The quake, which hit at 11:32 pm local time (1432 GMT), was initially measured at 7.4-magnitude, according to the US Geological Survey, which said it struck 66 kilometres (40 miles) east of Sendai at a shallow depth of 25.6 kilometres (15.9 miles).

USGS seismologists later downgraded its strength to 7.1, and revised its depth to 49 kms.

Workers battling to control the stricken Fukushima nuclear plant on Japan's northeast coast were ordered to evacuate, plant operator Tokyo Electric Power Co. said.

"After the earthquake and the tsunami warning, all the workers evacuated to a safe area. The company confirmed all the workers have cleared the plant safely," a TEPCO spokesman said.

"We have no information immediately indicating any abnormality at Fukushima Daiichi plant," a spokesman told a press conference.

Japan's weather bureau issued a tsunami alert for its Pacific coast, saying that waves of up to two metres could hit the shoreline.

"Please be warned that a tsunami as high as two metres is expected in some areas," Japan's meteorological agency said.

In a statement on its website the Hawaii-based Pacific Tsunami Warning Center said it did not expect a Pacific-wide tsunami.

Footage from broadcaster NHK showed that the power was off in parts of Sendai, a regional commercial hub that was heavily affected by the March 11 quake.

The broadcaster said gas and water leaks were being reported in some areas of the city.

Although the epicentre was at a distance of 333 kilometres (207 miles) from Tokyo, it caused buildings to shake in the Japanese capital.

"Please do not hesitate to leave for higher ground, nor try to return to the coast line. Please do not try to check the status of the coastline," broadcaster NHK said repeatedly.

Its advice not to go to the coastline was supposedly addressed to fishermen worried about their boats.

Workers have been grappling to tame runaway reactors at the Fukushima plant, which was badly damaged by the massive tsunami that hit Japan's northeast on March 11.

Cooling systems were knocked out, leaving the temperature of the nuclear cores to rise and setting off a scramble to prevent a meltdown.

earlier related report
Police search for bodies in Japanese nuclear zone
Tokyo (AFP) April 7, 2011 - Armed with radiation meters and protective gear, police launched an intensive search Thursday for people missing inside the exclusion zone around Japan's disaster-stricken nuclear power plant.

About 250 officers from the Tokyo police force joined 50 local officers who have now been searching for four days within a 10-20 kilometre (six-12 mile) arc around the plant, a Fukushima police spokesman said.

More than 2,400 people who lived in the search area are listed among the 15,000 people still unaccounted for nearly four weeks after the earthquake and tsunami struck on March 11.

A further 12,600 people have been confirmed dead in the double catastrophe that ravaged the country's Pacific coast northeast of Tokyo and crippled the Fukushima Daiichi nuclear plant.

"The search team is using dosimeters to make sure they are safe," the spokesman said.

There were no immediate reports of any bodies being discovered in the search, which was due to last a week.

A group of police officers wore white protective clothes with hoods as they moved into an area where only one house still stood amid the rubble of other homes, television footage showed.

They also wore goggles, masks and boots while some donned white helmets as well.

In the area, some 18 kilometres from the plant, a number of uprooted trees were scattered over what used to be rows of rice paddies.

About 10 diggers and bulldozers were used to clear rubble while searchers used long sticks to sort through debris.

A crew from broadcaster NHK said they had stayed inside the area for about 90 minutes and their dosimeters registered little change.

Japan's military have so far avoided searching inside the 20 kilometre boundary, as radiation levels were considered too high.

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