(Undated) -- Japan is bracing for a possible tsunami after a strong earthquake. The quake struck about 200 miles east of Japan's eastern coast. Japanese media sources say it had a magnitude of six-point-eight, while the U.S. Geological Survey says it had a magnitude of seven-point-three. It struck at a depth of about six miles. The Japan Meteorological Agency has issued a tsunami advisory for the Fukushima prefecture.