M 7.6 - Pulau Pulau Tanimbar, Indonesia

Tectonic Summary

The January 9, 2023, M 7.6 earthquake northwest of Pulau Pulau Tanimbar, Indonesia, occurred as a result of oblique reverse faulting at intermediate depth, approximately 105 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 an oblique reverse and right-lateral strike-slip fault striking northwest, or an oblique reverse and left-lateral strike-slip fault striking east. 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 January 9th 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.

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.

Eastern Indonesia and the islands of the Banda Sea host frequent moderate to large earthquakes—the region within 250 km of the January 9th event has hosted three other M 7+ intermediate depth earthquakes over the past 20 years, including a M 7.3 earthquake ~150 km to the northwest, on June 24, 2019. None of these M 7+ earthquakes are known to have caused significant damage or fatalities. In November 1963, a M 8.1 earthquake occurred ~50 km to the west of the January 9, 2023 event, at a similar depth of ~110 km.