Tectonic Summary

The February 18, 2009, M 7.0 earthquake in the Tonga-Kermadec subduction zone occurred as the result of shallow strike-slip faulting near the Tonga-Kermadec Trench. Focal mechanism solutions indicate that rupture occurred on either a near-vertical north-striking left-lateral fault or on a moderately dipping, west-striking right-lateral fault. At the location of the earthquake, the Pacific plate moves to the west with respect to the Australia plate at a velocity of about 60 mm/yr, and begins its subduction at the Tonga-Kermadec Trench. 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, depth, and focal mechanism solutions of the earthquake are consistent with it occurring within the subducting Pacific plate, rather than on the overlying thrust interface between the two plates. Slip on a fault aligned with either nodal plane of the focal mechanism solution is consistent with this intraplate setting.

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. Over the preceding 40 years, the surrounding 400 km of the Tonga-Kermadec subduction zone that includes the epicenter of the February 18th earthquake has produced 15 earthquakes of M 7 or larger. The largest event was a M 8.0 earthquake in January 1976, roughly 150 km to the southwest, with no recorded 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

Tectonic Summary

Tectonic Summary

Historical Magnitude 4.5+ (100km)