There were no immediate reports of damage or casualties. However, shallower earthquakes are more likely to cause damage.
The USGS originally categorized the quake as a magnitude 8.2, and later an 8.1, before lowering it to a 7.9.
Indonesia has issued a tsunami warning for West Sumatra, North Sumatra, and Aceh, according to the National Meteorological Agency.
In 2004, a 9.1 magnitude earthquake struck 160 kilometers (99 miles) off the western coast of northern Sumatra, resulting in a tsunami. A total of 230,000 people were killed across a dozen countries, including Thailand.
Indonesia straddles the so-called 'Pacific Ring of Fire,' a highly seismically active zone where neighboring tectonic plates violently clash, resulting in a large number of earthquakes and volcanoes.
