TOKYO - A strong earthquake jolted northeastern Japan on Sunday, and workers at the crippled Fukushima nuclear power plant were evacuated after an alert for a half-meter tsunami was issued by the Japan Meteorological Agency.

The quake monitoring agency said the earthquake had a magnitude of 7.1 and occurred at a depth of 10 kms (6 miles) off the northeastern coast of Japan.

There were no immediate reports of injuries or damage, public broadcaster NHK said. The same area was hit by a massive earthquake and tsunami on March 11 which cut power to the Fukushima power plant and triggered a radiation crisis.

Tokyo Electric Power said all of the workers at Fukushima had been evacuated to higher ground, adding that there was no sign of any immediate further damage at the nuclear plant where workers have been struggling to build a cooling system to stabilize the reactors.

The US Geological Survey initially estimated the quake's magnitude of 7.3, but then revised that estimate down to a magnitude 7.