An 8.7 magnitude earthquake has hit below the ocean off the northern Aceh province of Indonesia, causing families in the area to flee their homes.
Originally reported as 8.9 magnitude, the earthquake has been downgraded by the U.S. Geological Survey (USGS), but strong aftershocks of 6.5 magnitude have been reported, and tremors have been felt as far as India and Sri Lanka.
A tsunami warning has been triggered across the Indian Ocean area, with warning sirens ringing in Phuket, which had been set up in the wake of the tragedy of 2004. It is reported that people are calmly evacuating to safety zones.
The Pacific Tsunami Warning Center has stated its uncertainty as to whether a tsunami will strike, but have recommended that authorities in vulnerable areas “take appropriate action.”
The earthquake struck 33km (20 miles) under the sea, about 495km from Banda Aceh, the provincial capital. This is roughly the same area as the December 26, 2004 disaster, which had a magnitude of 9, when 230,000 people perished overall.
Indonesia straddles an area known as the Pacific Ring of Fire, for its volatile seismic activity.
However, Bruce Presgrave, a USGS official, has reported to the BBC that a tsunami being generated is unlikely, due to the quake moving the earth horizontally rather than vertically: “We can’t rule out the possibility, but horizontal motion is less likely to produce a destructive tsunami.”
Indonesian officials suggest that there is no immediate damage of rising water levels in the region, and the National Disaster Management Authority has asserted that there is “no likelihood” of a tsunami occurring.
It was initially reported as 8.9 magnitude but was later revised down to 8.6 by the USGS. Quake officials said a tsunami had been generated and was heading for the coast of Aceh.
A PTWC alert said that sea level readings indicated a tsunami was generated and that it "may already have been destructive along some coasts," without specifying where.
A Thai disaster official said a 10cm wave had been recorded on Koh Miang island, off Phang Nga.
People fled for higher ground in Aceh province
Indonesian President Susilo Bambang Yudhoyono had told reporters in the capital, Jakarta, that there had been no tsunami reports so far, "but we remain vigilant".
"Our warning system is working well, and I have ordered the national relief team to fly immediately to Aceh to ensure the situation is under control and to take any necessary action," he said.
A few hours later, the PTWC renewed its warning after a major aftershock measuring 8.2 struck 16km (10 miles) beneath the ocean floor and 615km from Banda Aceh.
An AFP correspondent in Banda Aceh said the aftershock lasted four minutes,
"People are panicking and running outside their home and from buildings," he said.
The PTWC issues advisory alerts across the region, which state authorities can use to issue their own emergency procedures. Indonesia straddles the Pacific Ring of Fire, a zone of major seismic activity.
'Minute of chaos'
Sutopo, a spokesman for Indonesia's disaster mitigation agency, said electricity had been cut in Aceh and there were traffic jams to access higher ground.
"Sirens and Koran recitals from mosques are everywhere," he told Reuters.
Tremors were felt as far away as Singapore, Thailand, Sri Lanka, Malaysia, Bangladesh and India. The French island of Reunion was also on alert.
"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.
But seismic experts said the nature of the first quake meant it was unlikely to generate a tsunami on the scale of the 2004 one.
Roger Musson, a seismologist from Britain's Royal Geological Survey, said it was unlike the quakes seen off Indonesia in recent years, where ground had been pushed under the continental plate, "flipping up" the seabed.
"It seems to be a large earthquake within the Indian Plate and the plate has broken in a sort of lateral way," he said. "It's a sort of tearing earthquake, and this is much less likely to cause a tsunami because it's not displacing large volumes of water."
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