Study reveals what set off the 2011 Japan tsunami and where similar disasters could occur

A science team searching for what turned the 2011 Japan tsunami into one of the most devastating disasters in history has turned up some surprising results, and has revealed other parts of the world where similar disasters may occur in the future.

The international team, with members from 10 different countries around the world, spent 50 days on the Japanese drilling ship Chikyu last year, boring down into the sea floor where the Tohoku earthquake struck. This zone, which is where the western edge of the Pacific Plate pushes underneath the far western portion of the North American plate, is responsible for many earthquakes. However, the March 11, 2011 magnitude 9.0 quake produced the largest fault-slip ever recorded — measured at 50 metres, more than twice the previous record — and it was this incredible displacement of earth that set off the deadly, destructive tsunami.

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Drilling down into the fault zone in three locations, and pulling up sediment samples from inside, the research team discovered some surprising results that contributed to the size of the fault-slip.

One factor was how thin the fault turned out to be ... meaning the thickness of the rock that's deformed at the boundary of the two tectonic plates. Other faults around the world range between about 10 metres thick to several hundred metres thick or more. However, the fault where the Tohoku earthquake struck turned out to be less than five metres thick.

"To our knowledge, it's the thinnest plate boundary on Earth," said geologist Christie Rowe, a member of the science team from McGill University, according to a university press release.

In thicker faults, there are plenty of individual grains of rock, clay or other earth to rub against one another when the tectonic plates try to move. Thus they generate a lot of friction and resistance, and the plates typically move only a small amount. With a thinner layer of material, there's less friction, and this makes it much easier for the plates to move.

However, another factor for the Japan Trench made matters even worse. The thin material in between the two plates in that region is composed of an extremely slippery clay that generates almost no friction at all.

"It's the slipperiest clay you can imagine," Rowe said in the press release. "If you rub it between your fingers, it feels like a lubricant."

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The Japan Trench isn't the only fault that has this kind of clay. In the press release, Rowe identified two other regions where it can be found — along the Kamchatka peninsula, in far eastern Russia, and along Alaska's Aleutian Islands. Earthquakes happen in these regions fairly often, and the presence of this clay could enhance a quake's destructive potential, the way it did off the coast of Japan.

The expedition's website has full details on the mission, and three papers published from the results are available online at the journal Science (here, here and here).

(Photos courtesy: Canadian Press, JAMSTEC/IODP)

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