TOKYO (Kyodo) — An active fault running under the Nos. 1 and 2 reactors at Japan Atomic Power Co.’s Tsuruga nuclear plant in Fukui Prefecture is at least 35 kilometers long and could trigger an earthquake with a magnitude of around 7.4, much higher than previously anticipated, a team of government-affiliated researchers said Monday.

“The worst-case scenario should be taken into consideration” as other faults on the south side of the Urasoko fault could become activated together, warned Yuichi Sugiyama, leader of the research team at the National Institute of Advanced Industrial Science and Technology.

The government’s Earthquake Research Committee and Japan Atomic Power have estimated that the Urasoko fault, combined with other faults connected to it, is 25 km long and capable of trigging a quake with a magnitude of 7.2. But that may be an underestimate in view of the latest findings by the research team.

According to Sugiyama, a member of an expert panel of the Nuclear and Industrial Safety Agency under the Ministry of Economy, Trade and Industry, the research team analyzed sonic survey and other data from Japan Atomic Power, finding that multiple faults exist 2 to 3 km from the Urasoko fault.

The faults are highly likely to be activated together, extending the length of the Urasoko fault to 35 km, Sugiyama said.

The power company and the agency have not taken the presence of the oceanic faults into account in assessing the safety of the Tsuruga nuclear power plant.

The Urasoko fault, already displaced vertically and horizontally, is expected to cause a total displacement of more than 3 meters if it becomes active.

Applying a computational formula to calculate the length of a fault based on its displacement, the Urasoko fault is 39 km long, close to the length estimated from the sonic survey data, Sugiyama said.

Japan Atomic Power has previously estimated that the Urasoko fault could cause a total displacement of 1.7 meters if it becomes active.

Sugiyama said the Urasoko fault could be displaced by up to nearly 5 meters if activated together with other faults.

Japan Atomic Power is examining the possibility, pointed out by experts, that many faults located under a reactor building on the west side of the Urasoku fault will move simultaneously.

If confirmed, the location of the Tsuruga nuclear plant will be disqualified, an official of the nuclear safety agency said.

Kyodo Press, March 6, 2012
http://mdn.mainichi.jp/mdnnews/national/archive/news/2012/03/06/20120306p2g00m0dm081000c.html

Quake severity estimate for Tsuruga nuclear plant to be reassessed

TOKYO (Kyodo) — The Nuclear and Industrial Safety Agency plans to call on Japan Atomic Power Co. to reassess its worst-case scenario for an earthquake at its Tsuruga nuclear power plant in light of new research showing a bigger-than-expected quake could strike the plant in Fukui Prefecture, agency officials said Tuesday.

The planned move, which would require the plant operator to report the outcome of its reassessment to the agency, follows the recent discovery that a quake greater than previously anticipated could strike due to a fault running underneath the plant.

The agency would require Japan Atomic Power to evaluate possible quake damage to buildings housing two nuclear reactors as well as equipment, based on recent research conducted by the National Institute of Advanced Industrial Science and Technology, the officials said.

The Urazoko fault, running around 250 meters from the reactor buildings, could have a serious impact on the earthquake resistance of the power plant.

The agency is also likely to instruct two other nuclear power plant operators in the area — Kansai Electric Power Co., which runs the Mihama plant, and the Japan Atomic Energy Agency, which operates the Monju fast-breeder reactor, both in Fukui Prefecture — to perform similar reassessments.

The Nuclear and Industrial Safety Agency “takes seriously” the results of the institute’s studies, an official of the safety agency said, adding that it will ask Japan Atomic Power and other operators to implement measures swiftly.

The government’s Earthquake Research Committee and Japan Atomic Power have projected that the Urazoko fault, combined with other faults connected to it, is 25 kilometers long and capable of triggering a quake with a magnitude of 7.2.

But the National Institute of Advanced Industrial Science and Technology has found the fault is at least 35 km long and could trigger a quake with more than twice the energy previously considered possible, according to a team of researchers at the government-affiliated institute.

Kyodo Press, March 7, 2012
http://mdn.mainichi.jp/mdnnews/national/archive/news/2012/03/07/20120307p2g00m0dm051000c.html