18 Aug 2010

Pacific tsunami caused by rare double quake

7:15 pm on 18 August 2010

A deadly tsunami in the Pacific was caused by a rare double earthquake - not a single quake as first thought, according to new research.

Some 186 people died when the tsunami struck in Samoa, American Samoa and Tonga's northern island of Niuatoputapu in September last year.

New Zealand's GNS Science has been looking into the quakes that struck along the Pacific tectonic plate.

An earthquake expert at Victoria University, John Townend, said both quakes triggered the tsunami, but it is not yet certain which came first, and they occured in an area where large quakes are rare.

Dr Townend told Checkpoint the quakes took place a couple of minutes apart, when it usually takes at least weeks for one big quake to trigger another.

Global earthquake readings initially indicated a single large "normal faulting" quake of magnitude 8.0 had occurred, producing an extensional motion while the tsunami waves indicated a "thrust" event with compressional movement.

The scientists said they were unable to reconcile the conflicting data until six weeks after the event when measurements from the small Tongan island showed there must have been two large quakes.

The scientists determined that the quakes occurred under the ocean floor about 70km apart with one at magnitude 8.0 and the other of magnitude 7.9.

The GNS ings appear this week in the science publication Nature.