Tectonic Summary

The September 29, 2009, M 8.1 Samoa Islands earthquake occurred as the result of shallow normal faulting near the northern end of the Tonga-Kermadec subduction zone, and to the east of the Tonga-Kermadec Trench. Focal mechanism solutions indicate that rupture occurred on either a southeast or northwest-striking normal fault. Of these two possible fault orientations, finite-fault modeling of globally recorded seismic data is more consistent with slip on the northeast-striking fault. At the location of the earthquake, the Pacific plate moves westward relative to the Australia plate at a velocity of about 86 mm/yr, subducting at the Tonga Trench, a 3,000-km-long segment of this plate boundary that trends north-northeast. Farther north of the earthquake’s source region, the plate boundary trends northwest and then west. 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.

On the basis of currently available location and focal mechanism solutions, we infer that the September 29th earthquake occurred as a normal-fault rupture on or near the outer rise of the subducting Pacific plate.

While commonly plotted as points on maps, earthquakes of this size are more appropriately described as slip over a larger fault area. Normal-faulting events of the size of the September 29, 2009, earthquake are typically about 120x40 km (length x width); modeling of this earthquake implies dimensions of about 100x30 km, predominantly up-dip from the hypocenter. Studies since this event have also identified a complex source process for the earthquake, including at least one additional thrust-faulting sub-event of a similar size to the normal-faulting source. The second source is located to the southwest of that first source within the Tonga-Kermadec subduction zone.

The broad-scale Australia-Pacific plate boundary is one of the most active earthquake regions in the world. Earthquakes occur on the thrust fault boundary between the Australia and Pacific plates, within the Pacific plate on both sides of the trench, and within and on the boundaries of the small microplates that compose the eastern edge of the overall Australia plate. Over the preceding 50 years, there have been nine other earthquakes of M 7+ within 400 km of the September 29th event. The largest of these events was also the closest, a M 7.8 earthquake in December 1975, about 100 km to the southwest, on the opposite side of the plate boundary that had no associated casualties or damage recorded.

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)