ASAHIKAWA, Japan

Rescuers were rushing to unearth survivors and restore power today after a powerful earthquake jolted Japan’s northernmost main island of Hokkaido, buckling roads, knocking homes off their foundations and causing entire hillsides to collapse.

Residents in Sapporo were shaken from their beds when the magnitude 6.7 earthquake struck southern Hokkaido at 3:08 a.m. At least seven people were confirmed dead, Chief Cabinet Secretary Yoshihide Suga told a news conference. About 30 people were unaccounted for, disaster officials said.

Video cameras showed cities going dark as the quake disabled power systems, leaving nearly 3 million households on the island without electricity.

The island’s only nuclear power plant, which was offline, switched to a backup generator to keep its spent fuel cool and nuclear regulators said there was no sign of abnormal radiation — a concern after a massive quake and tsunami in March 2011 that hit northeast Japan destroyed both external and backup power to the Fukushima Daiichi nuclear plant, causing meltdowns.

Japan’s Meteorological Agency said the quake’s epicenter was 24 miles deep. But it still wreaked havoc across much of the relatively sparsely inhabited island.

Rescuers were using small backhoes and shovels to sift through the tons of soil, rocks and timber in hopes of finding survivors in the town of Atsuma, where steep mountainsides collapsed, crushing homes and farm buildings and leaving scores of brown gashes in the deep green hills.

Airports and many roads on the island were closed and trains were idled by the power outages. NHK showed workers rushing to clean up shattered glass and reinstall ceiling panels that had tumbled down in the region’s biggest airport at Chitose.

Japan is used to dealing with disasters, but the last few months have brought a string of calamities. The quake came on the heels of a typhoon that lifted heavy trucks off their wheels and triggered heavy flooding in western Japan, leaving the main airport near Osaka and Kobe closed after a tanker rammed a bridge connecting the facility to the mainland. The summer also brought devastating floods from torrential rains in Hiroshima and deadly hot temperatures across the country.

Prime Minister Shinzo Abe said that up to 25,000 troops and other personnel would be dispatched to Hokkaido to help with rescue operations.

As Japan’s northern frontier and a major farming region with rugged mountain ranges and vast forests, Hokkaido is an area accustomed to coping with long winters, isolation and other hardships. But the blackouts brought on by the quake underscored the country’s heavy reliance on vulnerable power systems: without electricity, water was cut to many homes, train lines were idled and phone systems out of order.

In the prefectural capital of Sapporo, a city of 1.9 million, the quake ruptured roads and knocked houses askew. A mudslide left several cars half buried. By late today, stores were closed and the city’s streets were deserted.

Economy, Trade and Industry Minister Hiroshige Seko told reporters that the extensive power outage was caused by an emergency shutdown of the main thermal power plant at Tomato Atsuma that supplies half of Hokkaido’s electricity.

The hope had been to get power back up within hours and some electricity was gradually being restored. However, damage to generators at the Tomato Atsuma plant meant that a full restoration of power could take more than a week.

He said utilities were starting up several other thermal and hydroelectric plants but even with those stopgap supplies thousands would still be without power for some time.

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