Japan hit by 7.1 aftershock on earthquake anniversary

A tsunami warning issued after a magnitude-7.1 aftershock rattled Japan has been lifted.

The warning was in place for only about an hour. The aftershock at about 8.30pm New Zealand Time shook buildings and briefly forced Tokyo's main international airport to close both of its runways, but there have been no immediate reports of damage or injuries.

The warning came as Japan observes the one-month anniversary of the massive wave that killed as many as 25,000 people.

The epicenter of the aftershock was just inland and about 160 kilometres north of Tokyo.

The operator of the crippled Fukushima Dai-ichi nuclear complex says the latest quake briefly cut power but it was quickly restored. Operations there are not endangered.

People at a large electronics store in the northeastern city of Sendai screamed and ran outside, though the shaking made it hard to move around. Mothers grabbed their children, and windows shook. After a minute or two, people returned to the store.

Last Thursday's 7.1-magnitude aftershock, which had been the strongest tremor since the day the original quake hit, did sink hundreds of thousands more households into darkness, however. Most of that electricity has been restored.

The 9.0-magnitude earthquake and the tsunami it generated on March 11 are believed to have killed more than 25,000 people and caused as much as $310 billion in damage. The nuclear power plant they disabled has been spewing radiation since, and even a month on, officials say they don't know how long it will take to cool reactors there.