An earthquake with a preliminary magnitude of 5.6 jolted Miyagi Prefecture in northeastern Japan early Thursday, the Japan Meteorological Agency said, issuing no tsunami warning. There were no immediate reports of major injuries, (Kyodo) news agency quoted local officials as saying. No abnormalities have been confirmed at three nuclear power plants in the region -- Onagawa in Miyagi Prefecture, and Fukushima Daiichi and Daini in Fukushima Prefecture, south of Miyagi, their operators said. The quake originated in the Pacific off the coast of Miyagi at a depth of about 60 kilometers and measured upper 5 on the Japanese seismic intensity scale of 7 in the northern and central parts of the prefecture, the agency said.