The U.S.G.S. says the earthquake with a preliminary magnitude of 6.4 struck at 3:53 a.m. Sunday in the Pacific Ocean, about 11 miles (18 kilometers) east-southeast of Iwaki on Honshu Island. Its depth was 27 miles (43.5 kilometers).
U.S.G.S. says its epicenter is 114 miles (184 kilometers) northeast of Tokyo.
About 23,000 people died or were left missing across wide swaths of Japan's northeast coastline after the March 11 earthquake and tsunami. Another 80,000 have been forced to evacuate their homes because of the radiation threat from the Fukushima Dai-ichi nuclear plant.