Scientists have discovered that the tsunami that devastated parts of Tonga and Samoa in September 2009 was caused by two simultaneous earthquakes.
186 people died when the tsunami struck in the Samoas and Tonga in September last year.
A principal scientist at New Zealand's GNS Science Dr John Beavan, says at first, scientists thought it was caused by one normal fault earthquake.
He says, the discovery that there were two different earthquakes was clinched when GPS data showed that the northern Tongan island, Niuatoputapu, which was badly hit, moved nearly 40 centimetres during the quake.
"We think that the two earthquakes kind of combined together there to make the tsunami bigger than it would have been otherwise. And it was big, my colleague Kate Wilson, visited Niuatoputapu, and they found that the tsunami wave, when it came in and hit the eastern end of the island, was 15 metres high."
Dr John Beavan says two earthquakes on different faults, at the same time is very rare.
Their findings appear this week in the science publication, Nature.