Panic is subsiding in communities along the Indian Ocean as a U.S.-based warning centre says the tsunami risk has diminished, or is over, in the aftermath of two earthquakes that shook Indonesia's coast.

The 8.6- and 8.2-magnitude earthquakes struck parts of Indonesia Wednesday afternoon. Residents in coastal cities fled to high ground in cars and on the backs of motorcycles, and many people ran into the streets.

The Pacific Tsunami Warning Center issued a sweeping Indian Ocean tsunami watch after the first quake, but the watch has since been lifted. 

"Sea level readings now indicate that the threat has diminished or is over for most areas. Therefore the tsunami watch issued by this center is now cancelled," a bulletin said.

National governments also issued warnings and watches, but many of those have been lifted.

Experts said Wednesday's quakes did not have the potential to create massive tsunamis because the friction and shaking occurred horizontally, not vertically.

Roger Musson, seismologist at the British geological survey who has studied Sumatra's fault lines, 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."

No major damage reported

Major damage or tsunami waves locally were not reported, but hours after the temblor, many people in Aceh were still standing outside their homes and offices, afraid to go back inside.

Memories of the 2004 tsunami that killed 170,000 people in that region alone are still raw among survivors.

"What did we do to deserve this?" cried Aisyah Husaini, 47, who lost both her parents and a son in the 2004 tsunami. "What sins have we committed?"

"I'm so scared, I don't want to lose my family again," she said, clinging to her two children in a mosque in Banda Aceh, where hundreds of people sought shelter.

The U.S. Geological Survey said the first 8.6-magnitude quake was a shallow 22 kilometres, hitting in the sea 435 kilometres from Aceh's provincial capital.

It was initially estimated at magnitude 8.9, but that was later revised down to 8.6. The aftershocks that followed about two hours later were in the 8.2-magnitude range.

There were no immediate reports of high water levels or any damage as a result of the quakes, which began at 3:38 a.m. ET.

Victor Sardina, a geophysicist with the PTWC in Oahu, Hawaii, told CBC News that the centre issued its most severe level of "watch" earlier based on the strength of the initial quakes.

"Once we issue a watch or a warning, we cannot cancel unless we have actual measurements that can attest there's no danger remaining from that tsunami …," he said. "In this case … the wave was relatively small. It's small compared to 2004. It's basically negligible in terms of height."

The largest wave observed was 1.06 metres above normal sea level, measured at Meulaboh, Indonesia — about 250 kilometres south of Banda Aceh — at 5:07 a.m. ET, 89 minutes after the first quake.

Despite diminished fears, authorities remain on alert. 

"Danger to boats and coastal structures can continue for several hours due to rapid currents," noted the Pacific Tsunami Warning Center.

"As local conditions can cause a wide varitaion in tsunami wave action, the 'all clear' determination must be made by local authorities."

Aftershocks could last for months

In Indonesia, "the quake was felt very strongly," Reuters quoted a spokesman for the country's disaster mitigation agency as saying.

The tremors were felt in neighbouring Malaysia, where highrise buildings shook. Singapore, Thailand, Bangladesh and India also were rattled.

In India, many people felt shocks along the east coast, freelance reporter Rohit Gandhi said.

"People were all out on the streets; they all came out of the building. There was literally traffic jams all across India's east coast," he said.

The BBC reported earlier in the day that the airport at Phuket, a popular south-Thailand tourist destination along the coast facing the Andaman Sea, had been closed to prevent more people from arriving. By 9:30 a.m. ET, the airport website showed it was functioning again, with flights delayed, but not cancelled.

Phuket was severely affected by the quake in 2004, and has since put in place warning systems and signage directing people to higher ground in the event of tsunami warnings.

Indonesia straddles a series of fault lines that makes the vast island nation prone to volcanic and seismic activity.

"There will be ongoing aftershocks — probably for quite some time," Randy Baldwin, a geophysicist with the U.S. Geological Survey, told CBC News. "It wouldn't surprise me if we don't see aftershocks for maybe a few months, if not even longer than that."

A giant 9.1-magnitude quake off the country on Dec. 26, 2004, triggered a tsunami in the Indian Ocean that killed 230,000 people, almost three-quarters of them in Aceh.

A tsunami watch means there is the potential for a tsunami, not that one is imminent.

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With files from The Associated Press