Japanese nuclear regulators inspected ground structures at the country's only operating nuclear power plant Friday to examine if an existing fault line is active.

The inspection will determine whether the Ohi plant in western Japan should close. Its No. 3 and No. 4 reactors went back online in July, becoming Japan's only operating reactors after all 50 Japanese reactors went offline for inspection after a March 2011 earthquake and tsunami triggered a meltdown crisis at Fukushima Dai-ichi nuclear plant.

A five-member team, led by regulatory commissioner Kunihiko Shimazaki, a seismologist, includes four independent seismologists and fault experts. Among them is Toyo University professor Mitsuhisa Watanabe, who initially brought to attention the question over the F-6 fault where he has found a zone of crushed rocks in bedrocks.

If the fault that cuts across the plant is judged active, Ohi must be closed. Government's safety guidelines ban a nuclear plant directly above an active fault.

Active faults are also thought to be under several other plants across Japan and are under investigation.

"There were things we learned only by coming to the site," Shimazaki told reporters after finishing the inspection. But the team may need another round of inspection to make a decision, he said.

The experts will meet Sunday to discuss their findings and possibly make a verdict.

The closely monitored inspection would be a major first test for the regulators to gain credibility over a safety decisions.

Safety concerns run deep among the public, which largely turned anti-nuclear since the meltdowns in Fukushima. While regulators are stepping up reactor safety standards and emergency plans for nuclear plant host communities, many people remain highly concerned about fault lines running underneath nuclear plants.

The Ohi plant's shattered zone in question won't trigger an earthquake but could move with active faults near the plant's perimeter. If that happens, the movement could damage a water pipe to bring in water to cool reactors in an emergency. The north-south fault cuts between the No. 1-2 reactors and the No. 3-4 pair.

Ohi's operator Kansai Electric Power Co., which had earlier failed to produce a document of the suspected fault despite repeated requests, submitted its preliminary findings of a government-ordered internal investigation that they found no further evidence suggesting an active fault.

Before the on-site inspection, top regulator Tanaka said the two operating reactors would have to be shuttered if the F-6 turns out to be an active fault.

Regulators sought to distance themselves from business or political leaders and promised to make a purely scientific ruling.

"Our decision will not be political, social or economic, but purely based on safety," Tanaka said recently.

He also has modified the definition of an active fault, saying any traces of ground movements as far back as 400,000 years ago should be considered active, more than tripling the current definition.

Prime Minister Yoshihiko Noda, who faced massive protests after ordering the restart for the two reactors, told reporters that he would abide by any decision by the regulators.