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London, Aug 19 (ANI): Scientists have found that the massive earthquake that struck the South Pacific on September 29, 2009 was a combination of two earthquakes - one with magnitude 8.1, which then triggered another magnitude 8 earthquake seconds later on a different fault.

“We know of no precedent for the Samoa triggered doublet,” said Thorne Lay at the University of California, Santa Cruz.

The earthquakes unleashed devastating tsunami waves that swept onto the islands of Samoa, American Samoa, and Tonga, killing 192 people.

The Tonga subduction zone in the South Pacific marks the boundary where the Pacific plate is sinking under the Australian plate.

On September 29, the first earthquake actually occurred within the Pacific plate at a site 50 to 100 kilometers (30 to 60 miles) east of the plate boundary.

The rupture occurred along a fault in the middle of the plate.

According Lay, as the leading edge of the oceanic plate sinks into the mantle, it pulls on the rest of the plate, bending it downward and causing it to break along the extensional fault.

However, Lay’s team observed that the aftershocks were spread over a huge area, including the Tonga subduction zone. In addition, when researchers used different methods to calculate the fault geometry, they came up with inconsistent solutions, which is rare for large earthquakes.

Chen Ji of UC Santa Barbara suggested that the event involved two earthquakes on different faults with different geometries.

“The huge signals from the extensional faulting made it difficult to resolve the triggered event, but we were able to confirm that secondary faulting had occurred, that it involved thrust faulting in the Tonga subduction zone, and that it released energy from about 50 to 130 seconds after the onset of extensional faulting within the Pacific plate,” Lay said.

The strong shaking from the initial fault rupture appears to have triggered the second event, which involved two major sub events of magnitude 7.8, with a total magnitude equal to 8.0.

“There may be far more interaction between earthquakes than we have understood,” he said.

“Studying these complexities is essential for improving our understanding of how earthquakes rupture, how they interact, and how we can mitigate their impact on humans,” Lay concluded.

The research is published in the August 19 issue of Nature. (ANI)