A thin layer of very fine clay with a consistency similar to some cosmetics made Japan's tsunami-causing earthquake of 2011 much more dramatic because it acted as a lubricant, scientists say.

The narrow strip of slippery, wet clay that sits between two tectonic plates off the country's northeast coast allowed them to shift past each other at tremendous speed and to travel much further than in most regular quakes, researchers said.

The finding sheds more light on a catastrophe that claimed more than 18,000 lives when a 9.0 magnitude undersea quake unleashed a towering tsunami that slammed into the coast, destroying whole communities and causing nuclear meltdowns at Fukushima.

Researchers say the stratum, made mostly of material as fine and moist as cosmetic foundation, is very thin, about one to five metres (about 3 to 16 feet), compared with the 40-or so metres found at other major seismically-active plate borders.

"The layer is 90 percent made of low friction clay, called smectite -- material similar to foundation, which tends to become runny and slippery," said Kotaro Ujiie, an associate professor at Tsukuba University.

Ujiie is part of the Japan Trench Fast Drilling Project, which was launched in the immediate aftermath of the 2011 quake, using a deep-sea drilling vessel that allows scientists to bore far under the sea bed.

Researchers used the drill to get samples from the point where the two plates meet, and to measure heat caused by friction after the quake.

They discovered that in areas where the layer of clay is sandwiched between strata of impermeable rock, it becomes even more slippery because of the pressure it is under.

Ujiie said the smectite was formed over a very long period of time from volcanic ash and 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," Ujiie said. "The geological conditions were already there before the quake."

Seismologists are now drilling deep beneath the seabed off the Kii peninsula, southwestern Japan, into a fracture in the Earth's crust known as the Nankai Trough.

Experts have warned the trough, which marks the place where the Philippine Sea plate slides under the Eurasian plate, is the likely source of a monster earthquake sometime in the near future.

Japan's government last year unveiled a worst-case scenario, warning a big quake in the area could kill more than 320,000 people, dwarfing the toll from the 2011 quake-tsunami disaster.

Ujiie and other researchers have found that the tectonic fault in the Nankai Trough is less slippery than that further north.

"Under the Nankai Trough, a relatively new plate is sliding, where there was not enough time to collect and form a smectite layer," Ujiie said.

"But that doesn't mean we can sit back and let our guard down."

The 56,752-ton Chikyu -- "Earth" in Japanese -- was built in 2005 at a cost of $500 million. It is equipped with a 121-metre drill tower that can descend 7,000 metres below the seabed, nearly three times as deep as its predecessors.

It uses GPS with pinpoint accuracy that allow researchers to target very specific areas of the Earth's crust.

Seismically-active Japan experiences 20 percent of the world's major earthquakes every year.

Building standards are high and Japanese people are well-practised at taking cover when quakes strike, meaning damage and death tolls are often much lower than in other parts of the world.

The research, part of a project run by Japan Agency for Marine-Earth Science and Technology (JAMSTEC), was to be published in the Friday edition of the US magazine Science.