The world's biggest nuclear power station faces an uncertain future after it emerged today that it may lie directly above the fault line that triggered this week's earthquake in which nine people died and more than 1,000 were injured.

The Kashiwazaki-Kariwa plant - the biggest in the world in terms of output capacity - has been shut down indefinitely after it shook violently when an earthquake measuring 6.8 on the Richter scale struck Niigata prefecture in northern Japan on Monday. The plant was not designed to resist shaking caused by earthquakes of greater than magnitude 6.5.

On another day of embarrassment for Japan's nuclear power industry, the Tokyo Electric Power company [Tepco], which operates the plant, said the amount of radioactivity in water that leaked into the sea during the earthquake was 50% higher than it originally said. The firm blamed a calculation error and said the levels were still well within safety standards.

It also said that 400 drums of low-level radioactive waste - not 100 originally reported - had toppled over during the quake. About 40 lost their lids, spilling their contents on to the ground. The spillage was one of more than 50 malfunctions the plant experienced in the immediate aftermath of the quake.

International nuclear inspectors said they were concerned by Tepco's apparent lack of preparedness for such a powerful earthquake.

"It is clear that this earthquake ... was stronger than the reactor was designed for," Mohamed ElBaradei, the head of the International Atomic Energy Agency, told reporters in Kuala Lumpur.

"It doesn't mean that the reactor structure or system has been damaged. I would hope and I trust that Japan would be fully transparent in its investigation of that accident. The agency would be ready to join Japan through an international team in reviewing that accident and drawing the necessary lessons."

The mayor of Kashiwazaki, Hiroshi Aida, ordered Tepco to close the plant indefinitely until its safety could be assured. "I am worried," he said. "It would be difficult to restart operations at this time. The safety of the plant must be assured before it is reopened."

The closure has forced the firm to ask six other power utilities to supply it with additional electricity until end of September to avoid power cuts when demand peaks later this summer.

Tepco is under pressure to explain why it took so long to inform the authorities of radioactive leaks and why only four employees were on hand to tackle a fire inside an electrical transformer that was only extinguished after firefighters arrived almost 90 minutes later.

The mishaps have raised questions about the wisdom of building nuclear power stations in a country where earth tremors are recorded, on average, every few minutes. New safety regulations were brought in last year, but upgrading ageing reactors so that they can withstand tremors of a greater magnitude than 6.5 will require huge investment.

Akira Fukushima, the deputy director general of Japan's Nuclear and Industrial Safety Agency, said no irregularities had been found in critical areas of the plant but added: "It is possible that the epicentre fault line does run beneath the power plant. Our decision on what to do in the future will depend on the report we get from Tepco."

Inspectors reportedly identified four fault lines in the area while conducting a geological survey before work began on the Kashiwazaki plant in 1980 but concluded that they were inactive, according to the Asahi newspaper.

The Citizen's Nuclear Information Centre said that the fault believed to have triggered the earthquake was not discovered during pre-construction surveys. "Clearly Japan's earthquake safety standards are inadequate," it said in a statement.

Tepco's president, Tsunehisa Katsumata, defended the firm's response. "It is hard to make everything go perfectly," he told reporters during a visit to the site. "We will conduct an investigation from the ground up, but I think fundamentally we have confirmed that our safety measures work."

Japan, which has very few indigenous energy sources, depends on 55 nuclear plants for 30% of its electricity. Despite several accidents and mounting public opposition to nuclear power, it plans to increase capacity to 40% by the end of the decade.

Backstory: nuclear accidents in Japan



Only France and the US have more nuclear power stations than Japan, where the first plant went into service in 1966. Japan's 55 plants have a total generating capacity of 49,470 megawatts, supplying just under a third of the country's electric power output.

The industry has been plagued by accidents and cover-ups in recent years.

September 30 1999: Two workers died after setting off an uncontrolled nuclear chain reaction at a uranium processing plant in Tokaimura, north of Tokyo. The workers had been mixing nuclear fuel in buckets. Thousands of residents were evacuated.



September 2002: The Tokyo Electric Power company, Japan's biggest power utility, is ordered to shut down all 17 of its reactors after it admitted falsifying safety data.



August 9 2004: Five workers are scalded to death when boiling water and steam leak from a badly decayed pipe at the Mihama No 3 nuclear power plant.



July 16 2007: Kashiwazaki-Kariwa nuclear power plant, on Japan's north-west coast, is closed indefinitely after a powerful earthquake causes a fire, minor radioactive leaks and dozens of other malfunctions.