M 7.3 - 125 km NNE of Lospalos, Timor Leste

Tectonic Summary

The December 29, 2021, M 7.3 earthquake north of East Timor in the Banda Sea occurred as a result of oblique thrust faulting at intermediate depth, approximately 165 km beneath the southeastern Banda Sea where the Australia plate subducts northward beneath the Sunda plate. Focal mechanism solutions indicate that rupture occurred on either a northeast dipping thrust plane with a small component of right-lateral strike slip, or a southwest dipping thrust plane with a small component of left-lateral strike slip. At the location of this earthquake, the Australia subducts northward relative to the Sunda plate at a velocity of about 76 mm/yr. The December 29 earthquake likely represents faulting within the interior of the subducted Australian 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. 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 December 29 earthquake has hosted 11 other M7+ earthquakes in the past century. Like the December 29 earthquakes, most of these previous M7+ earthquakes, including the most recent M7.3 on June 24, 2019, occurred at intermediate or deep-focus depths. None of these M 7+ earthquakes are known to have caused significant damage or fatalities. In November 1963, a M 8.1 earthquake occurred approximately 245 km east of the December 29 earthquake at a depth of 65 km.