A group of researchers has found tsunami traces believed to date back to between the 14th to 16th centuries near the Takahama nuclear power plant in Fukui Prefecture.

The plant's operator, Kansai Electric Power Co., said the finding does not affect its tsunami risk evaluation of the plant or its countermeasures, but the Nuclear Regulation Authority said it plans to ask the utility to look into the research.

Idled reactors 3 and 4 at the Takahama plant obtained safety clearance from the regulator in February, clearing a key hurdle toward resuming operations.

"We will examine the information without preconceptions, and will note whether it may relate to a threat to safety," an official from the regulator said of the finding.

Hirofumi Yamamoto, a professor of geology at Fukui University, and other researchers in the group found a layer of sand containing seashells in shallow ground at a location more than 500 meters from the shore of the bay. Carbon-dating analysis puts the formation of the layer at between the 14th and 16th centuries.

An ancient document says the Tensho earthquake that struck central Japan in 1586 caused major tsunami damage in the Wakasa Bay area, where the idled plant stands.

Yamamoto said it is possible the traces are from the Tensho tsunami, although there is not enough evidence to prove it.

All of Japan's commercial reactors were taken offline after the Fukushima nuclear crisis in 2011. The Takahama reactors are among several that recently received regulatory approval for restarting, after taking enhanced safety measures against earthquake and tsunami hazards and other severe accidents.