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Killer waves so hard to detect
The giant waves which periodically wreak havoc on coastal communities around the Pacific are
commonly called tidal waves.
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.
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.
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