Tectonic Summary

The December 10, 2012, M 7.1 earthquake northwest of Saumlaki, Indonesia, occurred as a result of predominantly strike-slip faulting at intermediate depth, 170 km beneath the southeastern Banda Sea near the complex plate boundary between the Australia and Sunda plates. Focal mechanism solutions indicate that rupture occurred on either a left-lateral northeast-striking fault or a right-lateral northwest-striking fault. At the location of this earthquake, the Australia plate moves towards the north-northeast with respect to Sunda at a velocity of about 76 mm/yr. Motion between the two plates is dominantly convergent, and sections of the Australia plate have subducted beneath Sunda; the December 10th earthquake likely represents faulting within the interior of that subducted slab. Slip on a fault aligned with either nodal plane of the focal mechanism solution is consistent with this intraplate setting.

Eastern Indonesia and the islands of the Banda Sea are no strangers to moderate to large earthquakes—the region within 250 km of the December 10th event has hosted more than 50 earthquakes of M 6+ over the past 40 years. Six of those events have been greater than M 7, including a M 7.0 earthquake in November of 1998, which was 105 km to the southwest of the December 10, 2012, event, and a M 7.2 earthquake in December 1992, which was 65 km to the east. None of these M 7+ earthquakes are known to have caused significant damage or fatalities.

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 beneath the Banda Sea to the northwest. Earthquakes have been reliably located to depths of about 500 km in this region.

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)

Event Details

Event Map

Tectonic Summary

Tectonic Summary

Historical Magnitude 4.5+ (100km)