Two quakes caused Samoan tsunami - scientists

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A rare combination of two massive earthquakes happening almost simultaneously triggered the tsunami that devastated parts of Samoa and Tonga.

New Zealand researchers have spent months trawling through seismic and GPS measurements, and today published their findings in the international scientific journal Nature.

Until now, it was believed a single earthquake caused the tsunami in September last year, which killed at least 120 people, said the article's lead author, John Beavan.

Dr Beavan, a GNS Science geophysicist, said that, in fact, a double strike of two large earthquakes within two minutes and 70 kilometres of each other increased the size and destructiveness of the tsunami.

He credited sheer "detective work" by the scientists who had initially been unable to reconcile two sets of perplexing data gathered during the tsunami, until they realised it was generated by two earthquakes, each about magnitude 8.

One quake initially hid the other, he said. "We know that one of the earthquakes triggered the other in some way. The thing we can't be certain about was which one came first."

Usually, secondary earthquakes did not happen until weeks or sometimes months after the initial quake they were triggered by.

The tsunami that followed the quakes almost devastated the Tongan island Niuatoputapu, and Dr Beavan said when scientists received GPS data a few weeks later, they saw it had moved nearly 400mm to the east.

The displacement was much bigger than expected and in a completely different direction.

Looking again at the GPS data, and comparing that with readings from tsunami gauges, they established that two different kinds of earthquakes happened.

One was a "normal" earthquake, where a break occurred in the tectonic plate; the other was a "thrust" event, which had a compressing effect on the Earth's crust.

It was possible scientists had not recognised "large double-whammy earthquakes" in the past.

It was also possible two large and nearly simultaneous earthquakes could happen on the plate boundary under New Zealand. "This is a rare phenomenon, but it is possible."

GPS stations which helped scientists predict the impact of earthquakes and the possibility of tsunamis were located around New Zealand. However, they did not record real-time data.

More investment and more time were needed to upgrade those GPS stations, Dr Beaven said.

Another research paper on the tsunami, written by American and French seismologists, was also published in Nature today.

Kenji Satake, from the Earthquake Research Institute at Tokyo University, said those scientists used different research methods but came to similar conclusions about the September quakes.

- © Fairfax NZ News