The powerful aftershocks set off a new blaze in an oil refinery, though no reports of any casualties were yet available.

"A fire started at a naphtha tank shortly before 11 am (0200 GMT)," an official at the Idemitsu Kosan Co. Ltd refinery said.

It was the second fire to hit the refinery. The powerful quake on Friday measuring 8.0 on the Richter scale had set off a major fire at a crude oil storage tank.

The tank for naphtha, used as a raw material for petrochemical products, has a storage capacity of more than 7.8 million gallons.

Since the first tremor on Friday, Hokkaido has been jolted by more than 110 quakes. Of them, about 70 were strong enough to be felt by humans.

Japan is vulnerable to earthquakes since it sits atop the junction of three tectonic planes, whose movements trigger tremors.

The worst quake to hit the country in recent times was in the city of Kobe on 17 January, 1995. That killed 6,432 people and injured some 43,000 others.