M 7.2 - Fiji region

Tectonic Summary

The January 2, 2006, M 7.2 earthquake in the region of Fiji occurred as the result of deep normal faulting approximately 580 km beneath the South Pacific Ocean and roughly 400 km west of the Tonga Trench, which defines the location where Pacific plate lithosphere begins its westward subduction beneath Australia. Focal mechanism solutions indicate that rupture occurred on either a steep, northeast-striking fault, or on a shallow, west-northwest-striking fault. Slip on a fault of either orientation would accommodate the down-dip extension of the Pacific slab that is implied by the normal-component of the faulting solution. At the location of the earthquake, the Pacific plate moves westward relative to the Australia plate at a velocity of about 78 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.

Earthquakes in this region mostly occur on the thrust fault boundary between the Australia and Pacific plates (at shallow depths), within the Pacific plate (at shallow-, intermediate-, and deep-focus depths), and within and on the boundaries of the small microplates that together comprise the eastern edge of the Australia plate. The location, depth, and mechanism of this earthquake indicate that it occurred within the interior of the subducted Pacific plate at depth. This subduction zone frequently hosts large-magnitude earthquakes, and is one of the most active regions on the planet seismically. Over the past 30 years, 12 events of M 7 or larger have occurred within 400 km of the January 2nd earthquake, including the July 15, 2004, M 7.1 event, which occurred approximately 200 km north and also resulted in no fatalities.

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 January 2006 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, 74 earthquakes with a magnitude of M 7+ have occurred at depths greater than 300 km globally; 23 of these were located in the same region as January 2nd event. The largest nearby event at these depths was a M 7.7 earthquake in August 2002, just 200 km to the southwest and nearly at the same depth as the January 2nd 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)