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Seismologists have warned Japan to stay vigilant for the next "Big One" after a powerful 7.8-magnitude earthquake hit off the coast of the quake-prone nation, injuring a dozen people.
Buildings swayed for about a minute in Tokyo and its vicinity on Saturday night as the quake struck at a remote spot in the Pacific Ocean about 874km south of the capital, the US Geological Survey said.
Despite its power, there was no risk of a tsunami as the epicentre was a deep 676km below the earth's surface, the USGS and the Pacific Tsunami Warning Centre said.
Twelve people were hurt, including a 56-year-old man who broke his ribs, but no one was killed, a Tokyo Fire Department official and local media said.
Some 400 people were trapped at the observation decks of Tokyo Tower as its lifts stopped for more than one hour.
Runways at Tokyo's Haneda Airport were closed for about 30 minutes, with trains also temporarily halted. A soccer match in the city was briefly suspended.
There were no reported anomalies at any of the region's mothballed nuclear power plants.
A massive undersea quake that hit 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 hit 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 said.
"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 a 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 about 20 per cent of the world's most powerful earthquakes every year.
But rigid building codes and strict enforcement mean even powerful quakes frequently do little damage.


Nervous Japanese warned of the next 'Big One' a day after powerful quake

Worried soccer fans react to a strong earthquake in Hiratsuka, near Tokyo, on Saturday. Photo: Reuters

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