New research by New Zealand Institute of Geological and Nuclear Science (GNS) has found that the deadly tsunami that struck the Pacific last year was not caused by a single quake, as previously thought, but by a rare double earthquakes.

Global earthquake readings and GPS recordings from Samoa initially showed the Sept. 29, 2009 tsunami as a single "normal- faulting" 7.9 magnitude quake but just six weeks after the event, measurements from a small Tongan Island showed that there were in fact two large quakes that occurred within a couple minutes of each other.

Scientists have determined that the two quakes happened beneath the ocean floor about 70 km apart with one measuring 8.0 on the Richter scale and the other a magnitude of 7.9. Their findings was published on Thursday in the international scientific journal Nature.

GNS geophysicist John Beavan said on Thursday that it was pure detective work that uncovered the two earthquakes.

"The files arrived from Tonga in early November 2009. When I processed the data and looked at the GPS results I was astonished to see that the island of Niuatoputapu had moved nearly 400 mm to the east," he told the New Zealand Press Association (NZPA).

"This is a rare phenomenon, but it is possible wherever there is a subduction-type plate boundary," he added.

The death toll for the devastating tsunami that hit Samoa, American Samoa and Tonga's northern island of Niuatoputapu in September last year stood at 186.