The U.S. Geological Survey says a magnitude 6.3 earthquake was reported off northern Sumatra's west coast, about 170 miles south of Banda Aceh.
There are no reports of casualties or damage, and no tsunami warning was put out.
Firefighters freed at least one survivor Wednesday. A man was found trapped in a crumpled house on remote Nias island, 36 hours after he was buried in rubble. As the first foreign military help arrived, officials said an estimated 1,000 people had died in the region's latest large earthquake.
Residents swarmed over collapsed buildings in Nias island's main town of Gunung Sitoli, searching frantically for survivors of the country's second catastrophe in three months, after December's massive quake and tidal wave.
There were occasional sounds of weeping as lifeless bodies were pulled from the debris, reports CBS News Correspondent Barry Petersen. Many were killed as they slept when the quake hit just before midnight.
"It's difficult to give an estimate, but we think that half of the buildings have collapsed, there are still people being stuck in those houses, and people are trying to get the survivors out," said Jeremy de Lage, an Oxfam coordinator who spent the night on the island.
The United Nations said Wednesday it had confirmed 518 deaths, but "That number is expected to rise," according to Masood Hyder, who is leading the United Nations relief operation in Banda Aceh.
French firefighters from the agency Firefighters Without Borders — who rushed to the island from Aceh province's west coast — used a car jack to free the legs of 25-year-old television repairman Jansen Silalalahi, who had been pinned between a motorbike and a cupboard.
As he was lifted out of the rubble of what was once a three-story building, Silalalahi smiled weakly and gave a thumbs-up.
"People knew I was there but it was difficult to reach me. I kept screaming whenever I heard anyone," said Silalalahi, who did not appear to be badly injured. "I feel relief because now I am safe."
The improvised rescue highlighted the crisis situation officials face here: there are thousands of collapsed buildings and no machinery to help search through the rubble for survivors.
There is virtually no medical care available on Nias, said de Lage.
"All the pharmacies are closed; we don't have access to medicine. The hospital is severely damaged. There is no medical staff there; only one doctor and two nurses," he said.
"We know there are many people critically injured," said Dr. Norman Peeler, a medical coordinator from the World Health Organization. "It is essential they get treatment, infections spread easily in open wounds."
Two Singaporean military helicopters landed Wednesday and distributed food and water to a frantic crowd of survivors. They also delivered a car, medical supplies, generators and 20 Singaporean troops and medics. A third helicopter was unable to touch down because there were so many survivors at the landing area.