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.