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Plans are under way to install a deep ocean tsunami warning system in the Indian Ocean.Paul Grad reports.

There is no deep ocean tsunami warning system for either the Atlantic or the Indian Oceans, and so, although devastating tsunamis have occurred in both, no offi cial warning was issued in any of the countries affected by the Boxing Day tragedy.

Several experts say a warning should have been issued based on seismic data alone, given the intensity of the earthquake that caused the tsunami - magnitude 9.

Geoscience Australia, the national agency for geoscience research and geospatial information, detected the earthquake and notified Emergency Management Australia, as it is obliged to do by law, but did not issue a tsunami warning.

Only the US attempted to alert countries in the region, in some cases through frantic phone calls.

Major tsunamis are relatively rare in the Indian Ocean, but after the damage and loss of life caused by last year's event, the authorities in the affected countries decided a deep ocean tsunami warning system had to be established.

Any warning system for the Indian Ocean will probably draw on the technology and experience of the Pacific Ocean system - the only one in existence - which is served by an international programme, the Tsunami Warning System (TWS).

This involves many seismic and tide monitoring and communication facilities operated by most of the bordering nations.

The US tsunami warning system evolved from one based solely on seismic and coastal sea level data.

While these remain essential to the system, they are limited. First, seismometer-based assessments are not based on direct measurement and second, coastal sea level stations do not provide a direct measurement of deep ocean tsunami energy moving toward a faraway community.

These limitations were the main motivation behind the development of the Dart (Deep Ocean Assessment and Reporting of Tsunamis) project for the early detection and real time reporting of tsunamis in the open ocean.

Dart is operated by the US National Oceanic and Atmospheric Administration (NOAA), as part of the larger US National Tsunami Hazard Mitigation Program.

The system includes six sets of seabed bottom pressure recorders (BPRs), each linked to a communications buoy, in locations across the Pacific . The battery powered BPRs are capable of detecting and measuring tsunamis with amplitude as small as 10mm at a depth of 6,000m.

Under normal conditions, a BPR sends four 15-minute values hourly, each of which are averages of readings taken every 15 seconds. The BPR predicts water height values and compares all new samples with predicted values. If two 15-second water level values exceed predicted values the system goes into tsunami response mode.

The BPR transmits to the buoy which radios the data to a geostationary satellite. This transmits to ground stations before the signal goes to warning centres such as the Pacific Tsunami Warning Center in Hawaii.

This is one of two tsunami warning centres operated by NOAA (the other is the West Coast and Alaska Tsunami Warning Center). Hawaii also receives data from land-based seismographs and then decides on a proper course of action to warn coastal communities.

But there has been criticism over the effectiveness of Dart, particularly maintenance procedures which have meant the BPRs and buoys are not always fully functional. Operation of the buoys in the high latitudes of the north Pacific is particularly challenging.

The US announced plans for an improved system earlier this year.

Additional funding has been committed over the next two years as NOAA deploys 32 new advanced technology Dart buoys.

According to NOAA, the new system will provide the US with nearly 100% detection capability for a US coastal tsunami, allowing response within minutes. It will also expand monitoring capabilities throughout the entire Pacific and Caribbean basins, providing alerts for regions bordering half of the world's oceans.

The Dart system could also be enhanced. NOAA Tsunami Research Program senior scientist Harold Mofjeld says he envisages using the Iridium low-orbit satellite constellation as a two-way communication system for the BPR and buoy sets.

A satellite could request a BPR to go into rapid response mode, greatly speeding up the warning process.

The information from the set would be processed at a land-based station, bypassing the BPR processing which would be retained as a backup.

But University of California deputy director for research at the Scripps Institution of Oceanography, Professor John Orcutt, has serious misgivings about the maintainability of the proposed Dart system.

In testimony to the US House Science Committee in January, he expressed concern over the maintainability of the Dart buoys in the light of high costs and the great length of time between tsunami events.

Within the next few years, tsunami warning could be enhanced by the National Science Foundation's Orion (Ocean Research Interactive Observatory Networks) programme, a project Orcutt recommends.

The scheme is in the US budget for 2007 with $269M (£140M) funding to be spread over six years. It envisages the development of a system of seafl oor observatories linked by wireless and optical networking.

Orcutt said this would use much more capable buoys and real-time telemetry. He said much better warning could be achieved with more, better distributed and high fidelity seismic stations.

In his testimony, he said a 1,200km length of seafloor ruptured in the Sumatra earthquake which caused the Boxing Day tsunami.

The rupture took at least six minutes to propagate.

The National Earthquake Information Center (NEIC) in Golden, Colorado, estimated its magnitude at 6.2 on the Richter scale, 16 minutes after it began. The low estimate was due to the limited information available in the early minutes after the earthquake.

Orcutt says: 'When a large earthquake occurs, a seismograph appears differently when viewed from different directions. If the fault breaks toward the station, the sum of the rupture velocity and the wave propagation velocity will cause the event to appear compressed in time.

If the rupture front propagates away from the seismic station, the two speeds will combine to lengthen the seismogram in time.' To fully understand the magnitude of a great earthquake, seismic stations with high fidelity must be available from as many directions as possible, he believes. The higher the density of seismic stations, the more rapidly the accurate size of an event can be determined, as well as whether it is deep and not likely to cause a tsunami, or shallow and likely to do so.

'Because of the sparse distribution of high-quality seismic stations around the Sumatra earthquake, what is theoretically possible in 14 minutes took considerably longer, ' says Orcutt.

He believes an alternative may lie in exploiting the Global Positioning System (GPS). This would use ocean buoys and ships but would not require communications with the seafloor.

Horizontal tsunami motions are substantially larger than vertical displacements and should be detectable from a buoy or a ship under way. Horizontal resolution is also better than vertical in these instruments. 'Horizontal resolution with errors less than 3cm has been achieved on

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