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Wednesday, December 28, 2005

Indian Ocean tsunami warning system

When the Indian Ocean tsunami struck, the only warning most people in the region had was the sight of a giant wave heading towards them.

Unlike the Pacific, the Indian Ocean did not have a system to alert residents of coastal areas that a tsunami was imminent.

In the aftermath of the disaster, scientists and governments, under the auspices of the UN, began working on an early warning system for the region.

One year on from the tsunami, this is a guide to what is planned and what is already in place.


Seismic gauges can detect the earthquakes or volcanic eruptions which may cause a tsunami.

But as only a small proportion of strong earthquakes produce a tsunami, a warning system based solely on seismic data is prone to producing false alarms.

Other sea-based instruments are needed to help scientists decide if a tsunami has been triggered.

These fall into two main types: pressure recorders in the deep ocean and tide gauges monitoring sea-level at the coast.

The Deep-ocean Assessment and Reporting of Tsunami (Dart) system uses buoys and sensors stationed far out to sea.

A pressure recorder on the sea bed measures the weight of the water above it - which varies according to wave height - and sends its findings to a buoy on the surface.

The buoy monitors the surface conditions and sends this, plus the data from the sea bed, to a satellite which relays it back to a receiving station.

Germany is working on a joint project with Indonesia to put in place 10 of these buoys, the first two of which were installed in November 2005.

India, Thailand and Australia are also planning to install Dart buoys along the Sunda Trench, the site of the earthquake that triggered the tsunami.

The advantage of the Dart system is that it can detect tsunami far out to sea and give enough time to warn countries in the region. However, the buoys are expensive to install and maintain.

Unesco's Intergovernmental Oceanographic Commission (IOC) is also focusing on a network of tide or sea-level gauges.

Unlike Dart buoys, tide gauges in the Global Sea Level Observing System (GLOSS) are sited on land, either on mainland coasts or on islands out to sea.

The most basic form of gauges monitor the surface of the water with a system of tubes and floats (as shown right).

More modern versions "ping" the surface of the water from above with radar or sonar; or use sea-bed pressure sensors attached to the sea-level observing station with a cable.

There are almost 70 GLOSS stations in the Indian Ocean. Before the tsunami, they were used to measure the sea level for longterm climate change studies, and their data was transmitted only periodically.

Now, the stations are being upgraded so they can send real-time data via satellite to newly set up national tsunami centres.

They are also being fitted with solar panels so they can continue to operate even if the mains power supply is interrupted by severe weather.

Twenty-three stations should be fully upgraded by the end of June 2006, according to the IOC, and more will follow in the next few years (see pop up map).



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