An earthquake last month that forced the closure of the world's largest nuclear plant in Japan was about 2.7 times stronger than the maximum considered in the plant's design guidelines, a report said yesterday.
Tokyo Electric Power, which operates the Kashiwazaki-Kariwa nuclear power plant, registered the July 16 quake at 993 gals, the Mainichi Shimbun reported. A gal is a unit measuring the speed of acceleration.
The government's guidelines for the design of nuclear power plants only envisioned quakes of up to 370 gals, the report said, adding that eight of the country's 17 plants had been built based on that estimate.
Experts are now calling for a review of the guidelines in the wake of the 6.8-magnitude quake in central Japan last month.
The earthquake shut down the plant, which supplies 10 percent of the electricity company's power.
The giant facility northwest of Tokyo caught fire and leaked a small amount of radiation following the quake.
Japanese nuclear authorities expect the plant to be offline for about a year for safety checks, although the UN International Atomic Energy Agency said last week the plant had no major safety problems.
Japan relies on nuclear plants for nearly one-third of its power needs, but it is one of the world's most earthquake-prone countries and sits atop four tectonic plates.
The company and government have already acknowledged that they never anticipated such a strong quake in the area near the Kashiwazaki-Kariwa plant.
Meanwhile, a moderate earthquake with a preliminary magnitude of 5.6 jolted northern Japan yesterday, the Japan Meteorological Agency said. No tsunami warning was issued.
The quake struck at 2:31pm and was centered off the eastern coast of Hokkaido, Japan's northernmost island, the agency said.
No injuries or damage were immediately reported.