M 7.3 - Fiji region

Tectonic Summary

The September 15, 2011, M 7.3 earthquake near Fiji occurred as the result of oblique reverse faulting, approximately 645 km beneath the Earth’s surface in the South Pacific Ocean, within the subducting Pacific slab. Focal mechanism solutions indicate reverse slip occurred on either a northeast-striking, near-vertical fault or on a northwest-striking, moderately dipping 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 this earthquake, the Pacific plate moves approximately due west relative to the interior of the Australia plate at a rate of 75 mm/yr. The Pacific plate begins its westward descent beneath the eastern edge of the Australia plate at the Tonga Trench (to the east of this earthquake), and is seismically active west of the September 15th epicenter to depths of greater than 650 km. The geometry of the Pacific plate at depth is known to be quite complex, and may involve interactions between the Pacific plate proper and one or more relict slab fragments. The stresses generating the September 15th earthquake likely result from the slow deformation within the interior of one of these slabs, near the base of the mantle transition zone.

The subducting Pacific plate beneath Fiji, which initiates its westward descent beneath the eastern edge of the Australia plate at the Tonga Trench, is one of the most seismically active regions in the world. Over the past 37 years, more than 3,000 deep-focus earthquakes have occurred within 200 km of the September 15th event. Of these, 14 had magnitudes of M 6.5+. The largest was a M 7.7 earthquake in August 2002, which occurred about 15 km to the southwest and 50 km shallower than the September 15th event; that M 7.7 earthquake had no recorded casualties or damage.

Earthquakes that have focal depths greater than 300 km are commonly termed “deep-focus” earthquakes. Deep-focus earthquakes cause less damage on the ground surface above their foci than similar-magnitude shallow-focus earthquakes, but large deep-focus earthquakes may be felt at great distance from their epicenters. The largest recorded deep-focus earthquake prior to this September 2011 earthquake was a M 8.2 event that occurred at a depth of 630 km within the subducted Nazca plate beneath South America near the northern Bolivian border in 1994. A larger event has since occurred—namely the M 8.3 earthquake that occurred at a depth of 600 km within the subducted Pacific plate beneath the Sea of Okhotsk offshore of northeastern Russia in 2013. The M 8.3 Sea of Okhotsk earthquake was felt all over Asia, as far away as Moscow, and across the Pacific Ocean along the western seaboard of the United States (though at distant locations, individuals reporting having felt the event were likely very favorably situated for the perception of small ground motions). The M 8.2 Bolivian deep-focus earthquake in 1994 had similarly been reported by individuals in North America at great distance from the epicenter.

Over the past century, 83 earthquakes with a magnitude of M 7+ have occurred at depths greater than 300 km globally; 26 of these were located in the same region as the September 15, 2011, event. The largest nearby event at these depths was a M 7.7 earthquake in August 19, 2002 (11:01:01 UTC), just 10 km to the south-southeast and 65 km shallower than the September 2011 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)