M7.8 - SOUTH OF THE FIJI ISLANDS

Tectonic Summary

The December 9, 2007, earthquake occurred in the Tonga Kermadec subduction zone. The subduction zone, which is situated from the North Island of New Zealand to the north-northeast, is about 2500 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/y. 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 plate 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 9 earthquake occurred at 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, such as this one, that have focal depths between 70 and 300 km are commonly termed intermediate-depth earthquakes. Deep-focus earthquakes, 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.