The U.S. Geological Service said the earthquake struck Wednesday evening and had a preliminary magnitude of 7.9. It was centered under the ocean at a depth of 10 kilometers (6.2 miles), it said.
Shallow earthquakes are more likely to cause damage, but the USGS said the quake was located quite far from land, about 662 kilometers (411 miles) from the city of Muara Siberut.
Indonesian officials warned that the quake might trigger a tsunami.
Australia's Bureau of Meteorology issued a marine warning for distant Cocos and Christmas islands. It did not advise evacuations, but said strong and dangerous currents were possible and people should secure boats and avoid waterfront areas. The warning noted that a tsunami had not been confirmed.
A monster quake off Indonesia in 2004 triggered a tsunami that killed 230,000 people in a dozen countries. Most of the deaths were in Aceh province.
High impact earthquake has been reported in Indonesia. Tremors of magnitude 8.2 have been reported from capital city Jakarta and the south-western region of Sumatra. According to United States Geological Survey (USGC), the magnitude of earthquake is around 8.2. Tsunami alert has been issued in the region.
The epicentre of the earthquake is centred around south-west region of Sumatra in the Pacific Ocean. The ferocity of the quake is expected to be strong since the tremors are originating from 10 kilometres beneath in the seabed
Evacuation process has begun in the coastal regions of Indonesia. Rescue operations are currently underway. Indonesian police has dispatched the disaster relief team to the affected areas.
Large-scale impact of the earthquake is estimated. Local media has reported heavy loss to properties in the coastal city of Sumatra. As of now, no casualties have been reported.
In the past month, a 6.9 earthquake struck Indonesia, leaving 12 people dead in the Molucca islands. The region is increasingly turning prone to earthquakes.