M 7.6 - 191 km S of ‘Ohonua, Tonga

Tectonic Summary

The March 19, 2009, M 7.6 Tonga region earthquake occurred as the result of shallow reverse faulting on or near the subduction zone interface between the Pacific and Australia plates to the southeast of Tonga. Focal mechanism solutions indicate that rupture occurred on either a south-southwest- or a north-northeast-striking, moderately dipping reverse fault. Of these two possible fault orientations, finite-fault modeling of globally recorded seismic data is more consistent with slip on the south-southwest-striking fault. At the location of the earthquake, the Pacific plate moves west with respect to the Australia plate at a velocity of about 77 mm/yr, beginning its descent into the mantle at the Tonga Trench, just east of this earthquake.

The broad-scale tectonics of the earthquake region are dominated by the relative convergence of the Pacific and Australia plates. The eastern edge of the Australia plate may itself be viewed as a collection of small microplates whose relative motions help to accommodate the overall Pacific-Australia convergence and associated back-arc spreading. In terms of numbers of earthquakes, the broad-scale Australia-Pacific plate boundary is one of the most active in the world. Earthquakes occur on the thrust fault boundary between the Australia and Pacific plates, within the Pacific plate, and within and on the boundaries of the small microplates that together compose the eastern edge of the overall Australia plate. On the basis of currently available information, including the earthquake depth and the relative steepness of focal mechanism solution nodal plane dips, we infer that the earthquake of March 19, 2009, occurred within the subducting Pacific plate, rather than on its overlying thrust interface with the Australia plate.

While commonly plotted as points on maps, earthquakes of this size are more appropriately described as slip over a larger fault area. Reverse-faulting events of the size of the March 19, 2009, earthquake are typically about 90x40 km (length x width); modeling of this earthquake implies dimensions of about 70x40 km, predominantly down-dip of the hypocenter.

Over the preceding 50 years, a dozen earthquakes with M 7 or higher have occurred within 400 km of the March 19th event. The largest and most recent earthquake was the M 8.0 event of May 2006, which occurred about 300 km to the north and resulted in one injury, some minimal damage, and a tsunami that was recorded as far away as La Push, Washington. That 2006 earthquake was also likely an intraplate event.

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)