The quake, the latest episode in a miserable summer of natural disasters in Japan, knocked out power to all of Hokkaido's 5.3 million residents.

Public broadcaster NHK said the death toll had doubled to four and that six people were found in cardiopulmonary arrest, a term commonly used in Japan before death is formally confirmed. Another 120 were injured after the 6.7-magnitude quake struck before dawn.

Prime Minister Shinzo Abe said 25,000 Self-Defense Force troops would be deployed for rescue operations.

Aerial footage showed dozens of landslides exposing barren hillsides near the town of Atsuma in southern Hokkaido, with mounds of reddish earth and toppled trees piled up at the edge of green fields. The collapsed remains of what appeared to be houses or barns were scattered about.

"It came in four big jerks - bang! bang! bang! bang!" one unidentified woman told NHK. "Before we knew it our house was bent and we couldn't open the door."

The entire island lost power for the first time since Hokkaido Electric Power Co was created in 1951 when the utility conducted an emergency shutdown of all its fossil fuel-fired power plants after the quake. It said almost 12 hours later power was restored to parts of Sapporo, Hokkaido's capital, and Asahikawa, its second-most populated city.

All trains across the island, which is about the size of Austria, were also halted.

The government said there was damage to Hokkaido Electric's Tomato-Atsuma plant, which supplies half the electricity to the island's 2.95 million households. It could take a week to restore power fully to all residents, Industry Minister Hiroshige Seko said.

The quake hit at 3:08 a.m. local time at a depth of 40 km (25 miles), with its epicenter about 65 km (40 miles) southeast of Sapporo, according to the Japan Meteorological Agency (JMA). It registered a strong 6 on Japan's 7-point quake scale.

It struck near Hokkaido's main airport, New Chitose Airport, which would be closed for at least Thursday. Roof tiles and water could be seen on the terminal floors.

Chitose airport is a major gateway to the island, known for its mountains, lakes and abundant farmland and seafood. More than 200 flights and 40,000 passengers would be affected, Kyodo News agency said.

The closure comes just days after Kansai Airport, another major regional hub, in western Japan, was shut by Typhoon Jebi. The airport's operator said it would resume some domestic flights on Friday.

Japan is situated on the "Ring of Fire" arc of volcanoes and oceanic trenches that partly encircles the Pacific Basin.

A 9.0 magnitude earthquake, the most powerful ever recorded in Japan, struck on March 11, 2011, off its northeastern coast, triggering a tsunami that devastated communities along the Pacific coast and killed nearly 20,000 people.

The tsunami also damaged the Fukushima Daiichi nuclear power plant, leading to a series of explosions and meltdowns that spewed radiation into the air and ocean.

REUTERS