There were no immediate reports of damage or casualties.
Global earthquake monitoring agency USGS originally categorized the quake as a magnitude 8.2, and later an 8.1, before lowering it to a 7.9. (see report here.)
Indonesia has issued a tsunami warning for West Sumatra, North Sumatra, and Aceh, according to the National Meteorological Agency.
The earthquake struck at depth of only 24 kilometres, about 1,500km southwest of Phuket, reports the USGS.
UPDATE: As of 9:25pm Phuket time, the Pacific Tsunami Warning Centre posted a notice stating "No tsunami observed… Watch canceled for Australia coast." (See website here.)