By Masayuki Kitano

Tokyo - A series of powerful earthquakes shook the northern Japanese island of Hokkaido early on Friday, injuring more than 270 people, causing widespread damage and blackouts and prompting officials to issue tidal wave warnings.

Japan's Meteorological Agency measured the initial quake at 8.0 on the Richter scale - strong enough to cause major damage - and warned there could be aftershocks for up to 10 days.

Roads and buildings cracked, roof tiles fell and gravestones tumbled over. A storage tank at an oil refinery caught fire and the plant had to be closed.

Quake-generated waves measuring about one metre (three feet) in height struck the eastern Hokkaido coast, washing away some empty cars, but no major wave damage was reported. More than 40,000 people left their homes at least temporarily in response to the tidal wave warnings, Kyodo news agency said.

The only quake-related death reported was that of a 61-year-old man who was struck by a car as he picked up broken beer bottles on the street, officials said. NHK said at least 278 were injured in the lightly populated area.

The focus of the first quake, which was felt in Tokyo about 975km to the south, was 42km below the seabed in the Pacific Ocean near the port of Erimo. A second quake measuring 5.8 on the Richter scale jolted Hokkaido about an hour later, followed by another measuring 7.0.

The airport in the eastern town of Kushiro was closed after the ceiling of the control tower collapsed. Roads were closed and rail services were halted in many areas.

One person was injured when a passenger train derailed.

The first quake struck at 4.50am (1950 GMT) as most of the population was sleeping.

Residents said they were shocked by the quake's intensity.

"Everything was falling over in the house," a man at a hospital told NHK. "A shelf hit my wife on the back."

Officials at Hokkaido Electric Power Co were quoted by Kyodo as saying that some 24,300 homes near Kushiro lost power.

Hokkaido Electric's Tomari nuclear power station was not affected, NHK said.

A fire broke out in a storage tank at an oil refinery owned by Idemitsu Kosan Co Ltd in Tomakomai, a coastal city in southern Hokkaido, sending flames and black smoke spewing into the sky.

The refiner closed the plant for safety reasons but the fire was put out around midday, Idemitsu said.

The latest earthquake follows one measuring 6.2 on the Richter scale that hit the northeastern Miyagi prefecture in July, injuring around 500 people. Miyagi was also struck by an estimated 7.0 earthquake in May that injured more than 100.

Tsuneo Katayama, a seismologist and president of the National Research Institute for Earth Science and Disaster Prevention, said the quake was unlikely to be a trigger for larger ones.

"There's no real connection between this earthquake and the possibility of a larger one, say in a place such as Tokyo. The location is totally different," Katayama told Reuters.

He added that it was not particularly significant that there had been three earthquakes in northern Japan since May.

"In a seismically active place like this, it's not strange to go through a period where there are several small earthquakes close together about once every 10 years," he said. "It's a fairly frequent phenomenon. Just one of the things we have to put up with for living here."

Hokkaido, about the size of Austria, is the second largest of Japan's four main islands and has a population of more than five million. The capital, Sapporo, hosted the 1972 Winter Olympics.

Memories are still vivid in Japan of the earthquake in the western city of Kobe that killed more than 6 400 people eight years ago. That quake measured 7.2 on the Richter scale.

The Great Kanto earthquake of September 1, 1923 measured 7.9 on the Richter scale and killed more than 140 000 people in Tokyo and the neighbouring port city of Yokohama.

Until Friday's quake, none of 8.0 or higher had been recorded in 2003. There were none of that magnitude in 2002.

The meteorological agency issued a tsunami warning for the coastal areas in eastern and central Hokkaido and a tsunami alert for the coastal areas in western Hokkaido as well as the northern Japanese prefectures of Aomori, Iwate, Miyagi and Fukushima.

The "warning" was later downgraded to an "alert".

Minoru Kasahara, a seismologist at Hokkaido University, said on television: "We should be alert for tsunami for a day or two."

In 1993, a tsunami caused by a quake measuring 7.8 killed about 200 people on the island of Okushiri, off western Hokkaido.

The U.S. National Oceanic and Atmospheric Administration (NOAA) issued a tsunami warning for Russia as well as Japan. It also issued a lower-level tsunami watch for the Philippines, Taiwan, Guam, and a number of islands in the Pacific Ocean.

Japanese financial markets were mostly unaffected, although the yen briefly dipped to 112.25 per dollar in early trade. At 0500 GMT it was trading around 112.00.

(Additional reporting by Elaine Lies and Teruaki Ueno)