The Sunda Megathrust is the vast subduction fault where the India-Australia Plate dives beneath the Sunda Plate along the Sunda Trench, curving for thousands of kilometres offshore from Myanmar and the Andaman Islands past Sumatra and Java. It is a low-angle thrust interface, the type of fault capable of producing the planet's largest earthquakes.
Here oceanic crust is forced downward at around 40 to 60 millimetres per year, locking the interface until stored strain releases in sudden slip. When a locked patch ruptures, the seafloor can lift by several metres, displacing the ocean above and generating tsunamis.
This fault produced the magnitude 9.1 Sumatra-Andaman earthquake of 26 December 2004, whose rupture ran some 1,300 kilometres and triggered a tsunami that killed around 230,000 people across the Indian Ocean. It struck again with the magnitude 8.6 Nias event in 2005 and the magnitude 7.6 Padang quake in 2009.