Scientists: 2011 tsunami originated from slip in seabed clay layer

The towering tsunami created by the Great East Japan Earthquake in 2011 originated from a slip in a clay layer along a plate boundary beneath the seabed off the Pacific coast of northeastern Japan, according to researchers.

Working as part of the international Integrated Ocean Drilling Program, scientists from Japan, the United States, Europe and elsewhere said frictional heat generated during the earthquake may have pushed temperatures to 1,250 degrees, creating conditions for the slip.

"Frictional heat likely exerted intense pressure on the water content of the clay layer, turning it into fluid form and causing the large-scale slip," said Kohtaro Ujiie, an associate professor of structural geology at the University of Tsukuba and a leading member of the research team.

It is believed the tsunami that devastated the Tohoku region originated from displacements of 50 meters along a tectonic plate boundary that separates the Pacific plate from the North American plate, which carries the Japanese islands.

Chikyu, a deep-sea drilling vessel of the Japan Agency for Marine-Earth Science and Technology, drilled holes in the seabed 220 kilometers east of Miyagi Prefecture, part of the source area of the Great East Japan Earthquake, in April and May 2012. The scientists discovered a layer of clay, less than 5 meters thick, at a depth of 821 meters beneath the seabed.

Analysis showed it contained abundant smectite, a clay mineral with low friction that creates ideal conditions for a slip.

The researchers also installed 55 thermometers in the drill holes, which they operated for nine months from July 2012, and detected a temperature rise, likely a residue of frictional heat that arose from the plate boundary slip and was being dissipated in the soil.

The research results were published Dec. 6 in the U.S.-based Science magazine.