Learn · Hazards · 7 minute read
Tsunamis
A wave as fast as a jet that you could not see from a ship. What sets tsunamis off, and the signs that one is coming.
A tsunami is not a big version of an ordinary wave. Wind waves stir only the top few metres of the sea. A tsunami moves the whole depth of the ocean, from the surface to the floor. That is why it carries so much energy, and why it can keep coming ashore for minutes at a time, more like a flood tide than a breaking wave.
Most tsunamis, 88% of those on record, are set off by earthquakes or by landslides that earthquakes trigger. The biggest come from subduction zones, where one plate is forced under another. When the locked plates suddenly slip, the sea floor jumps up or down by metres over a huge area, and the water above it moves too. Volcanoes, weather and even asteroids can also set off tsunamis.
Fast in deep water, tall in shallow
The speed of a tsunami depends on the depth of the water. In the open ocean it can travel at over 800 km an hour, as fast as a jet plane, and ships may not notice it pass beneath them because the wave is so long and low. As it reaches shallow water near land it slows to about 30 to 50 km an hour. The front slows first, the water behind keeps arriving, and the wave grows taller. Most tsunamis are less than 3 metres high, but in extreme cases they can top 30 metres.
Warning signs
Near the source there may be no time for an official warning. The natural warnings are a strong or long earthquake, a loud roar from the ocean, like a train or a plane, and the sea doing something strange, such as rushing out to expose the sea bed. Any one of these means go to high ground or inland straight away, and stay there. Tsunamis come as a series of waves that can be five minutes to two hours apart, and the first is often not the biggest.
26 December 2004
A magnitude 9.1 earthquake off northern Sumatra, Indonesia, sent tsunami waves across the Indian Ocean. They struck 15 countries in southern and south-east Asia and eastern Africa. About 230,000 people died and 1.7 million were displaced. It was the deadliest tsunami in history, made worse because the Indian Ocean had no official tsunami warning system at the time. It has one now.
Britain is not immune. About 8,150 years ago, an enormous underwater landslide off Norway, the Storegga slide, sent a tsunami across the North Sea. A study in 2020 found it struck what was left of Doggerland, the low land that then joined Britain to Europe. You can read about it in Ice ages.