Al Jazeera's Step Vassen said that residents of the Indonesian island were in a state of "immediate panic" and have been advised to stand along the beach to observe sea levels.
She said that if residents saw the water pulling back, then they should evacuate given the possibility of a tsunami.
Vassen also reported that foreign aid was arriving at the island.
Residents of Sumatra took shelter in tents outside their damaged homes on Friday, traumatised by the latest of nearly 40 aftershocks since the first earthquake.
|The quake has inflicted major damage on|
infrastructure in north Bengkulu [AFP]
The overnight quake triggered new panic among thousands camping out in makeshift shelters or tents, using torches and kerosene lights, and setting fires to keep warm in drizzling rain.
Wednesday's quake and the ensuing aftershocks have killed at least 13 people and toppled hundreds of buildings.
Bengkulu, the major town nearest to the epicentre of the quake, experienced the most powerful temblor to have struck this year.
Mike Turnbull, a seismologist at the Central Queensland university, said the region was lucky to have escaped a tsunami similar to the one that killed more than 280,000 people in 2004.
Susilo Bambang Yudhoyono, Indonesia's president, said on Thursday that damage from the initial quake was "relatively less" than feared.
He said that a thorough assessment was needed "people are better at responding to disasters than in previous years".
There have been no reports of major surges hitting coastlines.
The tsunami in 2004 was caused by a quake of more than 9 in magnitude. The quakes in the last few days in Sumatra were felt in neighbouring Singapore, Malaysia and Thailand.