The Japanese government has attacked the operator of the crippled Fukushima nuclear plant for "unacceptable" mistakes.
The Tokyo Electric Power Company (Tepco) had said radiation levels were 10m times higher than normal before correcting the figure to 100,000 times.
Workers are battling to stop radiation leaks at a badly damaged nuclear plant.
Another earthquake off the north-east coast of Japan has shaken the already devastated region.
The 6.5-magnitude quake, 109km (67 miles) east of the badly-damaged port city of Sendai, prompted a brief warning of a possible small tsunami.
A much stronger earthquake on 11 March and the powerful tsunami it triggered killed more than 10,000 people and left many thousands more missing.
'Unacceptable'"Considering the fact that the monitoring of radioactivity is a major condition to ensure safety, this kind of mistake is absolutely unacceptable," the government spokesman Yukio Edano told a news conference.
"(The government) has ordered Tepco not to repeat this again," he said.
The government understood the workers were overworked and under stress, he added.
He also explained how high level of radiation in water flooding the basement of reactor 2 was probably to be due to melted fuel rods.
"The radiation seems to have come from fuel rods that temporarily melted down and came in contact with the water used to cool the reactor," Mr Edano said.
"Steam may have condensed ... carrying water from within the containment vessel."
The operators of the Fukushima-Daiichi nuclear plant on Sunday apologised for a "mistake" in reporting a radiation spike 10 million times above normal in one of the site's reactors.
Tepco said a worker taking the reading in a part of reactor 2's cooling system had no time to confirm with a second reading because the radiation level was so high anyway he had to leave the area.
"The number is not credible. We are very sorry," said Tepco spokesman Takashi Kurita.
There have been no reports of damage or injuries from the latest earthquake, which struck at 0723 on Monday (2223 GMT Sunday), according to the US Geological Survey.
The Japan Meteorological Agency warned that a tsunami of 50cm (18 inches) could hit Miyagi prefecture but later lifted the advisory.
UneaseA spokesman for Japan's nuclear watchdog, Hidehiko Nishiyama, said the level of radiation in puddles near reactor 2 was confirmed at 1,000 millisieverts an hour.
"It is an extremely high figure," Mr Nishiyama said.
The radiation levels are so high, that emergency workers near the contaminated water would have received four times their maximum annual dose of radiation in just one hour.
The BBC's Mark Worthington in Tokyo says the erroneous report has created more confusion around a crisis that is already causing widespread unease in the country.
On Sunday, anti-nuclear protesters held a large rally in Tokyo, calling for change in Japan's nuclear industry.
Meanwhile, efforts are continuing to locate the exact source of the radioactive water leak, amid concerns that the water is leaking directly from the reactor itself.
Earlier, Japan's nuclear agency said that levels of radioactive iodine in the sea near the plant had risen to 1,850 times the usual level.
The emergency workers are also trying to cool the reactors in an effort to prevent a meltdown. They have now switched to using fresh water as a coolant, rather than sea water.
There had been fears the salt in sea water could further corrode machinery. The fresh water is being pumped in so that contaminated radioactive water can be extracted.
The UN's nuclear agency International Atomic Energy Agency (IAEA) has warned the crisis could go on for months.
However, the government in Tokyo has said that airborne radiation around the plant is decreasing, so there is no need to extend the evacuation zone.
Tepco has been criticised for a lack of transparency and failing to provide information more promptly and for making a number of mistakes, including worker clothing.
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