Risk Wise

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The first tsunami early warning

centre in the Indian Ocean

Shailesh Nayak and T. Srinivasa Kumar, Indian National Centre for Ocean Information Services

he oceans are a cornerstone of the life support system for

all creatures on the planet Earth, and the source of several

natural phenomena that are beneficial to humankind.

However, oceans are also associated with disastrous events such

as tsunami, cyclones and associated storm surges. Tsunami is a

system of ocean gravity waves formed as a result of large-scale

disturbance in the sea that occurs in a relatively short duration

of time. A series of oscillations both above and below sea level

take place during the process of the water returning to an equi-

librium position by the force of gravity. Thus tsunami waves are

generated, propagating outwards from the source region.

Most tsunamis are caused by earthquakes (of the magnitude of more

than 7 on the Richter Scale), with a vertical disruption of the water

column generally caused by a vertical tectonic displace-

ment of the seabed along a zone of fracture in the Earth’s

crust which underlies or borders the ocean floor.

Tsunamis are also generated by volcanic eruptions and

submarine landslides. The speed of tsunami waves

depends on the depth of the water, typically varying

from 500 to 1,000 kilometres per hour in the deep ocean

and a few tens of kilometres per hour near shore. The

time period of the tsunami wave ranges from a few

minutes to an hour or even more.

The Indian Ocean tsunami (26 December 2004) was

one of the strongest in the world and the deadliest of all

time by an order of magnitude. The Pacific Rim coun-

tries are affected frequently by tsunamis (about 900

The tsunami early warning centre at INCOIS

Image: INCOIS