(Updates with PNG observatory comment) SYDNEY, June 7 (Reuters) - A strong earthquake struck in the Bismarck Sea off the northern coast of Papua New Guinea on Thursday, but residents on the mainland said there were no signs of a change in sea level and there had been no reports of damage. The 6.1 magnitude quake, which was 10 km (6.2 miles) deep, was centred 675 km (420 miles) north of Papua New Guinea's capital, Port Moresby, said the U.S. Geological Survey (USGS) said on its Web site. Residents in the towns of Madang and Wewak on Papua New Guinea's north coast said they had not felt the quake -- which hit at 10:40 a.m. (0040 GMT) -- and that there had been no change in sea levels. "We have not felt anything and we have not seen any change in the sea," the receptionist at the Windjammer Beach Hotel in Wewak told Reuters by telephone. Staff at the hospital on Manus Island, which lies north of the epicentre, also said they had not felt the quake. Port Moresby's Geophysical Observatory said it was awaiting reports from the area but was not surprised to hear that residents had not felt the quake. "Commonly these earthquakes in the Bismarck Sea are shallow and do not distribute a lot of energy and are not commonly felt. It (the quake) is a long way from anywhere," said Chris McKee, head of the observatory. McKee said earthquakes in the Bismarck Sea lie on a fault line which normally produces horizontal movements of the earth's crust and not vertical ruptures -- the later producing greater movement in the ocean. "The seafloor does not move up or down but sideways and there is unlikely to be a sudden movement of the sea," he said. In July 1998, two undersea quakes measuring 7.0 in the Bismarck Sea created three tsunamis that killed at least 2,100 people near the town of Aitape on Papua New Guinea's north coast. In April an undersea earthquake off neighbouring Solomon Islands created a tsunami which destroyed villages, killing 28 people, and leaving thousands of people homeless. Papua New Guinea and the neighbouring Solomon Islands lie on the so-called Pacific "Ring of Fire" where volcanic activity and earthquakes are common.
