PUBLISHED : Monday, 05 October, 2009, 12:00am
UPDATED : Monday, 05 October, 2009, 12:00am

Huge wave

Everybody knows a tsunami is a huge wave. But really they are a series of waves. Usually they are caused by an earthquake under the sea. But they can also happen because of a volcanic eruption, or a meteor hitting the sea, or by a large landslide. Tsunami is a Japanese word. Tsunamis are very common there - nearly 200 have been recorded in history.

Tsunamis are caused by lots of water suddenly being moved. Typhoons, cyclones and hurricanes can cause big waves. These are different from tsunamis. They are called storm surges. The worst one recently was from Cyclone Nargis in Mynamar. Nobody knows how many people were killed - tens of thousands, perhaps even hundreds of thousands.

Shallow waters

It is difficult to detect tsunamis out in the ocean. This is because in deep waters they are usually very small. Often a tsunami at sea will seem like a swell of water. They are usually only around 30cm in height. But they are extremely powerful.

The power of a tsunami becomes obvious when they reach shallow water. As the water gets shallower the wave gets bigger. Usually they don't look like big surf waves with foam. They're more like a rapidly moving hill of water. Before they arrive, water will be drawn out to sea. Some people who know about tsunamis survive because they escape when they see the water disappearing.

Recent tsunamis

The 2004 Indian Ocean tsunami was the biggest in recent times. It took place on Boxing Day - the day after Christmas. It was caused by an undersea earthquake of between 9.1 and 9.3 magnitude. It lasted longer than any other earthquake we have ever recorded - nearly 10 minutes. The tsunamis it caused killed nearly 230,000 people in 11 countries.

The south of Thailand was hit badly by the tsunami. A 10-year-old girl called Tilly Smith was on holiday in Phuket with her mother and father just before the tsunami struck. When she saw the water disappearing out to sea, she told them a tsunami might be coming. They told other people and saved dozens of lives. Tilly said her geography teacher had saved her life by teaching her about tsunamis at school.

Last week, a series of tsunamis pounded the South Pacific islands, killing more than 200 people. A magnitude 8.0 quake had struck off Samoa before the islands were soon engulfed by 4- to 6-metre waves. The waves reached up to 1.5 kilometres inland.

Interesting facts

The biggest-ever tsunami happened in Lituya Bay, Alaska. In 1958 there was a huge collapse of ice and rock into the sea. It caused a wave that was 500 metres in height - that's half a kilometre!

You cannot run away from a tsunami once you see it hit the shore. By the time they make landfall, tsunamis are usually travelling at around 70kmph. Also, if an earthquake happens not far out to sea, a tsunami can strike very quickly. In 1993, one hit Hokkaido in Japan two minutes after an earthquake. The wave was 30 metres tall.

now do this

1 A big wave caused by a cyclone or typhoon is called a ...

a. tsunami

b. deluge

c. storm surge

2 In deep water, a tsunami is usually around ... high.

a. 30cm

b. 30 metres

c. 70 metres

3 Just before a tsunami, the water on the beach ...

a. starts going out to the sea and disappearing

b. dries up

c. starts forming into a huge wave


September 28: 1. b, 2. c, 3. a