Jakarta, Indonesia (CNN) -- A massive earthquake struck off the coast of the Indonesian island of Sumatra on Wednesday afternoon, triggering a tsunami alert for the Indian Ocean.
The quake struck about 434 kilometers (270 miles) southwest of Banda Aceh, the capital of Indonesia's Aceh province, and had a magnitude of 8.6, the U.S. Geological Survey said. It took place at a depth of 23 kilometers (14 miles).
The tremor revived fearful memories of the catastrophic earthquake and tsunami in the region in 2004 that killed tens of thousands of people.
The Pacific Tsunami Warning Center said Wednesday that it had issued a tsunami watch for the entire Indian Ocean, and the Indonesian Meteorology, Climatology and Geophysics Agency said it had put up a tsunami warning.
The Indonesian president, Susilo Bambang Yudhoyono, said on local television that there were no reports of casualties or damage in Aceh so far.
The areas most at risk of a tsunami are coastal areas of Aceh, particularly the island of Simeulue, Prih Harjadi, an official for the Indonesian geophysics agency said on Metro TV.
The earthquake appears to have involved a horizontal movement rather than a vertical movement, so it is less likely that it will generate a tsunami, said Gary Gibson from the Seismology Research Center in Melbourne, Australia.
He also said that the tremor took place a long way offshore and was therefore unlikely to have caused much damage itself.
The power has gone out in Banda Aceh and residents are moving to higher ground, said Sutopo Purwo Nugroho, a spokesman for the Indonesian National Disaster Management Agency.
The authorities in India's Andaman and Nicobar Islands have ordered people to move out of low-lying areas.
The region's chief secretary, Shakti Sinha, said that there are "a few hundred people" in the areas where the evacuation order had been issued.
In 2004, a 9.1-magnitude underwater earthquake struck off the coast of Sumatra, triggering a tsunami that killed more than 200,000 people in 14 countries. The majority of the deaths were in Indonesia, with Aceh bearing the brunt.
That quake took place 250 kilometers (155 miles) south-southeast of Banda Aceh at a depth of 30 kilometers (19 miles).
The tsunami, which washed away entire communities, caused nearly $10 billion in damage and more casualties than any other tsunami in history, according to the United Nations.
Indonesia is on the so-called Ring of Fire, an arc of fault lines circling the Pacific Basin that is prone to frequent earthquakes and volcanic eruptions.
The earthquake Wednesday comes just over year after a magnitude 9 quake off the northeast coast of Japan caused a devastating tsunami. The death toll from that disaster stands at about 15,850.
