(Reuters) - An earthquake with a magnitude of 6.3 struck off Samoa in the Pacific Ocean on Monday, the U.S. Geological Survey (USGS) said, just hours after a major tremor rattled nearby Papua New Guinea. The epicentre of the quake was measured 77 miles (124 km)west-southwest of the capital Apia, at a depth of 72.2 miles (116 km). USGS reported a 7.7 quake near the town of Rabaul in the northeast of Papua New Guinea earlier on Monday.