A team of geoscientists drilled three boreholes in the ocean floor 150 miles east of Japan about 13 months after the quake and giant tsunami it triggered devastated the country, and found the fault where the earthquake originated consists of a very thin layer of water-swelling clay that acts as a form of lubricant during an earthquake slip.
"Apparently, the slippery clay lining of the fault minimizes any braking action once the fault starts to move," geologist Fred Chester of Texas A&M University said. "This likely contributed to the very large offset of the seafloor at the trench that spawned the tsunami. It was more slippery than anyone had believed."
The earthquake occurred in a subduction zone, a boundary between two tectonic plates in which one plate is diving beneath another.
"We found that the fault itself is very thin, only about 15 feet thick in the area sampled," Chester said. "In comparison, the San Andreas fault in California is more than a mile wide thick in places."
However, because of the lubricating effect of the clay sediment, the earthquake created a "slip" of about 150 feet, "which in earthquake terms is among the largest ever measured, and it was unexpected by many earthquake scientists that the fault ruptured all the way to the seafloor," Chester said.
The findings strongly suggest the area could experience more quakes in the future, he said.
"When an earthquake releases stress in one area, it transfers it to another area," he said. "So the stress is released in the area of the Tohoku rupture, but it is increased in neighboring sections along the Japan Trench."
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