[Updated at 9:50 p.m.] Read the full CNN.com story
[Updated at 9:18 p.m.] The tsunami watches have been canceled for "all areas of the Indian Ocean," according to the Pacific Tsunami Warning Center.
[Updated at 9:12 p.m.] Residents in Banda Aceh said they were without power.
Residents in coastal towns fled inland to higher ground just after the quake, according to a local radio station.
The tsunamis, in Banyak Island and Teluk Dalam, were small and not dangerous, measuring just under a foot high, said Fauzi, chief of the Indonesian Meteorological and Geophysics Agency. Fauzi goes by only one name, which is common in Indonesia.
[Updated at 8:20 p.m.] The quake triggered two tsunamis, according to the Indonesian Meteorological and Geophysics Agency.
There were no immediate reports of damage or injuries. The initial shaking, which reportedly lasted for at least three minutes, prompted scores of people to run out of buildings into open air, witnesses said.
"The quake was felt quite strong, maybe about three four minutes," said Dadik, the head of Simeulue police who goes by only one name. "I've ordered my staff to check if there's any damage or casualties, but apparently no damage reported so far."
Measurements of sea levels indicated that tsunami waves "may have been destructive along coasts near the earthquake epicenter," the Pacific Tsunami Warning Center said.
One registered tsunami measured close to a foot from peak to trough and was considered a small tsunami, said geophysicist Gerard Fryer with the Pacific Tsunami Warning Center.
Indonesia announced the tsunami warning quickly, Fryer said, and officials at Indonesia's Meteorological and Geophysics Agency lifted it about two hours later.
[Updated at 7:50 p.m.] The depth at which the earthquake struck has also been changed. The USGS has revised the depth to 19.3 miles. It was previously reported to be 28.6 miles.
[Updated at 7:42 p.m.] A 9.1-magnitude underwater earthquake struck off the coast of Sumatra in 2004, triggering a tsunami that killed more than 200,000 people in 14 countries.
The tsunami, which washed away entire communities, caused nearly $10 billion in damage and more casualties than any other tsunami in history, according to the United Nations. Indonesia was among the hardest hit nations.
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[Updated at 7:38 p.m.] The USGS has downgraded the earthquake to magnitude 7.7. The temblor struck 125 miles from Sibolga, situated on Indonesia's Sumatra island.
[Updated at 7:29 p.m.] CNN meteorologist Chad Meyers talks to CNN's Wolf Blitzer about the number of earthquakes that have occurred recently.
[Updated at 7:16 p.m.] CNN's Lisa Sylvester has some of the early details about the quake.
[Updated at 7:07 p.m.] Scientists are monitoring data from ocean buoys to determine whether the earthquake already generated a tsunami, said geophysicist Gerard Fryer of the Pacific Tsunami Warning Center.
If a tsunami does erupt from the quake in shallow waters, Fryer said, it would be very small and not move much water.
Scientists said they do not expect the tsunami to affect the west coast of North America.
[Updated at 6:47 p.m.] The temblor struck 145 miles off the coast of northern Sumatra, Indonesia, at a depth of 28.6 miles, the U.S. Geological Survey reported.
A "destructive widespread tsunami" is not expected, the Pacific Tsunami Warning Center said, but a local tsunami could affect coastal areas near the quake's epicenter.
[Updated at 6:41 p.m.] A local tsunami watch has been issued for Indonesia and surrounding areas.
[Posted at 6:35 p.m.] A 7.8-magnitude earthquake struck northern Sumatra, Indonesia, early Wednesday according to the U. S. Geological Survey.