BBC Homepage World Service Education
BBC Homepagelow graphics version | feedback | help
BBC News Online
 You are in: World: Asia-Pacific
Front Page 
Middle East 
South Asia 
From Our Own Correspondent 
Letter From America 
UK Politics 
Talking Point 
In Depth 
Monday, 5 June, 2000, 07:16 GMT 08:16 UK
Killer waves so hard to detect

The giant waves which periodically wreak havoc on coastal communities around the Pacific are commonly called tidal waves.

In fact they have little to do with tides and are more properly called tsunami - a Japanese term meaning harbour wave.

They are caused by seismic shocks under the ocean.

Earthquakes, volcanic eruptions or landslides on the sea bed can produce a type of wave which travels thousands of kilometres at high speed, and causes massive destruction when it reaches shore.

  • Tsunami differ from ordinary, wind-generated waves in that they can be almost imperceptible out on the open ocean, with typical heights of less than one metre. Most occur in the Pacific.

  • When a massive tsunami hit Japan at the end of last century, killing thousands, fishermen 20 miles out at sea failed to even notice the wave as it passed underneath their boats at a height of less than half a metre.

  • Tsunami typically have distances of several hundred kilometres and intervals of up to an hour between waves, unlike normal, wind-driven waves which are usually separated by intervals of seconds.

  • These characteristics mean tsunami travel at speeds of hundreds of kilometres per hour - as fast as a jet airliner.

  • As they approach shallower water near the coast, they are slowed down, which causes them to move closer together and to rise in height.

  • They can reach heights of 200 feet (61 metres) when they arrive on shore, with speeds of 150 miles per hour (241 kph), producing huge destructive force.


  • Tsunami waves are difficult to detect, and warnings have often proved unreliable.

  • One early signal is the detection of any earthquake on the ocean floor over magnitude 6.5 on the Richter scale.

  • The first sign for people on the coast of an incoming tsunami is often a sudden outrush of water, exposing the sea bed offshore and leaving boats stranded.

  • This is followed after a few minutes by a series of huge waves which rush inland, the largest of which is usually between the third and the eighth to arrive.

  • Because of the speed at which the waves travel, it can be assumed that anyone on the shore who sees one approaching is unlikely to survive to tell the tale.
  • Search BBC News Online

    Advanced search options
    Launch console
    See also:

    20 Jul 98 | Asia-Pacific
    'Thousands' dead in PNG disaster
    22 Jul 98 | Asia-Pacific
    Eyewitnesses tell their story
    Internet links:

    The BBC is not responsible for the content of external internet sites

    Links to more Asia-Pacific stories are at the foot of the page.

    E-mail this story to a friend

    Links to more Asia-Pacific stories