Tectonic Summary

The April 11, 2012 (08:38:37 UTC), M 8.6 earthquake off the west coast of northern Sumatra, Indonesia, occurred as a result of strike-slip faulting within the oceanic lithosphere of the Indo-Australia plate, the first of two M 8+ earthquakes in this region on the same day. This first event was located 100 km to the southwest of the major subduction zone that defines the plate boundary between the India:Australia and Sunda plates offshore Sumatra. At this location, the India:Australia plates move north-northeast relative to the Sunda plate at a velocity of about 52 mm/yr.

Large strike-slip earthquakes are not unprecedented in the diffuse boundary region separating the India and Australia plates, southwest of the Sumatra subduction zone. Since the massive M 9.1 earthquake that ruptured a 1,300-km-long segment of the Sumatran megathrust plate boundary in December 2004, three earlier large strike-slip events had occurred within 50 km of the first large April 11, 2012, event. These earthquakes occurred on April 19, 2006 (M 6.2), October 4, 2007 (M 6.2), and January 10, 2012 (M 7.2). Focal mechanism solutions of the three earlier earthquakes and the two great earthquakes of April 11, 2012, are consistent in implying that each earthquake could have occurred as the result of left-lateral slip on a north-northeast-striking fault or right-lateral slip on a west-northwest-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.” Studies since these major events have shown that faults of both orientations were involved in their rupture processes, breaking a network of conjugate faults over an area of about 200x200 km in size in the Wharton Basin.

Hayes et al. (2016) Tectonic summaries of magnitude 7 and greater earthquakes from 2000 to 2015, USGS Open-File Report 2016-1192. (5.2 MB PDF)

Event Details

Event Map