Japan has been hit by a strong earthquake and tsunami warning, with emergency workers at the stricken Fukushima nuclear plant ordered to evacuate.
The Tokyo Electric Power Company (Tepco), which operates Fukushima, says it is checking on the situation, but efforts to assess any damage were complicated by the evacuation.
"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 spokesman for Tokyo Electric Power said.
"We don't know if there is any impact to the facilities as all the workers have cleared the area."
The Japan meteorological agency issued a tsunami warning for a wave of up to 6 feet (two metres) after the magnitude-7.4 aftershock. The warning was issued 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.
"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.
Officials say Thursday's quake was a 7.4-magnitude and hit 40km under the water and off the coast of Miyagi prefecture. The quake that preceded last month's tsunami was a 9.0-magnitude.
Buildings as far away as Tokyo shook for about a minute.
In Ichinoseki, inland from Japan's eastern coast, buildings shook violently, knocking items from shelves and toppling furniture, but there was no heavy damage to the buildings themselves. Immediately after the quake, all power was cut. The city went dark, but cars drove around normally and people assembled in the streets despite the late hour.
U.S. Geological Survey gave the preliminary magnitude as 7.4 and it struck off the eastern coast 100km from Sendai and 140km from Fukushima. It was about 345km from Tokyo.
The depth was 40km. Shallower quakes tend to be more destructive.
Hundreds of aftershocks have shaken the northeast region devastated by the March 11 earthquake, but few have been stronger than 7.0.