M 8.6 - northern Sumatra, Indonesia

Tectonic Summary

The March 28, 2005, M 8.6 northern Sumatra, Indonesia, earthquake occurred as the result of thrust faulting 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 location, 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 towards the northeast at a rate of about 50 mm/yr 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 March 28th earthquake occurred as the result of northeast-oriented thrust faulting.

While commonly plotted as points on maps, earthquakes of this size are more appropriately described as slip over a larger fault area. Thrust-faulting events of the size of the March 28, 2005, earthquake are typically about 340x125 km (length x width); modeling of this earthquake implies dimensions of about 360x200 km, predominantly surrounding the hypocenter, and up-dip to the south.

The March 28th earthquake was likely triggered by stress changes caused by the December 26, 2004 (M 9.1) earthquake. However, it occurred on a segment of the fault 100 miles (160 kilometers) to the southeast of the rupture zone of the M 9.1 Sumatra earthquake. At least 1,000 fatalities and a great deal of damage resulted from the shaking related to the March 28th event; it also spawned a local tsunami, much smaller in size than the devastating tsunami that followed the M 9.1 December 2004 event.

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

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)