Tsunami causes

Panic: People flee as a Tsunami wave crashes into trees in Indonesia on Boxing Day, December 26, 2004

Wave: Japan has frequent Tsunamis

What is a Tsunami?

Tsunami is a Japanese word meaning ''harbour wave'' that refers to a series of large ocean waves that hit a shoreline. These waves may be as long as 100km and travel across the ocean at speeds of up to 800kmh. There may be a constant stream of waves that that batter the shore for between 10 and 60 minutes.

What causes a Tsunami?

Tsunamis are caused by sudden movements of the earth that happen under the sea. Often the most destructive Tsunamis are caused by earthquakes but causes can also include volcanic eruptions, landslides or even a comet hitting the sea.

Friction: Tectonic plates rub against each other

How do undersea earthquakes start?

The earth sits on about a dozen tectonic plates. These are large floating peices of hard rock that are constantly moving and fit together around the world like a jigsaw. Undersea earthquakes happen when one of these plates is rubbing against another at a plate boundary. The two plates may become stuck as the heavier plate tries to slide under the lighter other. This causes a build up of pressure in a process knows as subduction.
As the heavier plate continues to slide beneath the lighter plate, it causes the lighter plate to bend downwards with the pressure. A point comes when the lighter plate can no longer take the intense pressure and suddenly snaps back up to the surface where it had been before.
The incredible force of the earths plate shooting upwards in the water causes huge rise in sea level. A vast body of water moves upward - like a huge mountain of water in the sea.

How does the Tsunami develop?

Everybody knows that what goes up must come down. This is particularly true for water which always likes to form a nice flat surface. So once the mountain of water has risen up the next step is for the sea to level itself out.
The mountain of water comes back down. This pushes the water that was underneath it outwards. The force of the water moves through the ocean causing an underwater force that travels for hundreds of Kilometres. The force of the water can reach speeds of up to 800kmh as it surges through the ocean. The energy is underwater and is not noticeable on the surface.
As this force travels through the ocean it may eventually reach the shore. At this point, the sea becomes shallower. However, the energy in the water is still the same. The enegery is compressed and the water is pushed upwards. This is how the energy is transferred from being underater into waves on the surface.

• National Earthquake Information Center - NEIC
• NOAA - National Oceanic and Atmospheric Administration
• USGS - U.S. Geological Survey
• NOAA Pacific Tsunami Warning Center
• PNSN - Pacific Northwest Seismic Network

Can anything be done?

 Unfortunately nothing can be done to prevent Tsunamis. However, there are several organisations that use complex technology to monitor movement of the earths plates and sudden changes in water movement. There are also warning and evacuation procedures in place around countries like Japan and Hawaii where Tsunamis are frequent.
Any sudden earthquake that happens underwater will be detected in the same manner of on on-shore earthquake. These are measured in the Richter scale. If this is recorded then warning systems can sometimes be activated to evacuate people.

Wreckage from Aleutian Islands

Tsunami history

• Indonesia (mentawai) - 25 october 2010
• Indonesia - 26 December 2004
• Papua New Guinea  - 17 July 1998
• Sea of Japan - 26 May 1983
• Alaska Britis Columbia - 27 March 1964
• Chili - 22 May 1960
• Aleutian Islands  - 1 April 1946