Poster of the Northern Sumatra Earthquake of 28 March 2005 - Magnitude 8.7

Tectonic Summary

The earthquake of 28 March 2005 occurred principally on the interface of the Australia plate and Sunda plate and was caused by the release of stresses that develop as the Australia plate subducts beneath the overriding Sunda plate. The Australia plate begins its descent into the mantle at the Sunda trench, which lies to the southwest of the earthquake's epicenter. At this latitude, the trench is the surface expression of the plate interface between the Australia plate and the Sunda plate.

In the region of the earthquake, the Australia plate moves toward the northeast at a rate of about 5 cm/year relative to the Sunda plate. This results in oblique convergence at the Sunda trench. The oblique motion is partitioned into thrust-faulting, which occurs on the plate-interface and which involves slip directed perpendicular to the trench, and strike-slip faulting, which occurs several hundred kilometers northeast of the trench and involves slip directed parallel to the trench. The 28 March earthquake occurred as the result of thrust faulting.

This earthquake was likely triggered by stress changes caused by the 26 December 2004 (M9.0) earthquake. However, it occurred on a segment of the fault 100 miles (160 kilometers) to the southeast of the rupture zone of the M9.0 Sumatra earthquake.

The most recent magnitude 8+ earthquake in the immediate region of this earthquake occurred in 1861. The 1861 earthquake generated a regionally destructive tsunami. Another earthquake of similar magnitude occurred farther south in 1833.