M 7.6 - 30 km WSW of Pariaman, Indonesia

Tectonic Summary

The September 30, 2009, M 7.6 earthquake of southern Sumatra occurred as a result of oblique reverse faulting at an intermediate depth, approximately 80 km beneath the Indian Ocean off the western coast of Sumatra, and near the subduction interface plate boundary between the Australia and Sunda plates. Focal mechanism solutions indicate that oblique rupture occurred on either a northeast- or south-striking, moderately dipping reverse 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 Australia plate moves north-northeast relative to the Sunda plate at a velocity of about 60 mm/yr.

On the basis of the currently available fault mechanism information and earthquake depth of 80 km, it is likely that this earthquake occurred within the subducting Australia plate rather than on the plate interface itself. The recent earthquake was deeper than typical subduction thrust earthquakes that generally occur at depths less than 50 km.

The subduction zone surrounding the immediate region of this event has not witnessed a megathrust earthquake in the recent past, the last great-sized rupture being an earthquake of M 8.5 in 1797. Approximately 350 km to the south, a 250-km-long section of the plate boundary slipped during a M 8.4 earthquake in September 2007, while approximately 300 km to the north, a 350-km-long section slipped during the M 8.6 earthquake of March 2005. Both of these events resulted in numerous fatalities and damage to surrounding areas. In early 2008, the plate boundary up-dip of the September 30th earthquake was active in a sequence of M 5–6 earthquakes. It is not clear how the September 30th earthquake is related to the sequence of megathrust subduction zone events on the shallower section of the plate boundary.

Earthquakes like this event, with focal depths between 70 and 300 km, are commonly termed “intermediate-depth” earthquakes. Intermediate-depth earthquakes represent deformation within subducted slabs rather than at the shallow plate interface between subducting and overriding tectonic plates. They typically cause less damage on the ground surface above their foci than is the case with similar-magnitude shallow-focus earthquakes, but large intermediate-depth earthquakes may be felt at great distance from their epicenters. “Deep-focus” earthquakes, those with focal depths greater than 300 km, also occur in the subducted Australia plate beneath Sumatra and the Java Sea to the east. Earthquakes have been reliably located to depths of about 650 km within the subduction zone.

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)