TOKYO (AFP) -- Seismologists Sunday warned Japan to stay vigilant for the next "Big One" after a powerful 7.8-magnitude earthquake struck off the coast of the quake-prone nation, injuring a dozen people.

Buildings swayed for around a minute in Tokyo and its vicinity Saturday night as the quake struck at a remote spot in the Pacific Ocean 874 kilometres south of the capital, the US Geological Survey said.

Despite its power, there was no risk of a tsunami as its epicentre was a deep 676 kilometres below the Earth's surface, the USGS and the Pacific Tsunami Warning Center said.

Twelve people were injured, including a 56-year-old man who broke his ribs, but no one was killed, an official of the Tokyo Fire Department and local media said Sunday.

Some 400 people were trapped on the observation decks of Tokyo Tower as its elevators stopped for more than one hour.

Runways at Haneda Airport in Tokyo were closed for about 30 minutes, with trains also temporarily halted, while a football match in the city was briefly suspended.

There were no reported problems at any of the region's mothballed nuclear power plants.

A separate 6.2-magnitude quake hit Sunday morning off the Izu Islands south of Tokyo, the USGS said, but there were no reports of damage or injuries.

A massive undersea quake in March 2011 sent a tsunami barrelling into Japan's northeast coast, killing thousands of people and sending three reactors into meltdown at the Fukushima nuclear plant.

The nuclear disaster, the world's worst since Chernobyl, displaced tens of thousands of people and rendered tracts of land uninhabitable, possibly for decades.

Saturday's quake was the second sizeable tremor Tokyo has experienced in a week, after a much less powerful -- but far shallower -- earthquake close to the capital on Monday.

Some experts warn recent quakes and volcano eruptions may be signs that areas near the country are entering an active phase of crustal changes.

"I can say Japan is in an active stage now," said Toshiyasu Nagao, head of Earthquake Prediction Research Centre at Tokai University.

"Considering the geographic location of Japan, we can say the current activities are rather normal and it was too quiet" before the 2011 jolt, Nagao told AFP.

"We should be vigilant by knowing that it is no wonder that an earthquake sizeable enough to affect our society can occur anytime in the future," he said.

Kazuki Koketsu, professor with the Earthquake Research Institute at the University of Tokyo, said the latest tremor was unlikely to be a sign of a potential big jolt in the capital, which was devastated by an massive earthquake in 1923.

"But it is important to regard it as an opportunity to prepare for a future quake," Koketsu told TV Asahi.

Japan sits at the meeting place of four tectonic plates and experiences around 20 percent of the world's most powerful earthquakes every year.

But rigid building codes and strict enforcement mean even powerful quakes frequently do little damage.

On Friday a volcano in the far south of Japan erupted, spewing a huge column of ash high into the sky and forcing authorities to evacuate the island on which it sits.

The eruption caused no injuries and no damage was reported, but it was another reminder of the country's volatile geology.