On Simeulue island, off Aceh, there was panic and a blackout, a photographer with the Reuters news agency said from the island.
Local media reports said there was also panic in North Sumatra's capital Medan and other cities in the province.
Witnesses reported phone and power lines down in several areas, including in Medan, and Indonesia's Metro TV reported that people rushed to higher ground in some areas.
Dale Grant, of the US Geological Survey, told Al Jazeera that "having a little bit of depth here will help mitigate the strength of this major quake".
"The deeper the quake the less damage it does," he said.
Grant added that the recent spate of earthquakes in Haiti, Chile and now Indonesia, were not connected in any way, adding that there are about 18 major quakes 7-7.9 in magnitude recorded every year.
He said Wednesday's quake off Sumatra occurred in one of the world's most seismically-active zones and close to areas where major quakes had previously struck.
A magnitude 7.6 underwater earthquake struck off the coast of Sumatra in September last year, killing around 1,000 people and devastating the city of Padang on the island's western coast.
The island sits on one of the world's most active fault lines along what is known as the Pacific Ring of Fire, the same one that cracked off Aceh in 2004 to trigger the Indian ocean tsunami.
That disaster killed more than 220,000 people in Indonesia, Thailand, Sri Lanka and India, among other countries.