Aftershocks as strong as magnitude 5.4 continued rattling the area in the wake of Friday's magnitude 8 quake.
The fire at the refinery in the northern town of Tomakomai broke out in a 80-foot-high storage tank containing naphtha, a flammable liquid produced when petroleum is distilled. It was still burning eight hours later, sending a plume of black smoke into the air.
It was the second time in three days that a tank went up in flames at the refinery, run by one of Japan's largest oil companies, Idemitsu Co. Friday's earthquake set off a fire in a separate tank that consumed 30,000 kiloliters, or 188,700 barrels, of crude oil.
The lid of the tank was shaken partly open, and the naphtha ignited when it was exposed to the air, national broadcaster NHK and Kyodo News reported.
A disaster official in Tomakomai said the cause was still being investigated, and an Idemitsu spokesman declined to comment.
Firefighters worked into the night to douse the blaze with chemicals and drain the tank, a task expected to last through Monday, the disaster official said on condition of anonymity.
About 570 people were hurt in Friday's quake _ the strongest quake recorded in the world in the past 2 1/2 years. Two fishermen were missing and believed to have been swept out to sea by waves triggered by the quake.
But most of the injuries were minor. The temblor was centered deep under the seabed about 60 miles off the coast.
Only 5,600 of the 34,000 people in 14 towns covered by the evacuation advisory showed up at shelters, the Mainichi newspaper said, citing its own tallies. Northern Japan has been rattled by several magnitude-7-class earthquakes in the past few decades.
