The shutdown of a key thermal power plant may mean Hokkaido could see reduced electricity supply for over a week, a state minister said Thursday, following a powerful earthquake that rattled Japan's northernmost main island in the early morning.

With the coal-fired power plant that provides roughly half of Hokkaido's electricity offline, around 2.95 million households served by Hokkaido Electric Power Co. experienced blackouts.

(People buy food and other supplies at a convenience store in the dark)

Efforts to restart the Tomatoatsuma power station have been hampered by infrastructure damage and a fire at the plant after the magnitude 6.7 quake rocked Hokkaido at 3:08 a.m. on Thursday.

Hours after the predawn quake, the restart of some hydropower and thermal power generation restored electricity supply to around 340,000 households late Thursday but large-scale blackouts revealed vulnerability in the critical infrastructure systems.

The operator of the Tomatoatsuma plant had initially planned to restart power generation at one of its three units, but a fire related to an oil leak meant it was forced to remain idle.

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The other two units suffered quake damage, according to the industry ministry. When fully operational, the plant can generate a combined 1.65 million kilowatts of electricity.

As work to restore power continues, available capacity will likely increase to around 2.90 million kw through the restart of some hydropower and thermal power generation sources by the end of Friday -- roughly 76 percent of Hokkaido's peak electricity demand on Wednesday.

But Economy, Trade, and Industry Minister Hiroshige Seko told reporters it will likely take over a week for electricity supplies to return to a level that can meet peak demand.



The shutdown of the thermal power plant caused electricity supply and demand to lose balance, triggering a series of other power plants to halt operation in Hokkaido.

(A blackout in central Sapporo)

A government official said one factor behind the power outage was Hokkaido's "high dependence" at the time of the quake on one specific power station, adding that the power loss seen Thursday was "beyond expectations."

Hokkaido Electric had estimated a loss of around 1.3 million kw, smaller than the 1.65 million kw due to the complete shutdown of the Tomatoatsuma plant.

Electricity was also cut to the idled Tomari nuclear power plant in Hokkaido, prompting the operator to switch to backup diesel power generators to keep spent nuclear fuel cool.

Japan is prone to natural disasters and the 2011 nuclear accident at the Fukushima Daiichi power plant caused by a massive earthquake and tsunami has led to a rethink of safety measures.

As the village of Tomari that hosts the nuclear plant recorded only 2 on Japan's seismic intensity scale of 7 -- a level that would not affect people's daily life -- the prefecture's massive power outage, rather than the shaking itself, cut external power supply.

External power supply to the plant was restored nine hours after the quake, according to the government.