M 8.0 - 47 km SSE of Pangai, Tonga

Tectonic Summary

The May 3, 2006, M 8.0 Tonga earthquake occurred as the result of reverse faulting less than 100 km west of the Tonga Trench in the South Pacific Ocean. Focal mechanism solutions indicate that rupture occurred on either a steeply dipping reverse fault or on a shallowly dipping thrust fault. Of these two possible fault orientations, finite-fault modeling of globally recorded seismic data is more consistent with slip on the shallowly dipping thrust fault. At the location of the earthquake, the Pacific plate subducts westward beneath the Australia plate at velocity of about 77 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.

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 comprise the eastern edge of the Australia plate. On the basis of currently available information (in particular, the depth of the event relative to the local subduction thrust interface), we infer that the earthquake of May 3, 2006, occurred within the subducted Pacific plate, rather than on the overlying plate boundary between the Pacific and Australia plates.

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 May 3, 2006, earthquake are typically about 155x65 km (length x width); modeling of this earthquake implies dimensions of about 100x70 km, predominantly surrounding the hypocenter, and to the northeast.

The subduction zone surrounding Tonga and Fiji hosts large earthquakes quite regularly. Over the preceding 40 years, nine other events of M 7 or larger have occurred within 400 km of the May 3rd event. The largest was a M 7.8 earthquake in January 2000, about 200 km to the north that had 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)