However dozens of people could be buried under the remains of hundreds of collapsed buildings on the island of Nias, close to the quake's epicenter.
"Hundreds of buildings have been damaged or have collapsed. People who were standing fell over. We're not sure about casualties, but there may be dozens of people buried in the rubble," said Agus Mendrofa, deputy district head on Nias island which was nearest to the epicenter of the quake.
The U.S. Geological Survey said the temblor had a magnitude of 8.7 and was an aftershock of the Dec. 26 quake that sent giant waves crashing into coastlines around the Indian Ocean's rim and killing more than 174,000 people in 11 countries.
The USGS had earlier said the earthquake had a magnitude of 8.2.
"It seems this earthquake did not trigger a tsunami," Prihar Yadi, a scientist with the Indonesia Geophysics Agency. "If it had, the tsunami would have hit the coastline of Sumatra by now. And if there's no tsunami on the coastline near the epicenter of the quake, there will not be one heading in the other direction."
A Thai expert also said the threat of a tsunami had probably passed. Smith Thammasaroj, a former chief of the country's meteorological department, told a local television station that it appeared that southern Thailand would be spared from any possible tsunami from the quake.
In Sri Lanka, the military said it had no sign of a tsunami yet but wanted to wait a bit longer before declaring that things were safe.
The quake cut electricity and sent people scurrying for high ground in Banda Aceh on Sumatra's northern tip, the area hardest hit by December's tsunami. The quake lasted for about two minutes, far longer than most of the daily aftershocks that have rocked Aceh since Dec. 26.
"People are still traumatized, still scared, they are running for higher ground," said Feri, a 24-year-old recovery volunteer who goes by one name.
Sirens blared along Sri Lanka's devastated east coast as the government warned seaside residents to evacuate immediately.
"The government has ordered coastal areas to move to higher ground. We are giving priorities to eastern coast," said Brig. Daya Ratnayake, the military spokesman.
In Washington, State Department deputy spokesman Adam Ereli said U.S. diplomatic missions in Asia and Africa are in "battle mode" so that they can respond quickly to any contingency.
He said embassy officials in the area have been asking host governments to inquire about any causalities to permit an early U.S. response if the situation calls for it.
The International Red Cross in Geneva said all their mobile phone systems were down so they haven't been able to talk to anyone on the ground in Indonesia.
The quake was felt as far away as Malaysia, about 300 miles from the epicenter, sending panicked residents fleeing their apartments and hotels in Kuala Lumpur and Penang after authorities activated fire alarms.
The quake occurred at 11:09 p.m. local time (11:09 a.m. EST) at a depth of nearly 19 miles, the USGS in Golden, Colo., said.
The quake was centered 125 west-northwest of Sibolga, Sumatra, and 150 miles southwest of Medan, Sumatra, the agency said.
Japan's Meteorological Agency said the quake registered 8.5.
Tremors were felt throughout peninsular Malaysia's west coast, causing thousands of residents to flee high-rise apartment buildings and hotels. There were no immediate reports of any casualties or major damage.
"I was getting ready for bed, and suddenly, the room started shaking," said Kuala Lumpur resident Jessie Chong. "I thought I was hallucinating at first, but then I heard my neighbors screaming and running out."
The USGS said in a statement that the quake occurred on a segment of the same fault line that triggered the magnitude-9 earthquake on Dec. 26, the world's biggest in 40 years.
The Dec. 26 quake triggered the huge tsunami that swept across the Indian Ocean at the speed of a passenger jet, killing more than 174,000 people and leaving another 106,000 missing.
More than 1.5 million people were left homeless in 11 countries.
Tremors form the quake could be felt in the Thai capital Bangkok for several minutes beginning at about 11:20 p.m.