The epicenter of the quake was about 16 miles from the Hokkaido's main airport in the city of Chitose - about 70 miles south of Sapporo.
The quake triggered a blackout across a wide area in Hokkaido.
Japan's Prime Minister Shinzo Abe speaks to reporters after a powerful quake hit Japan's northern island of Hokkaido, at Abe's official residence in Tokyo.
The natural disaster came just hours after areas in southern Japan were hit by Typhoon Jebi.
The International Tsunami Information Centre said there was no tsunami threat from the quake.
Efforts to restore power to almost 3 million households were underway but it was not clear when supplies would be restored, a company spokesman said.
In the town of Atsuma, a massive landslide occurred on the side of a mountain, crushing houses at lower levels.
And the national meteorological agency warned that more bad weather could be on the way for Hokkaido, urging people to be vigilant for landslides, high tides and heavy rain. Sounding haggard, Abe said saving lives was his government's top priority.
NHK showed a brick wall collapsed and broken glass in a home and quoted local police as reporting there had been power outages and some people trapped in collapsed structures. Altogether peak demand on the island averages 3.8 million kilowatts, stopgap supplies from smaller plants and from the nearby Tohoku region could bring supplies up to 2.9 million kilowatts within days, Seko said.
The Japanese news outlet NHK said two people have been confirmed dead, 140 people were injured and another 40 have been reported missing.
A fire broke out at a Mitsubishi Steel Mfg Co (5632.T) plant in the city of Muroran after the quake but was mostly extinguished with no injuries, a company official said.
Additionally, Japan Meteorological Agency officials told NHK that risks of aftershocks are substantial for as long as the next week.
On March 11, 2011, a devastating 9.0-magnitude quake struck under the Pacific Ocean, and the resulting tsunami caused widespread damage and claimed thousands of lives.
Almost two million people live in Sapporo.
TV footage showed some people coming out of their houses following the quake which struck at around 3:00 am local time.