An ultra-thin fault zone loaded with slippery clay was what gave off the mammoth March 2011 Japan quake and tsunami, which killed around 16,000 and injured 6,150 across twenty prefectures in the country.
Around 27 scientists from 10 countries said it was a fine sediment clay within the fault that caused the destructive earthquake two years ago. The fault is called Japan Trench plate boundary megathrust fault.
"It's the slipperiest clay you can imagine," geologist Professor Christie Rowe, from McGill University in Canada, said. "If you rub it between your fingers, it feels like lubricant."
The research team drilled three holes in the ocean floor to study the earthquake's rupture zone, in a fault line where the Pacific and North American tectonic plates meet.
Researchers said the narrow strip of slippery, wet clay was what pushed the phenomenon to move past between two tectonic plates at tremendous speed and to travel much further than in most regular quakes.
What's frightening is that the deadly clay and ultra thin fault zone is capable of generating other huge earthquakes similar to the one that hit Tohoku, Japan.
Scientists said the same kind of clay can be found in other vulnerable weak spots in the Earth's crust in the northwest Pacific, from Russia's Kamchatka peninsula to the Aleutian islands.
They said the layer they found was very thin about one to five metres and mostly composed material as fine and moist as cosmetic foundation.
"The layer is 90 per cent made of low friction clay, called smectite - material similar to foundation, which tends to become runny and slippery," Kotaro Ujiie, an associate professor at Tsukuba University, said.
The smectite came from volcanic ash, formed over a very long period of time, Mr Ujiie said. It was originally on the surface, before it slid underneath the other plate.
"This layer of smectite existed in the area before the sinking of the plate started," he said. "The geological conditions were already there before the quake."
Moreover, the material becomes even more slippery in areas where the layer of clay is sandwiched between strata of impermeable rock because of the pressure above it.
"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," Virginia Toy, of the geology department at Otago University, said.
The new research was reported in the journal Science.
To report problems or to leave feedback about this article, e-mail:
To contact the editor, e-mail: