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By Kentaro Hamada

TOKYO, July 20 (Reuters) - Japan's trade minister asked industry to limit electricity use at peak periods and ordered nuclear utilities to make strict safety checks, after a strong quake forced the world's biggest nuclear power plant to shut.

Economy, Trade and Industry Minister Akira Amari said on Friday Tokyo Electric Power Co. (TEPCO), the operator of the closed plant, had told him there would be enough power if summer temperatures were average, but there may be shortages if it got too hot.

"Since we can't rule out an unusual rise in demand due to a big jump in temperatures, it is necessary to be on the safe side," Amari told reporters.

"We need to ask industry to limit power use during peak periods."

TEPCO's Kashiwazaki-Kariwa nuclear plant was closed indefinitely after Monday's 6.8 magnitude quake in northwestern Japan caused radiation leaks. Ten people were killed by the quake and hundreds of houses were flattened.

Fears about the safety of the nuclear industry -- which supplies about one-third of Japan's electricity -- have been renewed by the leaks -- and one expert said planned, tightened rules were still too lax.

TEPCO supplies power to the greater Tokyo area, where peak demand of 68 million kilowatts is forecast during the capital's humid summer.

The firm has asked six utilities to help replace lost production and said power supplies were sufficient for now.

Shutting the quake-hit plant deprives TEPCO of up to 8.2 million kilowatts of capacity -- around 12 percent of Tokyo's forecast peak demand -- which means more power must come from coal- or oil-fired thermal plants and it may need to restart some mothballed plants.

Summer has so far been reasonably cool in Japan, and the meteorological agency said on Friday it expected average or below-average temperatures in late July and early August -- the peak period for air conditioning demand.

SAFETY CHECKS, FIREFIGHTERS

Amari also ordered the nation's 11 nuclear power utilities to make strict checks on safety and set up firefighting units at their plants.

Quake-proofing regulations for Japan's 17 nuclear power stations were tightened last year, requiring utilities to reassess risks to their plants -- a process that one official said could take more than two years.

But one expert, Katsuhiki Ishibashi, a professor at Kobe University's Research Center for Urban Safety and Security, said the new rules still had loopholes and needed a further review.

"If we don't make fundamental improvements in nuclear power plants' earthquake countermeasures, Japan will suffer from a nuclear power plant-earthquake catastrophe in the near future," Ishibashi told reporters.

TEPCO has acknowledged that the tremor was stronger than the plant, whose first reactor came on stream more than 20 years ago, had been designed to withstand.

The utility also said a fault found about 30 years ago and now believed to have caused this week's quake had not caused concern at the time.

But based on recent scientific findings, it now appears that the fault is 20-30 km long -- about three times longer than what TEPCO thought previously -- and runs directly under the plant.

The quake also damaged some auto parts factories, forcing Japan's 12 automakers, including Honda Motor Co. and Nissan Motor Co., to cut production. (Additional reporting by George Nishiyama, Osamu Tsukimori, Emi Foulk and Isabel Reynolds)

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