September 6, 2018 Earthquake: Fiji Islands

The September 6, 2018, M 7.8 earthquake just offshore of the islands of Fiji occurred as the result of deep, oblique strike-slip faulting approximately 610 km beneath the South Pacific Ocean about 900 km to the west of the Tonga Trench. Focal mechanism solutions indicate that rupture occurred on a steeply dipping normal fault striking to the northwest or to the southeast. At the location of this earthquake, the Pacific plate moves approximately due west relative to the Australia plate at a velocity of about 81 mm/yr. This earthquake lies near the base of the inclined seismic zone that defines the subducting Pacific Plate, which subducts westward beneath the Australia plate at the Tonga Trench. The earthquake occurred in response to stresses generated by the slow distortion of the Pacific plate at depth, rather than on the shallow thrust fault that constitutes the interface between the two and which is seismically active near the Earth's surface. Slip on a fault aligned with either nodal plane of the focal mechanism solution is consistent with this intraplate setting.

The Pacific slab at this location shows evidence of significant distortion at depth, with deep earthquakes associated with this subduction zone spanning several hundreds of kilometers laterally. Some authors have proposed this is evidence for a large slab fragment in this region, with a sub-horizontal geometry lying to the west of the main, steep Wadati-Benioff zone of the currently subducting Pacific slab. In this context, the September 6, 2018 earthquake (and perhaps the August 19, 2018 M 8.2 event before it) lies within the sub-horizontal relic slab.

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. This earthquake is similar in size to the largest observed deep-focus earthquakes. 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, 96 earthquakes with a magnitude of M 7+ have occurred at depths greater than 300 km globally; 6 of these were located in the same region as the September 6, 2018, event. The largest nearby event at these depths was a M 8.2 earthquake on August 19, 2018, about 270 km to the east of and at a similar depth to the September 6, 2018 event. While today's M 7.8 earthquake demonstrates a strike-slip mechanism, the August M 8.2 event occurred as the result of northwest or southeast striking normal faulting. To date, the August event has been followed by close to 150 M 4+ aftershocks in the immediate vicinity of that event, the largest of which was a M 6.3 earthquake minutes after the M 8.2 mainshock. Further investigations will be required to reveal what, if any, connection exists between the August 19, M 8.2 earthquake, and today's M 7.8 event nearly three weeks later.

Levuka, Eastern, Fiji Islands - CDI: 7; Namosi, Central, Fiji Islands - CDI: 6; Nausori, Central, Fiji Islands - CDI: 4; Nadi, Western, Fiji Islands - CDI: 3; Dumbea, Sud, New Caledonia - CDI: 2

(all above from reference #13456)