WASHINGTON- A strong earthquake of 6.5 magnitude struck off the coast of Papua New Guinea, the US Geological Survey said on Sunday.The quake's epicenter was about 76 miles (122 km) west of Arawa, Bougainville, Papua New Guinea. It occurred at a depth of 27 miles (43 km), USGS said.The Pacific Tsunami Warning Center said there was no threat of a tsunami from the quake.