Slippery clay contributed to the devastating tsunami that struck Japan in March 2011 and could help trigger more disasters in the future, scientists say.
A research team that drilled to a depth of almost five miles discovered that the narrow fault beneath the earthquake zone was filled with extremely fine sediment.
"It's the slipperiest clay you can imagine," said geologist Professor Christie Rowe, from McGill University in Canada, who led the expedition. "If you rub it between your fingers, it feels like lubricant."
Other vulnerable weak spots in the Earth's crust in the northwest Pacific, from Russia's Kamchatka peninsula to the Aleutian islands, contain the same kind of clay, the scientists point out.
They may be capable of generating other huge earthquakes like the one that struck the coast of Tohoku, Japan, killing around 16,000 people and causing damage estimated to cost billions of dollars, they claim.
The research team drilled three holes in the ocean floor to study the earthquake's rupture zone, a fault line where the Pacific and North American tectonic plates meet.
Where the plates join they form what is known as a "subjuction zone", with the North American plate riding over the edge of the Pacific plate and pushing it down to form the Japan Trench, a deep abyss at the bottom of the ocean.
Until 2011, the largest displacement of plates ever recorded along a fault occurred in 1960 off the coast of Chile where a powerful earthquake displaced the sea floor plates by an average of 20 metres.
In the magnitude nine Tohoku earthquake, the slip amounted to 30 to 50 metres - and grew bigger as the subterranean rupture approached the ocean floor.
The bedrock was thrust up, triggering tsunami waves that reached a recorded height of 128 feet.
The new research, reported in the journal Science, found several factors that contributed to the unexpectedly violent event.
Besides the fine clay, another key element was the fact that fault itself was very thin - less than five metres thick in the region sampled.
"To our knowledge it's the thinnest plate boundary on Earth," said Prof Rowe.
In comparison, California's San Andreas fault is several kilometres thick in places.
The scientists used specially designed deep drilling equipment to obtain samples from more than 800 metres beneath the sea bed, in an area where the ocean is around 6900 metres deep.
It took more than six hours for each core sample to be transferred from the fault to the drilling vessel.
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