The Great Sumatran Fault is a 1,900-kilometre right-lateral strike-slip fault that runs the entire length of Sumatra, roughly following the volcanic spine of the Barisan Mountains. It sits inland from the Sunda Trench and takes up the sideways component of the oblique convergence between the India-Australia Plate and the Sunda Plate.
Because the plates collide at an angle, the offshore subduction zone handles the head-on thrusting while this onshore fault absorbs the trench-parallel sliding. It is highly segmented into around twenty pieces separated by step-overs, which tend to limit how far a single rupture can spread.
The fault has produced many damaging quakes, including magnitude 7.6 to 7.7 events in 1892, 1926 and 1943, and the magnitude 6.5 Aceh earthquake of 2013 that killed dozens. Its proximity to towns and cities strung along the Sumatran valleys makes even moderate ruptures dangerous.