Indonesia's Meteorology and Geophysics Agency said the quake would have "at least caused cracks in many buildings and houses, especially in areas closest to the epicentre".
But seismologist Paulus Prihandojo explained that the 46km depth of the quake and its epicentre being 200 km northwest of Sibolga in Aceh province, helped the big population centres of Medan and Banda Aceh to escape major damage.
Irwandi Yusuf, the governor of Aceh province in Sumatra's northwestern tip, said there had been no reports of damage or casualties and "the situation is under control", but he added it would take time to reach remote areas.
"Some people had gone to take refuge on higher ground but now they have returned to their homes," he told Indonesia's Metro TV.
Wednesday's quake triggered a tsunami warning for the region but that was later lifted, and Indonesia's meteorology agency said there were at least five strong aftershocks, measuring up to magnitude 5.2.
The quake, which struck at 5:15am (22:15 GMT) on Wednesday, caused panic in North Sumatra's capital of Medan and other cities in the region.
Electricity was cut in Medan and Banda Aceh, the provincial capital of Aceh.
People in several cities along the southeastern coast of Sumatra as well as Sinabang on Simeulue island and Gunung Sitoli on nearby Nias island poured on to the streets and rushed to higher ground after the quake.
"Rumours about a tsunami panicked villagers living near the beach," said Eddy Effendi, a Nias resident.
"They ran away on motorbikes and cars or by climbing the hills. There was panic and chaos everywhere, but I don't see serious damage or injuries in my village."
Residents in Sibolga said the shaking lasted more than a minute and utility poles in the area were knocked down.
Depth mitigates effects
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.