Samoan tsunami caused by 'shallow quake'
Scientists say the tsunami that devastated the islands of Samoa, American Samoa and Tonga was the result of a shallow rupture in the earth's crust.
The earthquake, which was measured as high as 8.3 on the Richter scale, occurred 190 kilometres southwest of American Samoa.
Gary Gibson, a senior seismologist at Environmental Systems and Services in Melbourne, says the region experiences several magnitude 7 earthquakes each year, but a magnitude 8 is quite rare.
"The earthquake had a magnitude of about 8, which corresponds to a rupture within the earth that may be 200 to 300 kilometres long, with a depth of 100 to 200 kilometres, with one side of the fault moving four to seven metres relative to the other," says Gibson.
He says the energy released in the earthquake was approximately one-thirtieth the size of the Boxing Day 2004 earthquake near the island of Sumatra, Indonesia.
"This meant that a devastating tsunami could be produced locally, but it was unlikely to seriously affect countries on the boundary of the Pacific."
Gibson says residents of the nearby islands would have felt the earthquake last for at least a minute, which is a sure sign of an impending tsunami.
"Anyone who feels an earthquake last for more than 10 seconds, by definition, should expect a tsunami. It's nature giving you a warning," he says.
Gibson estimates that the tsunami would have taken 20 minutes to reach the islands of Samoa and American Samoa.
Reports from American Samoa say waves of up to three meters washed onto the island, causing widespread destruction.
Authorities have confirmed that the tidal surge killed 28 people across the islands, but they fear the final toll could be as high as 100.
After the earthquake, the Pacific Tsunami Warning Center issued a tsunami warning for South Pacific nations, including New Zealand, which experienced a rise in sea level of 40 centimeters.
Dr Huilin Xing of the Earth System Science Computational Centre at the University of Queensland says that while the tsunami warning system worked well, it has its limitations.
"The problem is that there were a lot of false alarms, because not all large earthquakes can generate a tsunami," says Xing.
"From research we know we can expect that if an earthquake is larger than magnitude 6.5 there may be a tsunami, but this is not directly or linearly related to size.
"Currently no single model can model the whole process of an earthquake and its triggered tsunami generation.
"This means we really need to keep looking deeper to work out what kinds of earthquakes can generate tsunamis and how big the tsunami might be."
Dr Ray Canterford of the Joint Australian Tsunami Warning Center at the Bureau of Meteorology says no tsunami warning was issued for Australia.
But he adds, "This morning's tsunami is a reminder of the terrible force that can be unleashed by earthquakes under the sea."http://www.abc.net.au/science/articles/2009/09/30/2700995.htm