The leader of a team of earthquake researchers admitted Thursday that they made an erroneous announcement confirming the presence of an active fault near Tokyo after mistaking what appears to be a concrete column buried in the ground as natural stone.

Hiroshi Sato, a professor at the University of Tokyo's Earthquake Research Institute, told a news conference they were mistaken in saying there was an active fault based on the movement of stones, characteristic of a lateral fault, and that it was actually an artificial concrete structure.

Drilling work 10 meters deep was conducted on a vacant lot of what used to be an auto factory, raising the possibility the structure could be a concrete column used for the factory's foundations.

Public attention on the issue was high because the area is part of a fault zone stretching about 33 km from Tachikawa to nearby Saitama Prefecture that could cause an earthquake beneath central Tokyo.

Sato apologized for the hasty conclusion.

"The spot was where we had predicted an active fault, so we assumed that it was an active fault," he admitted.

But he also said the mistake doesn't mean the fault does not exist and it may be deeper underground.

Sato is an expert on geographical structure deep underground. According to the Nuclear Regulation Authority, he is among the experts who investigated faults under Tohoku Electric Power Co.'s Higashidori nuclear plant in Aomori Prefecture. The team has said faults under the nuclear plant are probably active, an assessment shared by other experts as well.