Slippery Clay at Fault in 2011 Japan Earthquake


Slippery clay that looks like scaly black dragon skin is the crucial clue needed to explain the 2011 Japan earthquake's surprising impact, according to three studies published yesterday (Dec. 5) in the journal Science.

You may have seen that image purportedly showing the alarming spread of Fukushima radiation across the Pacific. But dig deeper and you'll find that's not the case at all.

Scientists now have four lines of evidence (including a February 2013 study also published in Science) that help explain why Japan's earthquake-generating fault acted so weirdly during the 2011 temblor.

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"It seems that frictional resistance at this location is getting close to zero, and we never really thought it could go so low," said Patrick Fulton, a geophysicist at the University of California, Santa Cruz and lead author of one of the studies. "This is definitely providing new ideas and challenging our understanding of earthquakes and fault ruptures."

The clay, which resists slipping just slightly better than a banana peel, lines the shallow part of the massive plate boundary fault offshore of Japan, where the seafloor jumped eastward by a staggering 165 feet (50 meters) on March 11, 2011. The enormous surge gave the ocean a giant slap, generating the destructive tsunami that killed more than 15,000 people.

An international scientific team recovered the clay in May 2012, during the first-ever drilling project across a recently shattered subduction zone. Eastward of Japan, two of Earth's massive crustal fragments, called tectonic plates, bash into each other at a boundary called a subduction zone — where the Pacific Plate slides, protesting and groaning, under the Okhotsk Plate. The world's biggest earthquakes (those stronger than magnitude-9) strike on subduction zones.

Flawed fault

The Tohoku quake was a magnitude 9.0, but it was like no subduction quake ever seen before. Before the 2011 earthquake, scientists thought subduction zones concentrated their energy deeply, where rocks are strong and plates can stick together between quakes. (Faults store energy between earthquakes kind of like springs, slowly squeezing until the boundary unleashes and everything rips apart.) But the Tohoku temblor was a surprise — the shallow part of the fault shifted twice as much as the deeper part. These soft, muddy rocks were expected to be too weak to store energy between earthquakes. [Infographic: How Japan's 2011 Earthquake Happened]

"We'd never seen such large slip happen at very shallow depth in a subduction zone before," Fulton told LiveScience's OurAmazingPlanet. "It was unheard of."

Today's studies conclude the dragon-skin clay was the earthquake's weak link. The slippery clay helped the plates slide so far during the 2011 temblor.

"All of the tectonic motion was concentrated into this weak layer," said Christie Rowe, a study co-author and fault geologist at McGill University in Canada.

And because the clay layer is a distinctive marker found buried across the Northwest Pacific seafloor, scientists fear subduction zones near Alaska and Russia may also hide this clay. If so, their potential for powerful tsunamis could be greater than thought.

"We think other areas are at risk for this type of event, like Kamchatka and the Aleutians," Rowe said. "It's a sobering thought."

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