Tsunamis

FAQs

What is a tsunami?

A tsunami is a wave or series of waves generated at sea by the abrupt movement of a mass of seawater. This is usually caused by the sea floor moving suddenly. Tsunamis may also occur in large lakes.

What causes tsunamis?

A number of natural phenomena can cause the initial displacement of water that generates a tsunami.

Earthquakes that push up or depress the sea floor by a metre or more, large slippages of the sea floor down slopes, volcanic eruptions, and large volumes of ice breaking off glaciers or ice sheets can all cause tsunamis. 

Even large meteorite impacts may generate complex tsunamis as they enter the sea and then hit the sea floor.

How do tsunamis differ from ordinary waves?

Out in the open ocean a tsunami is only about 1m high and the distance between its crests or troughs (its wavelength) is typically 200km. In comparison, waves whipped up by winds during storms have peaks about 100m apart and heights ranging from a few metres up to 15m or more when generated by hurricanes.

Tsunamis travel fast in the open ocean, reaching 700km per hour or more. Wind-generated waves travel at only 30 to 100km per hour.

Why are tsunamis dangerous?

Tsunamis are generally unrecognisable at sea because the heights of their crests are relatively small, up to 1m, and successive crests are 15 to 20 minutes apart. However, when a tsunami enters shallower water near the coast it slows down dramatically and becomes compressed. The distance between peaks reduces by 10 times or more and its height increases up to 20 to 30m. 

A tsunami doesn't generally break on a beach like a typical wave. Instead, it moves rapidly over the coastline as a steep surging mass of water, destroying or damaging whatever is in its path as it moves forward and then retreats. 

Significant tsunamis often affect areas tens of metres above sea level. Some have reached even higher elevations.

If the trough of the tsunami reaches land first, the sea retreats dramatically before rising. If the crest arrives first, it causes a rush of water across the land with little or no warning. It can be travelling up to 100km per hour. 

Since tsunamis are usually a series of waves, multiple surges may flow across the coastline and the danger can last for many hours. The shape of coastlines may also deflect or reflect the tsunami causing complex arrivals of waves.

Where do tsunamis occur?

Tsunamis can occur in any ocean, sea or large lake. Most occur in the Pacific Ocean, originating in the zone around the Pacific known as the Ring of Fire where the destruction of tectonic plates by the process of subduction results in frequent earthquakes and volcanic eruptions.

Learn about subduction and plate tectonics

For instance, in March 2011, a tsunami hit western Japan causing 15,505 deaths, 5,389 missing persons, hundreds of thousands of refugees and widespread destruction of homes and industrial plants. 

Just over a year earlier, in February 2010, a tsunami caused more than 500 deaths and destruction in Chile, as well as some damage on the opposite side of the Pacific in Japan.  

Many tsunamis also occur along the eastern Indian Ocean between the Bay of Bengal and the islands of Indonesia, in another area where subduction is occurring. The most devastating of these occurred in December 2004 when 227,898 deaths were caused around the Indian Ocean by a tsunami originating off the coast of Sumatra.

Tsunamis have also resulted in deaths in the Caribbean and Mediterranean during the last 40 years.

How often do tsunamis happen?

Tsunamis are more frequent than you might think. Japan experiences tsunamis annually, but most are low magnitude events and do not cause serious damage or casualties.

More destructive large magnitude events are less frequent but seriously affect coastal communities. Between 1975 and 2000, 28 tsunamis are recorded as causing deaths around the world.

What warning do we have?

There is a tsunami detector network across the Pacific Ocean. These detectors send a message to the Tsunami Warning Centre in Hawaii. The centre checks that there is a tsunami and sends messages to local communities that might be affected.

People living near the epicentre of the earthquake and the site where the tsunami was generated will probably not get the message in time, but others living a couple of hours away may have time to evacuate.

Following the 2004 Indian Ocean Tsunami a warning system has been installed in Indonesia, but a coordinated system across the Indian Ocean is still being planned.

Land snails, Sri Lanka

Tsunamis preserve land snails in Sri Lanka

For 10 years two Museum scientists, Fred Naggs and Dinarzarde Raheem, have been studying Sri Lankan land snails.

They carry out surveys with local partners to find out which land snails live in the area and produce guides that local people can use to identify snails themselves. Find out more about the project and the illustrated guide to Sri Lankan snails they produced.

Snail shells on the beach at Unawatuna, Sri Lanka.

Fred was in Sri Lanka shortly after the Indian Ocean tsunami struck in December 2004.

When he returned he wrote an article about his experience. In it he describes the devastation wrought by the tsunami and the fortunate escape of one of his friends.

While Fred was there, he saw lots of land snail shells on the beach and realised that a tsunami helps to form and preserve fossils.

Tsunamis that happened millions of years ago dragged land snails out to sea and dumped lots of mud and sand on top of them. Under these conditions fossils are formed and can last for a very long time, whereas fossils on land can be lost due to erosion.

Sediments, Thailand

Tsunamis churn sand and mud in Thailand

Museum scientist Gordon Paterson has been investigating marine sediments like mud and sand on beaches in Thailand since 1997.

In December 2004 a tsunami wave hit the beaches of Thailand and elsewhere around the Indian Ocean. The beaches that Gordon had been studying were affected.

The accommodation block at Ranong station after the tsunami

Gordon and his colleague Mike Kendall from Plymouth Marine Laboratory, together with local partner Chittima Aryuthaka from Kasetsart University, Thailand, were asked to do a survey to look at the impact of the tsunami on natural resources.

Supported by funding from the European Union, this project was combined with a community monitoring programme to look at the impact on local people.

'We analysed data to see how the natural environment has changed,' says Gordon.

'The tsunami had caused a great deal of rearrangement of mud and sand. Lots of the organisms that live in mud can’t live in sand. Also, there had been an impact on what fisherman had been collecting.'

Many people died in the tsunami and it has taken a long time for communities to recover. But Gordon and his team found that the plants and animals have made a speedier recovery. Many species have evolved to recover after huge storms and to them the tsunami was no worse than a storm.