By Jerram Watts
New research into the earthquake responsible for the deadly 2009 tsunami in Samoa and Tonga has found it was caused by not one, but two near-simultaneous quakes.
The New Zealand-led research helps explain why the resulting tsunami did not fit predictions; that is, the waves were larger than expected in some areas and failed to arrive at anticipated times in others.
Dr John Beavan of GNS Science led the research team; the story will feature on the cover of the science journal Nature.
While initial seismic data recordings indicated a normal fault earthquake, tsunami wave observations from seafloor pressure sensors in the southwest Pacific were not consistent with a normal fault and pointed toward a thrust fault.
The research team found the tsunami was caused by a combination of a normal-faulting earthquake within the Pacific Plate (when the two tectonic plates meet, one descends below the other and a rupture occurs within the descending plate as it bends before entering the subduction zone) and a near simultaneous thrust on the subduction interface (sub-surface friction between the Pacific Plate and the Tonga microplate).
The normal fault earthquake was a magnitude 7.9 and the thrust earthquake 8.0.
The GNS team says the thrust earthquake was seen in the tsunami data but not in the seismic data because either the speed of the plate slip creating the thrust was so slow it did not generate seismic waves, or because it happened shortly after the normal fault and its seismic waves were hidden within those of the normal fault.
One hundred and eighty people were killed after the earthquakes struck in the Tonga Trench on September 29 at a depth of 10km.
The tsunami that hit was almost four storeys high, scientists say – 14m when it hit the beach in Samoa, 10m high in American Samoa and penetrated as far as 700m inland in some places.
Dr Beavan says there is the need for more GPS stations located on land by subduction zones so data can be continuously recorded and analysed so scientists can measure ground displacement to better predict the effects of any resulting tsunamis.
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