M 7.8 - southwest of Sumatra, Indonesia

Tectonic Summary

The March 2, 2016, earthquake occurred in oceanic lithosphere approximately 600 km to the southwest of the major subduction zone offshore Sumatra at which the India and Australia plates subduct to the north-northeast beneath the Sunda plate. The oceanic lithosphere in which the earthquake occurred is commonly viewed as part of a broad zone of deformation that separates the relatively undeformed interiors of the India and Australia plates and that accommodates the motion of the interior of the Australia plate north-northwestward with respect to the interior of the India plate at a velocity of roughly 11 mm/yr.

While commonly plotted as points on maps, earthquakes of this size are more appropriately described as slip over a larger fault area. Events of the size of the March 2, 2016 earthquake are typically about 185x20 km in size (length x width). Preliminary slip models indicate much shorter, but wider source dimensions, approximately 90x40 km.

Large strike-slip earthquakes are not unprecedented in the diffuse plate boundary region separating the India and Australia plates, southwest of the Sumatra subduction zone. In 2012, two events of M 8.6 and M 8.2 on the same day (04/11/2012) ruptured a series of oceanic strike-slip faults 650-850 km to the north of the March 2, 2016 event. On June 18, 2000, a M 7.9 earthquake ruptured an oceanic strike-slip structure about 1000 km southeast of the March 2, 2016 earthquake. The focal mechanisms of the all of these earthquakes are consistent in implying that each event could have occurred as the result of left-lateral slip on an approximately north striking fault or right-lateral slip on an approximately west striking fault. The two different orientations of strike-slip faulting are both possible under the same tectonic stress field; perpendicular strike-slip faults that are both compatible with the same stress field are called "conjugate faults". In 2012, in-depth studies of those major events showed that faults of both orientations were involved in their rupture processes, breaking a network of conjugate faults over an area of ~ 200x200 km in size in the Wharton Basin. In 2016, fault models favor rupture on the east-west (right-lateral) nodal plane. Because of the remote locations of these oceanic earthquakes, such events rarely cause shaking-related fatalities (though the 2012 M 8.6 event caused two). Similarly, strike-slip earthquakes do not typically generate tsunamis.