M 7.2 - Kermadec Islands, New Zealand

Tectonic Summary

The June 3, 2001, M 7.2 Kermadec Islands earthquake occurred in the Tonga-Kermadec subduction zone northeast of New Zealand as the result of oblique reverse faulting at intermediate depth, approximately 180 km beneath the South Pacific Ocean and South Fiji Basin. Focal mechanism solutions for the event indicate that rupture occurred on either a northeast- or southwest-striking, moderately dipping reverse fault. Slip on a fault of either orientation would accommodate the down-dip compression of the Pacific slab that is implied by the reverse component of the faulting solution. At the location of the earthquake, the Pacific plate moves westward relative to the Australia plate at a velocity of about 120 mm/yr, beginning its descent into the mantle at the Kermadec Trench, to the east of the June 3rd earthquake. This subduction zone extends north-northeast from the North Island of New Zealand for more than 2,500 km through Tonga to within 100 km of Samoa. 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. The location and focal mechanism solutions of the earthquake are consistent with it occurring as oblique thrust faulting within the interior of the subducting Pacific plate, rather than on the shallow interface between the Pacific and the overriding Australia plates.

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 of the June 3rd event. Earthquakes have been reliably located to depths of about 650 km in this region.

The interaction between the Pacific and Australia plates in this region creates one of the most active tectonic environments in the world, with a high level of associated earthquake activity. Since 1971, a 400-km-long section of the Tonga-Kermadec subduction zone that includes the epicenter of the June 3rd earthquake has produced 120 earthquakes of M 6 or larger; the largest to date was a M 8.0 earthquake in January 1976. None are known to have caused any associated fatalities 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)