A magnitude 7.1 tremor shook buildings in Tokyo and a wide swathe of eastern Japan on Monday evening, triggering a small tsunami alert. NHK state television said it caused the off-site power supply for two damaged reactors to shut down.
The U.S. Geological Survey said the aftershock struck 38 km (24 miles) west of the city of Iwaki, at a depth of 13 km (8 miles).
Tokyo Electric Power Co (TEPCO), which operates the plant, said workers had stopped pouring cooling water on reactors No. 1, No. 2 and No. 3 at Fukushima.
Chief Cabinet Secretary Yukio Edano said villages and towns outside the 20 km (12 mile) evacuation zone that have had more accumulated radiation would be evacuated. Children, pregnant women, and hospitalized patients should stay out of some areas 20-30 km from the Fukushima nuclear complex, he added.
The decision to widen the evacuation band around the Fukushima plant was "based on data analysis of accumulated radiation exposure information", Edano told a news conference.
"These new evacuation plans are meant to ensure safety against risks of living there for half a year or one year," he said. There was no need to evacuate immediately, he added.
Japan had resisted extending the zone despite international concerns over radiation spreading from the six damaged reactors at Fukushima, which engineers are still struggling to bring under control after they were wrecked by the 15-meter tsunami.
Residents of one village, Iitate, which is 40 km from the Fukushima Daiichi plant, have been told to prepare for evacuation because of prolonged exposure to radiation, a local official told Reuters by phone. It has a population of 5,000.
The International Atomic Energy Agency has urged Japan to extend the zone and some countries, including the United States, have advised their citizens to stay 80 km away from the plant.
TEPCO President Masataka Shimizu visited the area on Monday for the first time since the March 11 disaster. He had all but vanished from public view apart from a brief apology shortly after the crisis began and has spent some of the time since in hospital.
"I would like to deeply apologize again for causing physical and psychological hardships to people of Fukushima prefecture and near the nuclear plant," said a grim-faced Shimizu.
Dressed in a blue work jacket, he bowed his head for a moment of silence with other TEPCO officials at 2:46 p.m. (0546 GMT), exactly a month after the earthquake hit.
Fukushima Governor Yuhei Sato refused to meet Shimizu during his visit, but the TEPCO boss left a business card at the government office.
Sato has criticized the evacuation policy, saying residents in a 20-30 km radius were initially told to stay indoors and then advised to evacuate voluntarily.