Tectonic Summary

The December 9, 2007, M 7.8 earthquake occurred in the Tonga Kermadec subduction zone as the result of oblique reverse faulting at an intermediate depth, approximately 152 km beneath the South Pacific Ocean and 200 km west of the Tonga and Kermadec Trenches. Focal mechanism solutions indicate that rupture occurred on either a steeply dipping reverse fault or a shallowly dipping thrust fault. Of these two possible fault orientations, finite-fault modeling of globally recorded seismic data is more consistent with slip on the shallowly dipping thrust fault. The Tonga Kermadec subduction zone is situated from the North Island of New Zealand to the north-northeast and is about 2,500 km long. At a broad scale, tectonics of the subduction zone reflect the motion of the Pacific plate westward with respect to the interior of the Australia plate at a velocity of about 77 mm/yr. The Pacific plate subducts beneath the Australia plate at the Tonga and Kermadec Trenches and dips to the west. In detail, the eastern edge of the broad Australia plate may itself be viewed as a collection of small plates or microplates that move with respect to each other and with respect to the Pacific plate and the Australia plate interior.

The interaction between the Pacific and Australia plates creates one of the most active tectonic environments in the world, with a high level of associated earthquake activity. Earthquakes occur on the thrust fault interface between the Pacific and Australia plates, within the overriding Australia plate, and within the subducting Pacific plate. The December 9th earthquake occurred at a depth of about 150 km within the Pacific plate and reflects stresses generated by the deformation of the Pacific plate as it is subducted.

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 in the subducted Pacific plate in the Tonga Kermadec subduction zone. Earthquakes have been reliably located to depths of nearly 700 km within the subduction zone.

Fiji and the surrounding region are among the most seismically active areas on the planet, and frequently host large earthquakes. Eleven earthquakes of magnitude 7 and greater have occurred within 400 km of the December 9th event over the preceding 40 years. The largest of these events was a M 8.0 earthquake in January 1976, which was roughly 200 km to the south and had no reported casualties or damage.

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