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International research reveals causes of Fukushima tsunami

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Friday, 6 December 2013 9:09am

Research from the University of Otago along with a team of international scientists, has shown for the first time that fine sediment clay within the Japan Trench plate boundary megathrust fault, was a key factor in triggering the devastating Fukushima tsunami in March 2011.

Dr Virginia Toy from the Department of Geology at Otago University was part of a study team of 27 scientists from 10 countries who recovered three core samples by drilling into the fault off the Japanese coast in 2012 from the International Ocean Discovery Program (IODP) research vessel Chikyu.

Chikyu_research_ship

The researchers on this expedition, known as the Japan Trench Fast Drilling Project (JFAST), have produced three ground-breaking papers published in the leading international journal Science.

“Our main contribution to the research has been to determine the structure and composition of the plate boundary slip-zone which caused the undersea Tohoku-Oki earthquake and subsequent tsunami; and the results are quite a surprise!” explains Dr Toy.

“What the core samples show, for the first time, is that the fault, particularly near the seafloor, is composed of less than five metres of very fine volcanic sediment, highly altered to a special type of clay (smectite), which acted as an incredibly slippery lubricant and 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,” says Dr Toy.

The research team also measured the temperature of the fault zone. The temperature rises due to 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. This is partly because of the lubricating effect of the clay, and helps explain why the very large displacements of the seafloor were possible.

Finally, high velocity experiments, in which layers of the fault zone materials are subjected to extreme forces and sheared at earthquake rates (i.e. metres per second), also indicate that the Japan Trench megathrust clays have very low frictional strengths. Thus there are numerous lines of evidence suggesting the fault is frictionally extremely weak.

The Pacific and Eurasian tectonic plates meet at the Japan Trench, which is a ‘subduction zone’. Here the Pacific plate pushes under the Eurasian plate, plunging deep into the earth, forming the Japan Trench.

The discovery that the unexpected behavior of the Japan Trench megathrust fault is attributable mostly to the presence of such high concentrations of this particular type of clay is significant. Dr Toy says it will 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 says.

The international JFAST expedition broke other records too. Supported by 24 countries who are members of the International Ocean Discovery Program, it examined the fault rupture zone by drilling more than 800 metres below the sea floor, in a record water depth of nearly 7 kilometres.

"It is an honour to be part of the world-leading scientific team working on this project. I am inspired about the possibilities of these results to increase our understanding of the potential earthquake hazards in New Zealand.”

The University of Otago’s involvement in the expedition was funded by the Australian-New Zealand IODP Consortium (ANZIC).
Expedition website: www.jamstec.go.jp/chikyu/exp343/e/

For further information contact:

Dr Virginia Toy
Department of Geology
University of Otago
Tel 64 3 479 7506
Mob 64 21 127 1012 or from 7-14 Dec, 1 415 340 9065 (California = NZ+3 hrs)
Email virginia.toy@otago.ac.nz
Web www.otago.ac.nz/geology

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