The quake was centered 200 kilometers under the seabed of Siau island in the country's northernmost region but they've been no immediate reports of damage or casualties.

It comes as Indonesia prepares for the possible evacuation of nearly 30-thousand people living on the slopes of Java's simmering volcano Mount Merapi.

Vulcanologists have raised the alert to stage two, one level below ordering an evacuation from the country's second most active volcano.

All districts on the slopes of the two-thousand 900 metre volcano are preparing to coordinate relief and rescue efforts

Merapi's last eruption in 1994 produced heat clouds which killed more than 60 people and forced 6,000 others to evacuate.

ERCOT said it declared the emergency after concluding there was insufficient generating capacity in the region to reliably serve the public's electricity demand.

As much as 15 per cent of the state's power supply goes offline each spring so plants can perform seasonal maintenance before energy usage peaks in the summer, said Public Utility Commission spokesman Terry Hadley. He said maintenance is typically finished by mid-May.

But unusually high temperatures this spring have pushed demand for electricity, creating a shortage, he said.

The rollouts were limited to the ERCOT grid, which provides electricity to about 80 per cent of Texas.

Traffic backed up at intersections in Grand Prairie, just west of Dallas, during the afternoon rush hour.

CenterPoint Energy spokeswoman Emily Thompson said rolling blackouts every 15 minutes for the Houston area were ordered just after 4 p.m. Monday.

"We're hoping this will be taken care of very shortly," she said.

Austin Energy said it began its rotating blackouts about 4:20 p.m. to comply with its share of the load shedding requirement.

Around 3,000 residents of the village of Rast in Romania were evacuated on Monday before the surging river entirely submerged their homes. Some 115 houses were destroyed and another 600 damaged.

The district chief, Nicolae Giugea, said 10,000 residents of four villages in the area were on standby for evacuation if the water continued to rise.

According to the Romanian environment and water management ministry, the Danube was flowing at double the average speed for this time of year - a record 15,800 cubic metres per second.

Controlled flooding

In the southern Romanian port of Calarasi on Monday, a recently opened riverside hotel was flooded causing major damage to the building. Dozens of tourists and personnel were evacuated.

Romanian authorities were using controlled flooding of agricultural land to reduce the pressure of the swollen river on dykes built to protect populated areas.

Several border crossings over the river separating Romania and Bulgaria were closed temporarily.

Authorities restricted traffic at the Giurgiu border crossing, with hundreds of cars waiting on the Romanian side of the border.

In Bulgaria, a state of emergency was being maintained along the Danube, with the government warning the water levels were expected to reach a peak of up to 990cm in the northwestern town of Vidin on Wednesday.

The situation was made worse in Serbia by the high level of water in swollen smaller rivers that join the Danube. These include the Tisa and Begej, which meet at the town of Titel, north of Belgrade.

Over the weekend, the river reached an all-time high of 9.41m near Veliko Gradiste, 100km east of Belgrade.

In the agriculture-rich northern province of Vojvodina, the flooding and heavy rains submerged some 10,000 hectares of farmland and turned another 200,000 into mud and slush.

In the eastern Serbian town of Smederevo, authorities dispatched all city workers to fight flooding as the waters submerged parts of the town's train station, bringing rail traffic to a halt.

Dyke defence

In Belgrade, where the Danube meets the Sava, the situation was under control on Monday morning, although authorities there said they were working to maintain 250km of dykes.

"Our principal concern is the survival of the dykes," Nikola Marjanovic, of Serbia's water authority, told B92 radio.

Goran Kamcev, head of the Serbia flood prevention task force, told Reuters: "The situation is under control along the whole flow of the Danube through Serbia."

"We now have to watch out for the longstanding pressure on the barriers, with water expected to stay high for some 10 to 15 days. It could cause the dykes to leak or even break and our teams on the ground have to stay vigilant."

Much of the Balkans were hit by floods last year that drowned scores of people and caused damage to houses, farmland and infrastructure running into hundreds of millions of dollars.

He says they will be drawing on that experience.

"With our run-in with cyclone Ingrid last year, this year we had three clean-ups, so we were well-prepared this year,'' he said.

"The counter disaster group has already met today.

"We've put in train all the necessary actions that we can do and know it's a waiting game."

He says some residents will probably choose to shelter in the community's lock-up tonight.

State Emergency Service crews from Brisbane are due to arrive in Cairns tonight.