A powerful 7.8 magnitude undersea earthquake struck in the Scotia Sea, a remote region in the far south Atlantic near Antarctica, US earthquake monitors reported Sunday.

The quake struck at 0904 GMT in the ocean some 893 kilometers (550 miles) southwest of Grytviken, South Georgia, and 1,140 kilometers (710 miles) southeast of Ushuaia, Argentina, said the US Geological Survey, which monitors earthquakes worldwide.

The epicenter was at a depth of 10 kilometers (6.2 miles), and was near that of a 6.8 magnitude undersea earthquake that the USGS registered in the Scotia Sea some 30 hours earlier.

The quake ocurred at the boundary between the Antarctic tectonic plate and the Scotia Sea plate, said geophysicist Randy Baldwin at the National Earthquake Information Center in Golden, Colorado.

"They're sliding past one another horizontally, it's not a subduction zone," Baldwin told AFP. "There will be aftershocks probably for weeks."

There were no tsunami warnings since there were no vertical movements in the seafloor as occur in a subduction quake, when one tectonic plate moves under another one, Baldwin said.

Yet despite the enormous energy unleashed the area is so remote that there is little or no impact to humans, he said.

"You couldn't pick a more remote area for an earthquake," he said.

5.5 magnitude quake hits eastern Japan: USGS
Tokyo (AFP) Nov 16, 2013 - A 5.5 magnitude quake struck eastern Japan Saturday evening, according to the US Geological Survey, causing skyscrapers to shake in Tokyo and temporarily halting high speed trains.

The quake hit at 8:44pm local time (1144 GMT) at a depth of 63 kilometres (39 miles), in the Chiba prefecture which neighbours Tokyo.

Service on a high-speed train line was briefly halted but later resumed after a track inspection.

Neither the Tokyo-Narita airport nor any nuclear installations in the region were affected, public broadcaster NHK said.

The tremor came exactly one week after another 5.5 struck close to the capital, and three weeks after a major 7.1 magnitude quake generated small tsunamis on Japan's north-east coast, without causing damage or casualties.

More than 18,000 people died when a 9.0-magnitude sub-sea earthquake sent a towering tsunami barrelling into Japan's northeast coast in March 2011 in the country's worst post-World War II disaster.

Cooling systems at the Fukushima nuclear plant were knocked out, sending reactors into meltdown and forcing tens of thousands of people to flee.