The magnitude 8.0 quake hit just before dawn, cracking roads, capsizing fishing boats and causing the roof of a local airport to partially cave in. The temblor was followed by two strong aftershocks and several small tsunami waves.
Public broadcaster NHK reported that at least 236 people were hurt. Most of the injuries were caused by glass from shattered windows and falling objects in homes. Officials said at least two people suffered serious injury, but most of the other injuries reported initially were minor.
A 61-year-old man cleaning up broken beer bottles on a street immediately after the quake was hit by an oncoming car and died, Hokkaido police said. There were no reports of other quake-related deaths.
"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."
She said the city offices were a mess, with papers and books strewn around by the quake. But she added that there was no serious damage.
The quake struck at 4:50 a.m. and was centered in the Pacific about 100 kilometers (60 miles) off Hokkaido's eastern shore. Japan's Central Meteorological Agency initially estimated the quake's magnitude at 7.8, but later revised that to 8.0.
That would make it the most powerful to hit anywhere in the world this year.
A powerful aftershock of magnitude 7.0 followed shortly after 6 a.m., and another hit at about 8 a.m.
The government warned residents to avoid coastal areas due to the possibility of tsunami, or ocean waves caused by seismic activity.
Japan's meteorological agency said a tsunami of about 1.30 meter (4 feet) had struck Kushiro, a city of about 200,000. There were no immediate reports of damage from the wave.
Because of the threat of tsunami, aftershocks or the collapse of already damaged buildings, 41,000 people were evacuating their homes to local shelters, according to Kazuhiko Kunii, a spokesman for the National Fire Agency.
Hiroaki Tanaka, a Kushiro fire department official, said 50 people had been treated there for bruises and broken bones. Hokkaido government official Hiroyuki Nakao said 31 people had been injured, two of them seriously, in towns outside Kushiro.
Officials could not immediately confirm the NHK report of the 236 injured. But they said the number of injured was certain to rise as not all areas had submitted damage reports.
Officials said most of the injured were hit by falling shelves and other toppled objects. One 70-year-old woman was being treated at a hospital after breaking her leg as she was trying to leave her house through a window.
Huge cracks forced the closure of local highways, and regional airports were closed for inspection. Hokkaido Electric Power Co. official Yoshihiro Akiyama said it was unclear when power could be restored to the blacked out homes, mostly in and around Kushiro.
A fire broke out at an oil tank in the city of Tomakomai, but no workers were reported injured, said Hokkaido prefectural police official Kuniyoshi Omori. Black plumes of smoke and flames leapt from the tank, which police said belonged to Idemitsu Co.
The fire was contained within three hours.
Nuclear power facilities on Hokkaido were inspected after the quake, but officials said there was no evidence of any damage or safety breaches.
Omori said one person was injured when a local train carrying about 39 passengers derailed. Kushiro airport was temporarily closed after part of a roof caved in, and several roads were blocked by landslides.
The quake, which had its epicenter about 60 kilometers (36 miles) under the seabed, is the largest to strike so far this year anywhere in the world, according to the U.S. Geological Survey.
On average, there is one magnitude-8, or great, quake a year in the world, USGS geophysicist Brian Lassige said from Golden, Colorado. The amount of energy released in a magnitude-8 earthquake is equivalent to that contained in 1.01 billion tons of TNT, according to the USGS.
Japan is one of the world's most earthquake-prone countries. It sits atop four tectonic plates, slabs that move across the earth's surface.
Earlier this month, Tokyo marked the 80th anniversary of a magnitude 8.3 quake that devastated Tokyo and neighboring Yokohama, killing at least 140,000 people. In January 1995, a magnitude 7.2 temblor in Kobe killed more than 6,000 people.
Hokkaido is the northernmost and most sparsely populated of Japan's major islands. Sapporo is the prefecture's capital.
A quake generated by the same fault caused a major earthquake in 1956. Another quake and tsunami on the western side of Hokkaido killed 230 people in July 1993, most on the nearby isle of Okushiri.
By Eric Talmadge