Tsunami hazards: HSC Earth and Environmental Science Module 6

What this dot point is asking

NESA wants you to explain what generates a tsunami, how it behaves differently from an ordinary wind wave as it crosses the ocean and reaches the coast, the hazards it produces, and how warning systems reduce risk. You need the physics of propagation and shoaling, plus a sense of the Australian region's exposure.

The answer

A tsunami is a series of ocean waves generated by the sudden displacement of a large volume of water. It is not a single wave and is unrelated to wind or tides; the older term tidal wave is misleading. Understanding tsunamis ties directly to the plate tectonics of this module, because most are triggered by undersea earthquakes.

Causes

The most common cause is a large, shallow undersea earthquake at a subduction zone, where the sudden vertical movement of the sea floor lifts the entire water column above it. Other causes include submarine landslides, volcanic eruptions and caldera collapse (such as Krakatoa in 1883), and, rarely, meteorite impacts. The 2004 Indian Ocean and 2011 Tohoku (Japan) tsunamis were both generated by giant subduction-zone earthquakes.

Propagation across the ocean

In the deep ocean a tsunami has a very long wavelength (often over 100 kilometres) but a small height (less than a metre), so ships at sea may not notice it. Because its wavelength is far greater than the ocean depth, it behaves as a shallow-water wave, and its speed depends on water depth: in deep ocean it can travel at the speed of a jet aircraft, around 700 to 800 kilometres per hour. It loses little energy crossing open water, so it can strike coastlines thousands of kilometres from its source.

Shoaling and coastal impact

As the tsunami enters shallow water near the coast, it slows down. The front slows before the water behind it, so the wave compresses: wavelength shortens and the energy piles up into height, a process called shoaling. A wave a metre high at sea can build into a destructive wall several metres high at the shore, surging far inland as a fast-moving flood rather than a single breaking wave. A leading trough sometimes arrives first, drawing the sea dramatically back from the beach, a natural warning that the water is about to return. The hazards include drowning, structural destruction by the moving water and debris, contamination of fresh water and farmland by salt, and, as at Fukushima in 2011, damage to coastal infrastructure.

Warning systems and the Australian region

Australia is not on a plate boundary, but it faces subduction zones to its north and east (Indonesia, New Guinea, the New Hebrides and Tonga trenches), so a tsunami could reach the coast within hours. The Australian Tsunami Warning System, run jointly by the Bureau of Meteorology and Geoscience Australia, combines seismographs that detect large undersea earthquakes, deep-ocean pressure sensors (DART buoys) and coastal tide gauges that confirm a wave is travelling, and models that forecast arrival times. Warnings give time to evacuate low-lying coasts. Because deep undersea earthquakes cannot yet be predicted, the system relies on rapid detection and communication rather than forecasting the trigger.

Try this

Q1. Explain why a tsunami that is barely noticeable in the deep ocean can be devastating at the coast. [3 marks]

• Cue. As it enters shallow water it slows and compresses; energy transfers from wavelength into height (shoaling), building a destructive surge.

Q2. Describe two components of a tsunami warning system and the role of each. [4 marks]

• Cue. Seismographs detect large undersea earthquakes that may generate a tsunami; deep-ocean pressure sensors (DART buoys) and tide gauges confirm a wave is travelling so warnings and evacuations can follow.