Tokyo, April 21 -- A magnitude 7.7 earthquake struck off the northeastern coast of Japan on Monday, prompting authorities to urge residents to stay away from coastal areas where tsunami waves of up to 3 metres were expected. Two hours after the tremor, which struck at 4:53pm (0753 GMT), tsunami waves as high as 80 cm had been detected. A tsunami warning was later downgraded to a tsunami advisory. There were no immediate reports of casualties or major damage, Japan's top government spokesperson Minoru Kihara told a news conference as night fell in the capital Tokyo. Several port towns including Otsuchi and Kamaishi - both hard-hit by a massive earthquake and tsunami in 2011 - earlier issued evacuation orders for thousands of residents, accor...