Official evacuation orders have been issued to people on the Pacific coast of Japan and in Hawaii.
The first tsunami waves, at 30cm high, arrived at the northern coast of Japan's Hokkaido prefecture before 10.40 local time (02.40 BST), Japanese public broadcaster NHK reported. Waves at 40cm high have been observed in Tokachi, in Hokkaido.
US authorities have issued tsunami warnings for Hawaii and Alaska. Hawaii warns it could face "destructive" 3-meter high waves.