Tsunamis

Overview

How an earthquake generates a tsunami and what happens to a tsunami as it approaches the coast.

What you need to know

Answer each one before you reveal it.

Questions from this subtopic

Each one with the model answer underneath.

1. 1

   What is a tsunami, and why is 'tidal wave' the wrong name for it?

   Model answer

   A tsunami is a series of very long waves generated by the sudden displacement of a large volume of sea water, most often by the vertical movement of the sea floor during an earthquake. It has nothing whatever to do with the tide, which is produced by the gravitational pull of the moon and sun, nor with the wind, which produces ordinary waves in the surface layer only. A tsunami sets the entire depth of the water column in motion, from the sea bed to the surface, which is why it carries so much energy.

2. 2

   Explain how an earthquake generates a tsunami.

   Model answer

   At a destructive margin the overriding plate is dragged down and compressed as the subducting slab sticks against it. When the fault finally ruptures, the leading edge of the overriding plate springs back upwards, lifting the sea floor and the entire column of water above it by several metres over an area that may be hundreds of kilometres long. Gravity then pulls that displaced water back down, and the disturbance spreads outwards in both directions as a train of long waves. The essential requirement is vertical displacement of the sea bed, which is why great subduction thrust earthquakes generate tsunamis and horizontal slip on conservative faults generally does not.

3. 3

   What other events can generate a tsunami?

   Model answer

   Any sudden displacement of a large volume of water. Submarine landslides, often triggered by an earthquake, can displace water and generate locally very high waves. The collapse of the flank of a volcanic island into the sea does the same, as does a large explosive eruption or caldera collapse at a coastal or island volcano. A landslide or glacier calving into a confined bay produces a very large local wave. Meteorite impact is a theoretical cause. Of all these, subduction zone earthquakes are much the most common source of ocean-crossing tsunamis.

4. 4

   What does a tsunami look like in the deep ocean, and why is it almost undetectable there?

   Model answer

   In water several kilometres deep a tsunami has an extremely long wavelength, often 100-200 km, but a very small amplitude, frequently under a metre. A ship on the open ocean would rise and fall by that small amount over many minutes and the crew would notice nothing. The wave is undetectable to the eye because the energy is spread through the whole depth of the water column and along a very long wavelength rather than being concentrated at the surface. Only pressure sensors on the sea bed can reliably record it out at sea.

5. 5

   Why does a tsunami travel so fast in deep water?

   Model answer

   Because the speed of a very long wave depends on the depth of water it is crossing: the deeper the water, the faster it travels. In an ocean basin around 4000 m deep the wave moves at roughly 700-800 km per hour, comparable to the cruising speed of an airliner, so it can cross an entire ocean in less than a day. This is why distant coasts may have several hours of warning while those close to the source have only minutes.

6. 6

   Explain what happens to a tsunami as it approaches the coast.

   Model answer

   As the water shallows, the wave slows down sharply because its speed depends on depth. The energy in the wave cannot disappear, so as the front slows and the back catches up the wave shortens and the water piles up: the wavelength decreases and the amplitude increases greatly. A wave under a metre high in the open ocean can therefore build to several metres or, in the worst cases, tens of metres at the shore. This process is called shoaling, and it is why a tsunami is harmless at sea and catastrophic on the coast.

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