Double quake caused 2009 tsunami in Samoa and Tonga

[JAKARTA] The deadly tsunami that hit Samoa and Tonga in 2009 was caused by rare back-to-back earthquakes that occurred within a minute of each other, according to researchers.

They added that if this had been understood at the time, it may have significantly reduced the number of people who were exposed to the subsequent tsunami.

The findings, published last week (19 August) in two separate papers in Nature, add another layer of complexity to the early detection of tsunamis, say the authors. The 12-metre tsunami killed almost 200 people on 29 September last year.

The data initially indicated a large 8.1 magnitude quake occurred some 100km away from the nearest subduction zone — where two tectonic plates meet. Although large, it would not normally cause a tsunami of such size and force.

The deadly waves that hit Samoa and Tonga indicated a "thrust" event with vertical movement that displaces water in a manner known to cause large tsunamis, but unlikely to result from the first type of earthquake that had been detected.

It was not until scientists realised that two different types of quakes had occurred that the size of the tsunami made sense.

Because the earthquakes were so close together in time and location, they produced a confusing jumble of evidence that, initially, left scientists puzzled.  

"This was a bigger displacement than expected and the resulting wave moved in a totally different direction than we predicted," John Beavan, a geophysicist at New Zealand research institute GNS Science and lead author of one of the Nature reports, told SciDev.Net. "We’re now sure that one event led to the other, but it took some time to figure out."

Beavan said understanding the event might make it easier to predict the deadliness of future tsunamis.

He said the key to predicting tsunamis lay with sophisticated GPS devices that read ground displacement in millimetres in real time. "If we could have seen the second thrust quake in real time, perhaps we could have allowed a more accurate prediction of the tsunami’s effects and provided better warning to the people who were in harm’s way," Beavan said.

He said that covering the seabed with GPS devices would be a major undertaking but worth the cost and effort.

But Fauzi, head of the tsunami and earthquake centre at the Meteorology and Geophysics Agency in Indonesia, said that sophisticated GPS equipment was only of value if backed up by proper warning infrastructure on land.

"We can get real-time information on seismic events and issue warnings within minutes using this GPS equipment," said Fauzi. "The question, and the potential problem, is how to disseminate information to the public and whether local governments are prepared."

Link to first full paper in Nature

Link to second full paper in Nature