There were no immediate reports of deaths directly caused by the quake in the hours following the temblor that forced the evacuation of some 41,000 people and left some 16,000 homes blacked out.

The government warned residents to avoid coastal areas due to the possibility of tsunami, or ocean waves caused by seismic activity. The meteorological agency said tsunami as high as 3 feet hit the city of Kushiro, about 510 miles north of Tokyo. There were no reports of damage.

The quake struck at 4:50 a.m., cracking roads, capsizing fishing boats and causing the roof of a local airport to partially cave in. The temblor, centered in the Pacific Ocean about 60 miles off Hokkaido’s eastern shore, had a magnitude of 8, Japan’s Central Meteorological Agency said.

An earthquake of that magnitude is capable of causing tremendous damage. The meteorological agency said the earthquake was centered 36 miles under the seabed.

An aftershock of magnitude 7 followed shortly after 6 a.m., the U.S. Geological Survey said from Golden, Colo. Another aftershock was reported at 8 a.m. but its magnitude was not immediately known.

Television footage showed an office where books were knocked off shelves, and desks and computers swayed as the quake hit. Merchandise fell off store shelves and people sought shelter in schools.

“It shook hard and long and I was very frightened,” said Eri Takizawa, a city official in Kushiro, which was believed to be the hardest hit. “We have small quakes here from time to time, but this was completely different.”