Japanese Tsunami In 2011 Caused By Slippery Clay And Thin Tectonic Plates On The Ocean Seafloor

By Ajit Jha on December 5, 2013 2:43 PM EST

Scientists and geologists have always suspected that "dramatic displacement of sea floor" led to the devastating tsunami off Japan's Tohoku region in March of 2011. A series of studies published today in the journal Science show that researchers now have a much clearer picture of what actually caused this dramatic displacement of the seafloor.

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Twenty-seven scientists from 10 countries participated in a 50-day study in 2012 on the Japanese drilling vessel Chikyu. In order to study the rupture zone of the 2011 Japanese earthquake, the team drilled three holes in the Japan Trench area. The Japan Trench is a fault in the ocean floor, comprising what is called a "subduction zone" - the joint where the Pacific and North American plates meet deep beneath the surface of the Pacific Ocean. The Trench formed where the North American plate rides over the edge of the Pacific plate. The Pacific plate bends and plunges deep into the earth.

A violent tectonic clash of the plates, the largest ever, led to the disastrous 2011 tsunami. The largest before 2011 was a 1960 displacement of plates off the coast of Chile, where seafloor plates were displaced by an average of 20 meters; following the Tohoku earthquake there was a 30 meter slip that increasingly grew bigger as the subterranean rupture approached the seafloor, eventually reaching a 50 meter displacement.

The studies that resulted from the Chikyu expedition outline several factors that propelled the violent slip between the two plates. The fault was found to be extremely thin - less than 5 meters in thickness. "It is the thinnest plate boundary on Earth," said Dr. Christie Rowe, McGill University biologist and co-author on the three Science studies, said in a press release. In contrast, California's San Andreas Fault is several kilometers thick in places.

An international team of scientists has concluded that an unusually thin and slippery geological fault where the North American plate rides over the edge of the Pacific plate caused a massive displacement of the seafloor off the coast of Japan in March 2011, touching off the devastating tsunami that struck the Tohoku region.
An international team of scientists has concluded that an unusually thin and slippery geological fault where the North American plate rides over the edge of the Pacific plate caused a massive displacement of the seafloor off the coast of Japan in March 2011, touching off the devastating tsunami that struck the Tohoku region.

The researchers also found that the clay deposits that fill the narrow fault are made of extremely fine sediment. It's the slipperiest clay imaginable, feeling almost like a lubricant, according to Rowe. The discovery comes as a warning to other subduction zones from Russia's Kamchatka peninsula to the Aleutian Islands, in the northwest Pacific where this type of clay is found. Dr. Emily Brodsky, a geophysicist at the University of California, Santa Cruz, and coauthor of three papers on the Tohoku-Oki earthquake published in Science concurred that, "the Tohoku fault is more slippery than anyone expected".

erhaps the most challenging task of the project was the measurement of the fault's frictional resistance to slip. Since friction generates heat, taking the temperature of a fault after an earthquake can provide the frictional resistance. However, it is difficult to get this measurement because, "the signal is weak and it dissipates over time," said Brodsky. The logistically and technically challenging operation was successfully accomplished by the international Japan Trench Fast Drilling Project (JFAST) expedition by installing a temperature observatory in one of the three boreholes nearly 7 kilometers under the ocean surface. Eventually, the scientists for the first time measured the frictional heat produced by the fault slip. What they discovered explains the massive and unprecedented earthquake: the friction on the fault was amazingly low. It is the low resistance to slip on the fault, according to U.C. Santa Cruz researcher Patrick Fulton, that explains the unparalleled 50 meters of displacement, a slip of such huge magnitude that it could cause the horrendous earthquake.

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