The 2011 Japan Tsunami Was Caused By Largest Fault Slip Ever Recorded

The largest fault slip ever recorded produced the devastating 2011 Japan tsunami, according to three studies published today.

Two years ago, the sea off the coast of Japan reared up and swept away tens of thousands of lives in a devastating natural disaster.

The 2011 earthquake has been the subject of intense study ever since, and the trench that produced it is the best studied in the world. (See "Japan Tsunami: 20 Unforgettable Pictures.")

Now, three papers published today in the journal Science reveal the magnitude 9 earthquake off the east coast of Japan still has the capacity to surprise.

Experts calculate the fault—or the boundary between two tectonic plates—in the Japan trench slipped by as much as 164 feet (50 meters). Other similarly large magnitude earthquakes, including the 9.1 Sumatra event in 2004, resulted in a 66-to-82 foot (20-to-25 meter) slip in the fault.

"We've never seen 50-meter [slips]," said Kelin Wang, a geophysicist with the Geological Survey of Canada in British Columbia.

The next largest slip would probably be the Chile earthquake in 1960, said Wang, who was not involved in the research. Based on the limited data recorded from that earthquake, the fault slipped by 98 to 131 feet (30 to 40 meters).

Most of the movement occurred horizontally, he explained. But because the plates are wedged together at this trench, that horizontal displacement still managed to thrust up enough seawater to produce the killer tsunami that hit Japan.

Greasing The Wheels

Lubrication, specifically involving clay, is the key to such massive movement, said Frederick Chester, a geophysicist at Texas A&M University in College Station, and lead author of one of the studies.

The two tectonic plates involved are the Pacific plate, on which the Pacific Ocean resides, and a portion of the North American plate, on which parts of Japan sit.

A thick layer of clay sits atop the Pacific plate, which is getting dragged under a portion of the North American plate. As the Pacific plate dives into a trench off the coast of Japan, small portions of the clay get smeared along the plate boundary, Chester explained.

That clay traps water, rendering it quite slippery, he said. "We think that's responsible for allowing the incredibly large slip we observed near the trench."

Normally, when two plates collide, there is friction. You can think of friction like a brake, Chester explained. "But clay almost removes any braking properties."

Not a Lot of Heat

The unprecedented data haul that enabled Chester and colleagues to figure out what happened during the 2011 earthquake is courtesy of a rapid response by the Japan Agency for Marine-Earth Science and Technology, said Emily Brodsky, a geophysicist at the University of California, Santa Cruz, and a co-author of one of the studies.

The Japan Trench Fast Drilling project enabled researchers to get out to the fault zone about a year after the earthquake and drop instruments down to measure temperature anomalies—the sudden slips during an earthquake can generate vast amounts of heat—and to bring up samples of the fault zone itself for analysis.

Experts were able to take core samples of sediment and rock from the trench—located in 23,000 feet (7,000 meters) of water—thanks to a sophisticated drilling ship.

"[This] was right at the edge of what engineering could do," Brodsky said.

Not only did they find evidence of this thin layer of lubricating clay, but experts were also able calculate how much heat and friction was involved.

Even though the earthquake produced a 1,100° to 2,200°F (600° to 1,200°C) temperature increase, the amount of friction that had to be overcome to produce the fault slip wasn't as large as researchers expected, said Brodsky.

This helped confirm the fact that something else was going on—namely the clay lubrication.

It's difficult to say whether something like this could happen elsewhere, said Wang, because no other submarine trench has as many instruments monitoring it.

"Nowhere else do we have such a massive monitoring system."

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