The finding, announced Thursday by Nuclear Regulation Authority, is expected to prompt further study at nuclear-related operations in the northern end of the island of Honshu, The Asahi Shimbum reported.
Possibly active fault lines could force electric power companies to further postpone plans to restart reactors and require additional quake-resistance measures at the facilities, officials said.
Tohoku Electric Power Co., operator of a plant at Higashidori, had argued faults near the facility were not active.
"It is our shared understanding that (Tohoku Electric's) argument that they are not active faults is totally unacceptable," said Kunihiko Shimazaki, an NRA commissioner and head of the panel that studied the fault lines.
One fault 400 yards west of the Higashidori reactor building stretches for several miles, and another fault runs parallel to it, the panel said.
Tohoku Electric should be prepared for a temblor with a magnitude exceeding 7.0 and its epicenter directly below the plant, panel member Heitaro Kaneda, a professor of earth sciences at Chiba University, said.
Additional studies will be needed on the faults of the entire region, NRA Chairman Shunichi Tanaka said.