Sirens sounded along coastlines and warnings spread like wildfire by mobile phone text messaging. Though often chaotic, evacuations began immediately, with streets clogged with traffic, especially in Aceh province, where 170,000 people were killed in 2004.
Patients were wheeled out of hospitals, some still lying in their beds with drips attached to their arms. And at least one hotel guest was slightly injured when he jumped out of his window.
Countries all along the Indian Ocean from Australia and India to as far off as Africa received alerts from the Pacific Tsunami Warning Center in Hawaii saying they should prepare for the possibility of seismically charged waves.
Hour later the tsunami warnings were lifted, and damage from the tremors was minimal something experts attributed to the unique nature of the fault line.
The only wave to hit was less than 80 centimetres high, rolling to Indonesia's emptied coastline.
The U.S. Geological Survey said the first 8.6-magnitude quake was a shallow 22 kilometres, striking in the sea 435 kilometres off Aceh's coast making it the sixth-largest temblor in the last half-century.
Just as the region was sighing relief, an 8.2-magnitude aftershock followed, again causing only slight damage.
Experts said that's because both tremors were what are known as "strike slip'' quakes, where friction and shaking along the fault line occurs horizontally, creating more of a vibration in the water.
In contrast, mega-thrust quakes like the one that hit off Aceh in 2004 and off Japan just over a year ago cause the seabed to rise or drop vertically, displacing massive amounts of water and sending towering waves racing across the ocean at jetliner speeds.
Roger Musson, a seismologist at the British geological survey, said initially he'd been "fearing the worst.''
"But as soon as I discovered what type of earthquake it was ... I felt a lot better.''
The tremors were felt in neighboring Malaysia, where high-rise buildings shook, and Thailand, India, Singapore and Bangladesh.
Buildings and beaches emptied out, also in Sri Lanka and the Maldives.
But it was Aceh where the real chaos broke loose.
"I'm so scared, I don't want to lose my family again,'' cried Aisyah Husaini, whose parents and a son were killed in the 2004 tsunami. She clung to her two children in a mosque in the provincial capital, Banda Aceh, where hundreds of people sheltered.
Sitting nearby was 52-year-old Nasir Djamil, who said he and his family weren't taking any chances they planned to spend the night.
"For sure, this has reawakened all those horrible nightmares,'' he said. "But I do feel like we learned something, that we know much better now what to do.''
The World Meteorological Organization agreed, saying communication systems set up after the 2004 tsunami appeared to have worked well.
"Our records indicate that all the national meteorological services in the countries at risk by this tsunami have received the warnings in under five minutes,'' said Maryam Golnaraghi, the head of WMO's disaster risk reduction program.
New Zealander Caroline Preston, the World Vision operations director in Indonesia, said the shake lasted about three minutes.
"We had staff who experienced the shake and could see people panicking, running for the hills, traffic jams."
Ms Preston had felt at least five aftershocks in the hours after the first quake.
She said World Vision had its national emergency rapid response team on standby in case it was needed.
Indonesia, a sprawling archipelagic nation of 240 million people, straddles a series of fault lines that makes it prone to volcanic and seismic activity.
Aceh accounted for nearly three-quarters of those killed in the giant 9.1-magnitude quake and tsunami of Dec. 26, 2004.
Aftershocks from Wednesday's quake are likely to persist for weeks or months. Subsequent earthquakes can be as strong or stronger than the initial temblor, but they mostly weaken over time.
- AP
