Page last updated at 09:08 GMT, Wednesday, 30 September 2009 10:08 UK

Islanders act on tsunami alerts

By Jonathan Amos
Science reporter, BBC News

Screen display of earthquake location (AFP)
Sophisticated systems now support scientists in their forecasts

The earthquake and subsequent tsunami that struck the South Pacific in the early hours of Wednesday were significant on all scales.

Fortunately, it appears the long established Pacific Tsunami Warning System was able to give some advance notice of incoming waves - not just their arrivals times but their likely heights.

Scientists are now alerted immediately to the dangers by extensive seismometer networks.

Computer modelling is able to make forecasts about probable wave generation, and tsunami gauges connected by satellite and radio links can report the movement of water in real-time.

Reports on the ground in Samoa and American Samoa cited walls of water in places in excess of three metres - large enough for the water to move considerable distances inland, where the coastal areas are relatively flat.

However, there was good evidence that the warnings - some of which will have been broadcast on TV and radio and via sirens - were heeded, with people evacuating to higher ground where possible.

The speed at which tsunami move means the waves would have hit land within tens of minutes, even though Samoa and American Samoa were some 200km from the epicentre of the quake.

It is clear many people acted on one of the classic indicators of an imminent tsunami: the rapid withdrawal of sea-water at the shore that occurs just prior to the arrival of the main wave front (although not all tsunami arrive in this fashion).

Seismometers recorded the initial earthquake event at 0648 Samoa time (1748GMT).

The US Geological Survey's rapid reporting system registered the quake at Magnitude 8.1. Others put it slightly higher at 8.3; other calculations slightly smaller. Some uncertainty is not unusual because of the complexity of the calculations involved.

Map showing location of Samoan tsunami

To put the event in context, scientists expect something of this size or bigger to occur perhaps just once or twice a year.

The tremor occurred at the north end of a major north-south tectonic plate boundary, close to the 10,000m-deep Tonga-Kermadec Trench. This is where the Pacific Plate, which comprises much of the central ocean floor, dives under (subducted) the Australian Plate.

There will obviously be immediate comparisons made with the great Indian Ocean quake in December 2004 and the tsunami it generated. The waves initiated then stemmed from a Magnitude 9.3 event - considerably bigger.

For every step on the magnitude scale, the power of the quake increases 30-fold.

"The earthquake magnitude and deformation were much less than in the rare huge earthquakes of Magnitude 9.0 and higher, such as that off Chile in 1960, and the Boxing Day 2004 earthquake off Sumatra," explained Gary Gibson, the chairman of the executive committee of the International Seismological Centre.

"The fault length for those was greater than 1,000km, and the plates moved 10-20m or more relative to each other."

Early calculations on the Samoa event suggested the fault rupture may have been 200-300km long with movement of about 4-7m, he added.

Whether a tsunami is triggered by an earthquake, though, depends on a number of factors, but principally on the amount of movement that occurs right at the ocean-floor itself.

If a quake is deep in the Earth, this may not translate to the uplift or slump of billions of tonnes of rock and mud at the sea-bed - the mechanism that disturbs the entire column of water above and sets off the tsunami.

"The size and pattern of the tsunami depend on the amount and orientation of the sea-floor deformation," said Gary Gibson.

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