M 7.2 - off the west coast of northern Sumatra

Tectonic Summary

The January 10, 2012, M 7.2 earthquake off the west coast of northern Sumatra, Indonesia, occurred as a result of shallow strike-slip faulting within the oceanic lithosphere of the Indo-Australia plate, approximately 100 km to the southwest of the major subduction zone that defines the plate boundary between the Indo-Australia and Sunda plates offshore Sumatra. Focal mechanism solutions indicate that rupture occurred on either a right-lateral east-striking fault or a left-lateral south-striking fault. Finite-fault modeling of globally recorded seismic data for this earthquake is not able to distinguish between these two possibilities. At the location of this earthquake, the Indo-Australia plate moves north-northeast relative to the Sunda plate at a velocity of about 52 mm/yr.

While they are rare, large strike-slip earthquakes are not unprecedented in this region of the Indo-Australia plate. Since the massive M 9.1 earthquake that ruptured a 1,300-km-long segment of the Sumatran megathrust plate boundary in December 2004, two M 6.2 strike-slip events have occurred within 50 km of the January 10th event, on April 19, 2006, and October 4, 2007. These events seem to align with fabric of the sea floor in the diffuse boundary zone between the India and Australia plates.

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)