A fire burns on Oct. 1, 2009 in Pago Pago, American Samoa after a tsunami struck in September 2009.
Credit: Wikimedia
PARIS: A tsunami that hit the Pacific islands of Samoa and Tonga last year was generated by three earthquakes unleashed by a seismic mechanism that has never been observed before, scientists said.
The September 29, 2009 disaster killed 186 people in Samoa, American Samoa and northern Tonga when waves rearing as high as 15 metres flattened villages and resorts.
Seismologists initially pinned the blame on a massive quake of magnitude 8.0, later upgraded to 8.1, occurring 18 kilometres below the sea on the Tonga Trench.
Multiple earthquakes bring on tsunami
But within weeks, doubt soon set in, as a single quake could not explain the pattern of tsunami waves and their height variations.
Reporting in Nature, U.S. experts said they found the 8.1 temblor unleashed two more extremely powerful quakes in very quick succession.
Each measured 7.8 magnitude and occurred close to each other, at a location more than 50 kilometres from the initial event.
Quakes hidden behind first quake
"When we looked at the data, it turned out it wasn't just one great earthquake but three large earthquakes that happened within two minutes of one another," said Keith Koper, director of the University of Utah Seismograph Stations.
"The two quakes that were hidden by the first quake ended up being responsible for some of the damage and tsunami waves."
That one quake can cause another quake is not in itself new. "Domino" quakes are notorious, for instance, on the Sunda Trench off the Indonesian island of Sumatra and on Turkey's North Anatolian Fault.
Fastest plates on Earth
But what happened at the Tonga Trench occurred among distinctly different types of faults, with implications for understanding how tsunamis are caused, the researchers said.
The Tongan Trench lies at the conjunction of the Pacific plate and a microplate called the Tongan block.
Pushing westwards, the Pacific plate collides with the Tongan block before diving beneath it, a movement called subduction.
The plates are moving towards other at 20 centimetres a year, the fastest of any such convergence in the world.
