Three people have been killed and scores injured after a powerful aftershock struck north-east Japan.
Several buildings were destroyed and power was cut to 3.6 million homes.
It was the most powerful tremor since the 9.0-magnitude quake that triggered a devastating tsunami four weeks ago.
At the crippled Fukushima nuclear power station workers briefly retreated to a quake-proof shelter. The plant's operator later said there was no sign problems there were any worse.
The latest earthquake struck just before midnight on Thursday, at a depth of 49km (32 miles), close to the epicentre of the 11 March quake.
First reports said it had a magnitude of 7.4 but that was later revised to 7.1, according to the US Geological Survey (USGS).
A tsunami warning was lifted after about 90 minutes.
Last month's quake struck at 32km deep. More than 12,700 people are known to have died in the disaster and nearly 15,000 people remain unaccounted for. Hundreds of thousands have been made homeless.
In the latest earthquake, a 63-year-old woman died when the tremor knocked out power in Yamagata prefecture, shutting off her respirator.
In Miyagi Prefecture, two men, aged 79 and 85, died at a hospital. Fire officials say the quake may have brought on heart attacks.
Japan's nuclear safety agency said facilities along the north-east coast were under control. Back-up diesel generators kicked in at several plants after external power was lost.
Operations have been suspended at all nuclear power plants from Aomori to Ibaraki prefectures since the 11 March quake, but electricity is still crucial to keep the cooling systems operating.
Workers at the crippled Fukushima Daiichi nuclear power station were safe, a spokesman for plant operator, Tepco, told a news conference in Tokyo.
No new irregularities were detected in radiation readings or at the facilities, the firm said.
Workers are trying to keep the damaged reactors cool to stop further releases of radioactive material.
Work to discharge low-level radioactive water into the sea from a storage facility would continue on Friday, Tepco said.
The work is designed to make room for highly radioactive water that leaked into the basement of the turbine building next to the plant's No 2 reactor and an adjoining tunnel.
The company said it would also continue work to inject nitrogen into the containment vessel of the No 1 reactor to prevent a possible hydrogen explosion.
China has urged Japan to observe international law and adopt effective measures to protect the marine environment, amid concern over the discharge of contaminated water into the Pacific Ocean.
The foreign ministry also asked Japan for swift, comprehensive and accurate reports on the crisis.
Fish exports from Japan have been hit by the radiation leaks.