An 8.2-magnitude aftershock struck off the coast of Indonesia's Sumatra island on Wednesday, following a stronger quake that triggered an Indian Ocean-wide tsunami alert.
The US Geological Survey said the aftershock struck at 10:43 GMT, 615 km (382 miles) from Banda Aceh, where 170,000 people were killed in a 2004 tsunami.
Southeast Asian nations issued tsunami alerts and urged people to move to safety away from coastlines after the initial 8.6-magnitude earthquake struck off Sumatra earlier Wednesday.
"The aftershock continued for four minutes, and it was strong," an AFP correspondent in Banda Aceh said.
"People are panicking and running outside their home and from buildings," he said.
Indonesian President Susilo Bambang Yudhoyono said there was "so far no tsunami threat," after the first shock, but that the country remained on alert.
"So far, there is no report of significant damages and casualties," he added.