A 7.6 MW earthquake struck off the southern coast of Sumatra, Indonesia, killing 1,115 and impacting an estimated 1.2 million people.

The moment magnitude scale is a measure of an earthquake's magnitude based on its seismic moment. M_w  was defined in a 1979 paper by Thomas C. Hanks and Hiroo Kanamori. Similar to the local magnitude/Richter scale (M_L ) defined by Charles Francis Richter in 1935, it uses a logarithmic scale; small earthquakes have approximately the same magnitudes on both scales. Despite the difference, news media often use the term "Richter scale" when referring to the moment magnitude scale.

The first of the 2009 Sumatra earthquakes occurred on 30 September off the coast of Sumatra, Indonesia with a moment magnitude of 7.6 at 17:16:10 local time. The epicenter was 45 kilometres (28 mi) west-northwest of Padang, West Sumatra, and 220 kilometres (140 mi) southwest of Pekanbaru, Riau. Government and authorities confirmed 1,115 dead, 1,214 severely injured and 1,688 slightly injured. The most deaths occurred in the areas of Padang Pariaman (675), Padang (313), Agam (80) and Pariaman (37). In addition, around 135,000 houses were severely damaged, 65,000 houses were moderately damaged and 79,000 houses were slightly damaged. An estimated 250,000 families have been affected by the earthquake through the total or partial loss of their homes and livelihoods.

Sumatra is one of the Sunda Islands of western Indonesia. It is the largest island that is fully within Indonesian territory, as well as the sixth-largest island in the world at 482,286.55 km² (182,812 mi.²), including adjacent islands such as the Simeulue, Nias, Mentawai, Enggano, Riau Islands, Bangka Belitung and Krakatoa archipelago.