Indonesia Aceh quake triggers Indian Ocean tsunami alert
Indonesian TV showed pictures of people running from buildings in panic
History of deadly earthquakes
The Pacific 'Ring of Fire'
An earthquake with an magnitude of 8.7 has struck under the sea off Indonesia's northern Aceh province.
The quake triggered a tsunami watch alert across the Indian Ocean region.
The Pacific Tsunami Warning Center (PTWC) said it was not yet known whether a tsunami had been generated, but advised authorities to "take appropriate action".
The region is regularly hit by earthquakes. The Indian Ocean tsunami of 2004 killed 170,000 people in Aceh.
The US Geological Survey (USGS), which documents quakes worldwide, said the Aceh quake was centred 33km (20 miles) under the sea about 495km from Banda Aceh, the provincial capital.
Continue reading the main story
Sirens and Koran recitals from mosques are everywhere”
Indonesian emergency official
It was initially reported as 8.9 magnitude but was later revised down to 8.7 by the USGS. Strong aftershocks were also reported.
The PTWC alert said quakes of such a magnitude "have the potential to generate a widespread destructive tsunami that can affect coastlines across the entire Indian Ocean basin".
But Bruce Presgrave of the USGS later told the BBC that the nature of this quake made it less likely a tsunami would be generated, as the earth had moved horizontally, rather than vertically, therefore had not displaced large volumes of water.
"We can't rule out the possibility, but horizontal motion is less likely to produce a destructive tsunami," he said.
Sutopo, a spokesman for Indonesia's disaster mitigation agency, said the quake had been felt "very strongly".
"Electricity is down, there's traffic jams to access higher ground. Sirens and Koran recitals from mosques are everywhere," he told Reuters.
'Minute of chaos'
The earthquake monitoring agency in Indonesia said the tsunami warning would remain in place for another few hours, but that there had been no reports so far of a low tide, which would indicate the water was receding before building into a tsunami.
The tremor was felt as far away as Singapore, Thailand, Sri Lanka and India.
"There was a tremor felt by all of us working in the building," a man called Vincent in Calcutta, India, told the BBC.
"All just ran out of the building and people were asked not to use the elevator. There was a minute of chaos where all started ringing up to their family and asking about their well-being."
The Thai office of disaster management said people along the coasts of Phuket, Phang Na and Andaman province should heed warnings and evacuate.
Tsunami warning sirens, set up in many vulnerable areas after the 2004 disaster, were heard in Phuket, where correspondents said people were calmly following evacuation routes to safe zones.
Indonesia straddles the Pacific Ring of Fire, a zone of major seismic activity.
The BBC's Karishma Vaswani in the Indonesian capital, Jakarta, says there were reports of the ground shaking for up to five minutes. Contact with people in the immediate area around the quake has not been possible so far, says our correspondent.