Key factor in triggering Fukushima tsunami identified - study

Fine sediment clay within the Japan Trench plate boundary megathrust fault, was a key factor in triggering the devastating Fukushima tsunami in March 2011, according to a new study involving Otago University researchers.

A team of 27 scientists from 10 countries, including New Zealand, recovered three core samples by drilling into the fault last year.

They have now produced three papers published in the international journal Science.

One of the researchers, Dr Virginia Toy from Otago University's geology department, said the samples showed, for the first time, the fault, particularly near the seafloor, was composed of less than 5 metres of very fine volcanic sediment, highly altered to a special type of clay.

That acted as an "incredibly slippery lubricant", which allowed the huge quake to occur.

"The extreme frictional weakness of this material facilitated the huge vertical and horizontal displacements of the seafloor (up to 50m) during the magnitude 9 quake.

"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," Dr Toy said.

The research team also measured the temperature of the fault zone. The temperature rises because of frictional heating when a fault slips, generating an earthquake.

The temperature increase during the Tohoku-Oki event was lower than expected, despite the large magnitude of the earthquake, indicating the fault had very low frictional strength, Dr Toy said.

This is partly because of the lubricating effect of the clay, and helps explain why the very large displacements of the seafloor were possible.

Dr Toy said it would help scientists to work out if other major faults around the Pacific Rim, including beneath New Zealand's east coast, could generate similarly large tsunami due to very large slippage during future earthquakes.

"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," she said.