Japan braced for more aftershocks of giant 2011 quake

Seismologists said an earthquake that struck near Japan's shuttered Fukushima nuclear site early Saturday was an aftershock of the tremor that sparked 2011's deadly tsunami, and warned of more to come.

The strong 6.8-magnitude earthquake off the Pacific coast of northeastern Japan caused a minor tsunami in the early hours, though authorities lifted all weather warnings roughly two hours later.

Seismologist Yasuhiro Yoshida of the Japan Meteorological Agency said it was a delayed tectonic reaction to the 9.0-magnitude quake which left the Fukushima nuclear power plant in a meltdown crisis after the coast was ravaged by monster tidal waves in March 2011.

"There are fears that relatively large earthquakes will occasionally occur in the ocean area where aftershocks of the great earthquake continue," he said.

"The aftershock activity has been steadily declining on a long-term basis. But aftershocks, accompanied by tsunamis, will still occur."

The 2011 disaster killed more than 18,000 people.

Saturday's quake measured up to four on the Japanese scale of seven in terms of intensity, and Yoshida said there was a possibility aftershocks measuring a moderate three on that scale would occur in the next two weeks.

At least three were injured in the latest quake but there was no damage done to the crippled nuclear plant less than 24 hours after dwindling super typhoon Neoguri skirted the Fukushima region and veered off into the Pacific, officials said.

The quake occurred at a depth of 13 kilometres (eight miles) off Fukushima's coast, according to the US Geological Survey.

It was strong enough to be felt in Tokyo, more than 200 kilometres (125 miles) to the south, with many people woken by earthquake alerts sent by the Japan Meteorological Agency to their mobile phones.

Japan's islands are situated at the conjuncture of several tectonic plates and experience a number of relatively violent quakes every year.

But thanks to strict building codes, even powerful quakes that might wreak havoc in other countries frequently pass without causing much damage.

Fukushima plant operators Tokyo Electric Power (TEPCO) said there were no reports of any abnormality early Saturday.

Sea levels cannot be gauged near the nuclear plant as the tsunami monitoring system was destroyed by the 2011 disaster.

"We have not seen any damage or any change in radiation gauges after the quake," said TEPCO spokesman Masahiro Asaoka.

"Our temporary breakwater that was newly built at the plant is high enough to block a one-metre tsunami," he added.

The Fukushima plant's cooling systems were swamped by the tsunami three years ago, sparking reactor meltdowns and radiation leaks in the worst atomic crisis since the 1986 Chernobyl disaster.

Tens of thousands of people were evacuated from around the plant, with decommissioning of the site expected to take decades.

The facility is struggling to handle a huge and swelling volume of contaminated water at the plant.