TOKYO — Japan was rattled by a magnitude-7.4 aftershock and tsunami warning overnight nearly a month after a devastating earthquake and tsunami flattened the northeastern coast.

Announcers on Japan’s public broadcaster NHK told residents along the northeastern shore to run to move ground and away from the shore. An hour after the quake, there were no reports of a tsunami hitting the shore.

The Japan meteorological agency issued a tsunami warning for a wave of up to 6 feet for a coastal area already torn apart by last month’s tsunami, which is believed to have killed some 25,000 people and has sparked an ongoing crisis at a nuclear power plant.

Officials at the tsunami-ravaged Fukushima Daiichi nuclear plant said there was no immediate sign of new problems caused by the aftershock. Japan’s nuclear safety agency says workers there have retreated to a quake-resistant shelter in the complex. No one there was injured.

Officials say today’s aftershock hit 30 miles under the water and off the coast of Miyagi prefecture.