04/11/2012 11:47 GMT

by Nurdin Hasan

BANDA ACEH, April 11, 2012 (AFP) - A massive earthquake struck off Indonesia's Sumatra island on Wednesday, triggering an Indian Ocean-wide tsunami alert and evacuation orders to clear people away from the coast.

The 8.6-magnitude quake struck 431 kilometres (268 miles) off the Indonesian city of Banda Aceh, the US Geological Survey said, and was followed by a major 8.2-magnitude aftershock.

Terrified residents poured into the streets of Banda Aceh, which was devastated by a 9.1-magnitude quake in 2004 that triggered an Indian Ocean tsunami, killing 220,000 people including 170,000 in Aceh province.

The latest tremor was felt as far afield as Thailand, where skyscrapers in the capital Bangkok swayed. India, Indonesia, Malaysia, Sri Lanka, Thailand and Myanmar all issued alerts or evacuation orders.

Indonesian President Susilo Bambang Yudhoyono downplayed the threat of a tsunami and said so far there had been no reports of major damage or casualties, but that the country remained on alert.

US monitors who issued the tsunami watch said it appeared to have only generated small waves.

Victor Sardina, a geophysicist with the US Pacific Tsunami Warning Center in Hawaii, told AFP the tsunami was "not anywhere near" as large as the devastating tsunamis that struck Asia in 2004 and Japan last year.

He said the waves measured a mere 35 centimeters (14 inches) near Padang, Indonesia, but could swell to as high as a metre (three feet) near Sri Lanka, adding that US scientists were still carefully monitoring the situation.

In Banda Aceh, panicked residents grabbed their families and raced through crowded streets.

"There are people trying to evacuate, some are praying and children at a school were panicking as teachers tried to get them out," an AFP correspondent in Banda Aceh said.

"There are traffic jams everywhere as people are trying to get away from the coast -- many are on motorcycles," he said, adding that telephone connections and electricity were patchy.

Television images showed hundreds gathering at a large mosque in Banda Aceh, many weeping and searching for family members. Women and girls draped head-to-toe in white were praying on mats laid out on the ground.

Sri Lanka issued a tsunami warning across the island and the disaster management centre asked residents on the coast to move inland to avoid being hit by any large waves.

In the Sri Lankan capital Colombo, nervous crowds gathered on the streets after the strong quake.

"There was a first jolt for five seconds, then a pause and then a really big one. It was really frightening, the whole room was shaking," said 42-year-old tourist Maria Teresa Pizarro from the Philippines.

"You could hear the wood in the furniture cracking, the curtains were moving and the ceiling fan was rattling. I just picked up the children and ran downstairs," she said from the city's coastal Galle Face hotel.

Thailand issued an evacuation order for its Andaman coast, a popular tourist destination.

The National Disaster Warning Centre, which advised people in the area to move to higher ground and stay as far away as possible from the sea, said a 10cm (four-inch) tsunami hit the Thai coast.

India issued a red high-level tsunami warning for the Andaman and Nicobar Islands, located in the Indian Ocean, and lower alerts for several other coastal states.

Australian Bonnie Muddle, vacationing in the Thai resort island of Phuket, said people were being evacuated from popular tourist areas including Krabi and Phang Nga bay.

"Everyone is getting a little concerned over here," she told AFP.

The catastrophic tsunami of December 26, 2004, was generated by a 9.2-magnitude earthquake that hit at a location in the ocean about 200 kilometres away from the latest quake.

An expert with the British Geological Survey (BGS) said however that the risk of a giant tsunami being generated by the latest earthquake off Sumatra was low.

The earthquake's movement was horizontal, not vertical, and caused no drop in the sea floor, which is what triggers tsunamis, seismologist Susanne Sargeant told AFP.

"Although an earthquake of this magnitude has the potential to cause a large tsunami, the fact that we haven't seen any drop of the sea floor, which is what generates the wave, it looks like the possibility of a tsunami being generated is low," she said.

Last year, a 9.0-magnitude earthquake caused a tsunami and nuclear disaster in Japan, killing some 19,000 people.

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