An active fault running under reactors 1 and 2 at the Tsuruga nuclear plant in Fukui Prefecture is much longer than previously thought and could trigger a 7.4-magnitude earthquake, larger than earlier projections, according to a team of government-affiliated researchers.
"The worst-case scenario should be taken into consideration" as the Urasoko fault, now thought to extend at least 35 km, could activate faults on the south side of the Tsuruga plant, warned Yuichi Sugiyama, leader of the National Institute of Advanced Industrial Science and Technology's research team.
The government's Earthquake Research Committee and Japan Atomic Power Co., which runs the Tsuruga plant, have estimated that the Urasoko fault, combined with other faults connected to it, was 25 km long and capable of triggering a 7.2-magnitude quake.
But the research team's latest findings suggest that is an underestimate, possibly as Japan Atomic Power did not take oceanic faults into account when assessing the Tsuruga plant's safety.
The research team analyzed sonic survey and other data from Japan Atomic Power and found that multiple faults exist 2 to 3 km from the Urasoko fault, according to Sugiyama, who is also on an expert panel of the Nuclear and Industrial Safety Agency.
The faults are highly likely to be activated in combination, extending the length of the Urasoko fault to 35 km and magnifying its power, Sugiyama said.
Applying a computational formula to calculate the length of a fault based on its displacement, the Urasoko fault extends to 39 km, supporting the length estimated from the sonic survey data, Sugiyama said.