outward at high speed.
WA exposure. The Sunda subduction zone (Sunda Trench) lies to the northwest of Australia. A great earthquake there can generate a tsunami that crosses the eastern Indian Ocean and reaches the northwest WA coast within a few hours, as occurred in 2004. The exposed, relatively populated northwest coast faces this source directly, which is why a regional warning system was developed.
Markers reward the lock-rupture-uplift-displacement mechanism and the point that WA faces the Sunda subduction source across the Indian Ocean.