Scientists think they now know why the tsunami that devastated Japan following a major earthquake in 2011 was so much bigger than scientists would have predicted.

Three studies that explain the disaster were published online in the journal Science on Thursday by an international team of scientists. The findings also suggest that a large area of the North Pacific may be more prone to giant tsunamis than previously thought.

The magnitude-9.0 earthquake that struck off the coast of the Tohoku region in northeastern Japan on March 11, 2011, set off a monster wave, up to seven metres high that crashed over the coast, causing massive damage.

The seawater from the tsunami breached the Fukushima Daiichi nuclear plant, disabling its cooling system, causing a nuclear meltdown that the country is still trying to clean up. Altogether, the disaster killed 19,000 people and displaced more than 300,000.

The huge size of the tsunami was surprising because earthquakes around Japan are caused by tectonic plates that slide over each other almost horizontally, said Christie Rowe, a geologist at McGill University who studies faults and earthquakes. Tectonic plates are individual slabs of the Earth's crust separated by cracks called faults. An earthquake occurs when two of the plates slide past each other at a fault.

Because tsunamis are big waves generated by a vertical motion of the seafloor, horizontal plate motions don’t usually cause big ones.

'More than any other earthquake ever'

But in this case, the plates slid a record distance over each other — further than even other earthquakes of a higher magnitude.

Drilling Vessel Chikyu

Last December, an international scientific expedition aboard the Japanese drilling vessel Chikyu, shown here before departure, collected samples of the seafloor at the epicentre of the earthquake, almost 7 kilometres beneath the surface. (James Kirkpatrick/Colorado State University)

"This one was just so big — 50 or 60 metres is more than any other earthquake ever," said Rowe, who was part of the international research team behind the new studies.

Even though the plates were nearly horizontal — tilted just 10 degrees — they slid far enough to raise the ocean floor 10 metres. That's twice as high as might be expected from an earthquake of this type and magnitude.

The big mystery was how the plates managed to make such a record-breaking slide.

Scientists hypothesized that there must be something strange about the sediments or rocks on the sea floor at fault — the boundary between two tectonic plates — where the earthquake originated.

To find out if this was the case, last December, an international scientific expedition visited the site aboard the Japanese drilling vessel Chikyu to investigate.

Rowe was among 27 scientists from 10 countries that spent 50 days drilling 800 metres into the sea floor, which was itself 6,900 metres beneath the surface of the ocean. Long cylinders of rock and sediment collected by the drill from the sea floor were brought back up to the ship for analysis.

Rowe's job was to identify the faults in the samples using X-rays to look for signs of past earthquakes. The samples were then opened up and examined in more detail.

"We measured every tiny fracture in every layer of sediment," recalled Rowe, who along with Frederick Chester of Texas A & M University was one of the lead authors of the new paper describing the structure of the fault.

'Totally unlike any sediment material'

What they discovered is the sediment at that fault is "totally unlike any sediment material I've ever seen in any of my studies," Rowe said. She described it as a "very, very squishy, very slimy clay."

Christie-Rowe

Scientists Christie Rowe (left) of McGill University and Kohtaro Ujiie of Tsukuba University, aboard the Chikyu, celebrated with a handshake when they agreed they had found the ocean-floor fault in the drill core. (JAMSTEC/IODP)

The clay forms an extremely thin layer between the two plates – less than five metres thick, making it the thinnest known plate boundary on Earth. For comparison, California's San Andreas fault is several kilometres thick in some places, noted a  McGill University news release.

Based on frictional measurements and stress tests on the clay, described in two of the new papers, scientists think that as the plates near Japan slide over each other, the clay acts as a lubricant. It heats up and becomes even more slippery the further and faster they slide, causing the plates to slide even further and faster.

"It doesn't probably cause more great earthquakes, but what it causes is those earthquakes to be more efficient at producing tsunamis," Rowe said.

"The somewhat concerning thing about this is this is present across much of the northwest Pacific Ocean."

Fortunately, the slippery clay isn't present in the northeastern Pacific, near the coast of B.C., where the plate is much younger, Rowe said.

"That's a good thing for Vancouver."

The expedition was supported by member countries of the Integrated Ocean Drilling Program, including Canada, which funds its membership through the Natural Sciences and Engineering Research Council (NSERC), and Canadian academic institutions and non-governmental agencies.

Japan-fault

A graphic shows the structure of the fault in the sea floor off the coast of Japan, where the North American plate rides over the edge of the Pacific plate, and explains how a slippery layer of clay touched off the devastating tsunami that struck the Tohoku region in March 2011. ( JAMSTEC/IODP)