Explain how tsunamis are generated and how they propagate and amplify at the shore

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.

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.

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.

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.