The Oi nuclear plant in Fukui prefecture was the only nuclear facility reactivated since the Fukushima disaster last year, Kyodo reported.
Tokyo University professor Mitsuhisa Watanabe said there is evidence that the plant is situated on top of an active fault line.
"It's certain there is an active fault. Operations should be stopped and another investigation should be conducted" at the Oi plant, Watanabe said Sunday.
Many activists have called for an immediate shutdown of the Oi reactors, but the government has so far rejected their demands.
The Nuclear Regulation Authority panel is investigating the plant's safety, but has not yet reached a conclusion.