National

Experts suspect extension of active faults off Fukushima caused November quake

Kyodo

The powerful earthquake that struck northeastern Japan and caused tsunami in late November may have been caused by a slip in a zone that may be an extension of two known active faults under the seabed off the crippled Fukushima nuclear power plant site, according to a new analysis.

Shinji Toda, a professor at Tohoku University who compiled the analysis with a colleague, warned that the suspected extension of the two faults could cause an earthquake in the future and trigger tsunami if blocks of the seafloor shift vertically along either fault.

The geology experts are calling on the Nuclear Regulation Authority to conduct an emergency probe into the faults and the suspected extension. They will report their findings to a meeting of the government’s earthquake investigation committee on Friday.

Tokyo Electric Power Company Holdings Inc., the operator of the crippled Fukushima plant, is aware that the two faults are each 22 to 23.5 kilometers long and could potentially cause a magnitude-7.1 quake.

According to the detailed analysis of existing sea-depth data for the seabed off the coast of Fukushima Prefecture, there is a cliff 5 to 10 meters high that is likely to have been formed by past quakes.

Taking geological conditions into account, the experts have determined that there is a new 30-km-long extension of the known faults, and that this extension likely caused the Nov. 22 quake. The quake caused a 140-centimeter-high tsunami in Sendai in Miyagi Prefecture, which was the largest to hit the country since the devastating magnitude-9 quake on March 11, 2011.

Both known faults are located south of the suspected extension. The power company and the now-defunct Nuclear and Industrial Safety Agency had determined that the two faults were not active, but changed their view after the 2011 quake.

An official of the utility said that although the November quake originated in an area near the active faults believed to exist off the damaged Fukushima No. 1 plant site, the exact relationship between the quake and the faults is unknown.

The plant was catastrophically damaged by the 2011 quake and tsunami.

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