Fine sand led to Fukushima tsunami - NZ researchers

Last updated 10:33 06/12/2013

GROUND BREAKING: Rikuzentakata, northern Japan, after the magnitude 9.0 earthquake and tsunami struck the area, March 13, 2011.

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Kiwi scientists have helped reveal a key trigger in the devastating Fukushima tsunami that could help understand the tsunami threat in New Zealand.

As part of a team of 27 scientists from 10 countries, the researchers drilled into a fault line off the Japanese coast last year and collected three samples.

A fine sediment clay within the fault, which is called the Japan Trench plate boundary megathrust fault, was a key factor in the March 2011 tsunami.

"What the core samples show is that the fault, particularly near the sea floor, is composed of less than 5 metres of very fine volcanic sediment, highly altered to a special type of clay, which acted as an incredibly slippery lubricant and allowed the huge quake to occur," said Virginia Toy, of the geology department at Otago University.

The researchers on the expedition, known as the Japan Trench Fast Drilling Project, have produced three papers published in leading international journal Science.

"The extreme frictional weakness of this material facilitated the huge vertical and horizontal displacements of the sea floor, up to 50m, during the magnitude-9 quake," Toy said.

"It was the water displaced by this massive movement of rock that generated the much larger than anticipated tsunami waves which devastated Fukushima on the east coast of Japan."

The discovery that the unexpected behaviour of the Japan Trench megathrust fault was because of the high concentrations of this type of clay was significant, she said.

Other scientists could use the research to work out whether other major faults around the Pacific Rim, including beneath New Zealand's east coast, could generate a similarly large tsunami.

"If our local subduction megathrust faults have similar composition and fabric, we should be aware they may generate large tsunami when they do fail in future earthquakes," Toy said.

Otago University's involvement in the expedition was funded by the Australian-New Zealand IODP Consortium.

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