A weak fault zone and an unusually slippery clay were responsible for the enormous earthquake that caused the 2011 tsunami that devastated Japan's Tohoku region, scientists have found.
In a series of reports published in the journal Science Thursday, a team of 27 scientists from 10 different countries explain several factors that contributed to the magnitude 9.0 earthquake off the coast of Japan in March 2011.
For one, the earthquake occurred in a subduction zone, an area in which one tectonic plate is diving underneath another. In this case, the Pacific plate plunged beneath the North American plate. Typically, earthquakes displace plates by less than 20 meters (about 66 feet), but in the Tohoku earthquake, the slip was up to 50 meters (164 feet). Also unusual in this situation, researchers said, was the fact that the earthquake ruptured all the way up to the sea floor.
"That large slip at shallow depths contributed to the tsunami that caused so much damage in Japan," said Patrick Fulton, a researcher at the University of California, Santa Cruz, in a statement. "Usually, these earthquakes don't rupture all the way to the surface."
That large thrust then set off the tsunami that killed thousands of people and wiped out entire towns. In some regions of Japan, such as in Miyagi, the waves were as high as 20 feet.
To discover why the impact was so large, the researchers used deep-water drilling equipment to dig more than 800 meters (about half a mile) below the sea floor, which itself lies about 4 miles beneath the surface of the ocean.
The researchers found there were several reasons the earthquake reacted the way it did.
The fault itself was much thinner than others - about 5 meters, or 16.4 feet - and may be the thinnest in the world, researchers said. By comparison, the San Andreas fault, which spans the length of California, is several kilometers thick in some places.
Additionally, scientists discovered that the type of clay that fills the fault is extremely fine, making it slippery to the touch.
"It's the slipperiest clay you can imagine," said Christie Rowe, a geologist at McGill University, in a statement. "If you rub it between your fingers, it feels like a lubricant."
It was that slippery clay that made the Tohoku fault so resistant to friction, which could help explain why the slip was so large, scientists said. The team was also able to determine exactly how much friction occurred during the earthquake by taking the temperature of the fault. The "frictional heat" generated in the Tohoku earthquake was much smaller than expected.
Those findings suggest there are other regions in the northwest Pacific where that clay is present that may be at risk for similarly large earthquakes. Russia's Kamchatka Peninsula, located on the eastern edge of the country, and the Aleutian Islands, which extend from the southwestern part of Alaska, both have deposits of the slippery clay in their faults.
"Looking for something like that clay may give us a tool to understand the locations of earthquakes that cause tsunamis," said J. Casey Moore, a researcher at the University of California, Santa Cruz, in a statement. "It's potentially a predictive tool."