There were no immediate reports of casualties or major damage, Japan's top government spokesperson Minoru Kihara said. TOKYO, April 20 (Reuters) - 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 meters (10 ft) were expected. Two hours after the tremor, which struck at 4:53 p.m. (0753 GMT), tsunami waves as high as 80 cm had been detected. A tsunami warning was