Tectonic Summary

The February 11, 2009, M 7.2 earthquake near Kepulauan Talaud, Indonesia, occurred as the result of shallow reverse faulting on or near the plate boundary system separating the Philippine Sea and Celebes Sea basins. Focal mechanism solutions indicate that rupture occurred on a reverse fault dipping moderately to the east or west. Northeastern Indonesia is characterized by complex tectonics in which motions of numerous small plates are accommodating large-scale convergence among the Australia, Sunda, Pacific, and Philippine Sea plates. In the region of this earthquake, the Philippine Sea plate moves west-northwest with respect to the Sunda plate at a velocity of about 62 mm/yr. Locally, arc-arc collision is occurring between the Sangihe and Halmahera microplates, wedging between them the Molucca Sea microplate, which subducts beneath both (to the east and west) and forms an inverted-U-shaped seismic zone. The location and depth of the earthquake and the westward dipping plane of the focal mechanism solution are consistent with slip on the interface of the west flank of this subduction zone. Seismicity within the Molucca Sea plate is active to depths of approximately 260 km to the east and 400 km to the west. The tectonic setting of this region is unique in that it is the only global example of an active arc-arc collision consuming an oceanic basin via subduction in two directions.

The earthquake occurred approximately 30 km off the western coast of Pulau Salebabu (Indonesia) in an area that has hosted large earthquakes in the past. Within 400 km of the February 11th earthquake, 18 earthquakes of M 7 or larger have occurred over the preceding 40 years. The largest of these was a M 8.0 event in December 1972, about 200 km to the north of the February 11, 2009, earthquake.

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