*11:30 a.m. update: Earthquake downgraded to 7.1 magnitude.
(NECN) - The northeastern coast of Japan has been hit by a 7.4 magnitude earthquake -- a strong aftershock.
A tsunami warning for a wave of up to one meter has been issued by Japan's meteorological agency.
The warning has been issued for a coastal area already ravaged by last month's earthquake and tsunami.
Thursday's quake hit 25 miles under the water and off the coast of Miyagi prefecture.
Officials at the tsunami-ravaged Fukushima Dai-ichi nuclear plant said there's no immediate sign of new problems caused by the aftershock. Plant operator Tokyo Electric Power Co. said it evacuated two workers there and seven at a sister plant to the south that was not badly damaged.
The quake that preceded last month's tsunami was a 9.0 magnitude.
Last month's tsunami is believed to have killed some 25,000 people.