Exploring the power of deadly waves

Hong Kong - Tsunamis like those that wreaked havoc across Asia on Sunday, leaving thousands dead, are massive waves that are usually caused by earthquakes deep under the ocean floor and can travel vast distances.

Born of strong seismic shocks, tsunamis can reach huge heights and speeds, picking up strength as they cross the ocean - often with disastrous results - thousands of kilometres from their origin.

Despite their strength, they can be barely noticeable out at sea.

"If you are on a boat, you might not even feel a tsunami," said Wong Wing-tak, senior scientific officer at the Hong Kong Observatory.

"It becomes powerful only when it is near the shore and reaches shallow water, which then can push waves over 10 times higher than the sea water level."

While they can also be caused by landslides, volcanic eruptions, the most common cause is an undersea earthquake, especially in areas such as the Pacific where there is significant movement of the Earth's tectonic plates.

"Tidal waves are not a common phenomenon as usually only an earthquake that's over 7.7 on the Richter scale is capable of causing tidal waves," Wong said.

"Tsunamis travel outward in all directions from the epicentre of an earthquake and can savagely attack coastlines," he said. "It can easily roll people out to the sea, it causes flooding, devastates property damage."

"The speed of tsunami is linked to the depth of the water. It can travel at speeds of several hundred kilometres per hour," he said.

In 1960 a huge tidal wave travelling at 750km an hour smashed into Japan, having been caused by a series of quakes in Chile on the other side of the Pacific Ocean. Hundreds were left dead.

In September 1992 a tsunami destroyed the homes of about 13 million people on the Nicaraguan coast.

Two months later villagers in Bali in Indonesia were swept by a series of giant waves that left thousands dead.

On July 17, 1998 two quakes that measured seven on the Richter scale caused tidal waves of 10m-high that ravaged a 30km stretch of the northern coast of Papua New Guinea. Seven villages were destroyed, and the official death toll was 2 123.

Alert to the destructive capacity of tsunamis, Pacific Rim countries co-ordinate and share their observations of the ocean. A tsunami alert centre in Hawaii collects information about possible tidal waves.

Smaller tidal waves can also be caused by weather phenomenon, notably extreme thermal changes which can lead to depressions that cause strong winds.