Tsunami, not easily detected

Tsunami (pronounced soo-nah-mee) comes from the Japanese words ‘tsu’ (harbour) and ‘nami’ (wave).

It is usually caused by underwater earthquakes and volcanic eruptions occurring less than 50 km beneath the sea floor, with a magnitude higher than 6.5 on the Richter scale.

Once a tsunami is generated, its energy is distributed through-out the water column, regardless of the ocean's depth.

A tsunami is made up of a series of very long waves. The waves travel outward from the source area, like the ripples caused by throwing a rock into a pond.

The deeper the water, the greater the speed of tsunami waves.

In the ocean, tsunami wave amplitude is usually less than one metre.

The indication of tsunami waves may be more than a hundred km away.

Therefore, passengers on boats will not feel or see the tsunami waves as they pass by underneath at high speeds.

For the same reason, tsunami waves cannot be seen or detected from the air.

Tsunami may reach a nearby shore in less than 10 minutes and there is not sufficient time for the local authorities to issue a warning. For people living near the coast, the shaking of the ground is a warning that a tsunami may be coming.