M 7.9 - 87 km SW of Sungai Penuh, Indonesia

Tectonic Summary

The September 12, 2007, M 7.9 earthquake of the Kepulauan Mentawai region of Indonesia occurred as the result of shallow thrust faulting on the boundary between the Australia and Sunda plates. At the location of the earthquake, the Australia plate moves northeast relative to the Sunda plate at a velocity of about 60 mm/yr. The direction of relative plate motion is oblique to the orientation of the plate boundary offshore of the west coast of Sumatra. The component of plate motion perpendicular to the boundary is accommodated by thrust faulting on the offshore plate boundary. Much of the component of plate motion parallel to the plate boundary is accommodated by strike-slip faulting on the Sumatra fault, which is inland of Sumatra proper.

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 September 12, 2007, M 7.9 earthquake are typically about 135x60 km (length x width); modeling of this earthquake implies slip occurred on two patches, each about 50x40 km or smaller in size, but separated by about 120 km along strike. The first surrounds the hypocenter, and the second is at a similar depth to the northwest.

The September 12, 2007, M 7.9 event occurred about 225 km northwest of the M 8.5 earthquake that occurred almost 13 hours previously, and at the northern end of the aftershock zone of the larger shock. These two earthquakes and their aftershocks overlay the southern portion of the estimated 1833 rupture zone, which extends from approximately Enggano Island to the northern portion of Siberut Island. The great-sized M 9.1 earthquake of December 26, 2004, which produced the devastating Indian Ocean tsunami of that date, ruptured much of the boundary separating the India plate (the northern portion of the Indo-Australia plate) and the Burma plate (which can be considered a microplate of the larger Sunda plate). Immediately to the south of the 2004 earthquake, the M 8.6 Nias Island earthquake of March 28, 2005, ruptured a segment of the plate boundary separating the Australia and Sunda plates. Since the December 26, 2004, earthquake, much of the Java-Sunda Trench between the northern Andaman Islands and Enggano Island, a distance of more than 2,000 km, has ruptured in a series of large subduction zone earthquakes.

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)