M 7.0 - 116 km NE of Tobelo, Indonesia

Tectonic Summary

The May 26, 2003, M 7.0 earthquake near Halmahera, Indonesia, occurred as the result of shallow reverse faulting within the complex tectonic zone of eastern Indonesia. Focal mechanism solutions indicate that rupture occurred on either a north-northwest or south-striking, moderately dipping reverse fault. In this region, the overall convergence among the Australia, Sunda, Philippine Sea, and Pacific plates is taken up by the smaller-scale relative motions of local microplates, specifically the Birds Head microplate in the context of this event. At the location of the earthquake, the Australia plate is moving northward with respect to Sunda at a velocity of about 76 mm/yr; the Philippine Sea plate moves westward with respect to Sunda at a velocity of approximately 107 mm/yr. Locally, the May 26th earthquake reflects the convergence of the Philippine Sea plate with Sunda, and likely occurred within the inclined seismic zone that defines this plate boundary beneath Halmahera (in other words, the southward-striking plane of the focal mechanism solution).

Frequent volcanic eruptions and earthquakes are evidence of the active and complex tectonic processes in this region. The distribution of small ocean basins, continental fragments, remnants of ancient magmatic arcs and numerous subduction complexes that make up the Indonesian region point to an equally active and complex tectonic history. Over the preceding 40 years, the 300 km surrounding the May 26, 2003, earthquake has hosted six earthquakes of at least M 7. The largest was also the closest—a M 7.7 earthquake on September 8, 1966, less than 50 km to the west.

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)