M 7.1 - 25 km SSE of Wau, Papua New Guinea

Tectonic Summary

The December 14, 2011, M 7.1 eastern New Guinea region earthquake occurred as a result of oblique normal faulting at an intermediate depth, approximately 135 km beneath eastern Papua New Guinea, near the northern edge of the Australia plate. Focal mechanism solutions indicate that rupture occurred on either a near-vertical normal fault or a shallowly dipping normal fault. At the location of the earthquake, the Australia plate moves towards the east-northeast relative to the Pacific plate at a velocity of about 100 mm/yr. Earthquakes in this geographical region are generally associated with the large-scale convergence of these two major plates, and with the complex interactions of several associated microplates, most notably the South Bismarck plate, the Solomon Sea microplate, and the Woodlark plate.

The depth of the December 14th earthquake implies that it occurred in subducted or foundered lithosphere. Although there is still not complete geophysical consensus on the location and configuration of subducted plates in the region, the location of the earthquake is consistent with it occurring in a subducted fragment of the Solomon Sea microplate that is bent about an approximately east-west axis and subducting both to the north and to the south or southwest. In northern Papua New Guinea, approximately 80 km to the north of the December 14th event, clear north-directed subduction of the Solomon Sea microplate beneath the South Bismarck plate occurs, with the associated Wadati-Benioff zone active to depths of more than 600 km beneath the Bismarck Sea to the northeast of New Guinea. The location of the December 14th earthquake places it in the gently south-southwest-dipping segment of the Solomon Sea microplate. Slip on a fault aligned with either nodal plane of the focal mechanism solution is consistent with this intraplate setting.

Papua New Guinea experiences a high rate of seismic activity, with almost 50 events of M 6+ occurring within 250 km of the December 14th event since 1973. Nearby (within 100 km), only one large earthquake has occurred at depths greater than 100 km—a M 7.3 earthquake 60 km to the west at 180 km depth in 1963.

Earthquakes like the December 14th 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 in beneath Papua New Guinea and the Bismarck Sea to the northeast.

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)