M7.1 earthquake Fiji region

Felt at Nuku`alofa and Vaini, Tonga.

The November 1, 2014, M 7.1 earthquake occurred as the result of deep reverse faulting approximately 435 km beneath the Pacific Ocean, within the lithosphere of the subducted Pacific slab. Focal mechanism solutions indicate that rupture occurred on either a near-vertical reverse fault or a shallowly dipping thrust 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 the earthquake, the Pacific plate moves approximately westward with respect to the Australia plate at a rate of about 78 mm/yr, subducting in to the mantle at the Tonga Trench, about 300 km to the east of the November 1st earthquake.

The approximately 3000-km-long Australia-Pacific plate boundary in this region extends from south of Macquarie Island to the Samoa Islands. It includes an oceanic transform (the Macquarie Ridge); two oppositely verging subduction zones (Puysegur and Hikurangi); a transpressive continental transform, the Alpine fault, through South Island, New Zealand; and the Tonga-Kermadec subduction zone north of New Zealand, the northward continuation of the Hikurangi subduction zone. At the northern end of the Tonga-Kermadec subduction zone, north of the November 1st earthquake, the boundary curves sharply westward and changes along a 700-km-long segment from trench-normal subduction, to oblique subduction, to a left lateral transform-like structure.

The subducting Pacific plate beneath Fiji is one of the most seismically active regions in the world. Over the past century, nearly 100 M 6.5+ deep-focus earthquakes have occurred within 250 km of the November 1st event. Of these, 13 had magnitudes of M 7+. The largest was a M 7.8 earthquake about 50 km to the northwest and 50 km deeper in January 1919.

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 to date was the M 8.3 event 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 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, 87 earthquakes with a magnitude of M 7+ have occurred at depths greater than 300 km globally; 28 of these were located in the same region as the November 1, 2014, 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)