M 7.4 - Kermadec Islands region

Tectonic Summary

The May 16, 2006, M 7.4 earthquake in the Kermadec Islands region occurred as the result of reverse faulting at an intermediate depth, approximately 152 km beneath the South Pacific Ocean, less than 200 km west of the Kermadec Trench. Focal mechanism solutions indicate that rupture occurred on either a steeply dipping reverse fault or a shallowly dipping thrust fault. At the location of the earthquake, the Pacific plate subducts westward relative to the Australia plate at a velocity of about 55 mm/yr. The eastern edge of the Australia plate may itself be viewed as a collection of microplates whose relative motions help to accommodate the overall Pacific-Australia convergence and associated back-arc spreading.

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 beneath the Lau Ridge and South Fiji Basin to the northwest. Earthquakes have been reliably located to depths of about 650 km in this region.

Earthquakes in this region occur on the subduction thrust boundary between the Australia and Pacific plates, within the subducting Pacific plate, and within and on the boundaries of the small microplates that together compose the eastern edge of the Australia plate. On the basis of currently available information, we infer that the earthquake of May 16, 2006, occurred within the subducted Pacific plate beneath the overriding eastern edge of the Australia plate. Slip on a fault aligned with either nodal plane of the focal mechanism solution is consistent with this intraplate setting.

The broad-scale Australia-Pacific plate boundary is one of the most active in the world. Within 400 km of the May 16th event, 13 other earthquakes of M 7 or larger have occurred over the preceding century. The largest was a M 8.0 earthquake in January 1976, 375 km to the northeast, with no recorded damage or casualties.

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)