The 2018 Fiji Deep Earthquake and Regional Tectonics

On 6 September 2018 at 15:49 UTC, a magnitude 7.9 earthquake struck 45 km south of Levuka, Fiji, at a focal depth of 670.8 km. This event ranks among the deepest large earthquakes recorded in the Fiji region and highlights the intense seismic activity associated with the Pacific Ring of Fire.

Fiji lies at the complex junction of the Pacific, Australian, and Tonga plates. The archipelago sits atop the Fiji Plate, which experiences compression from the westward subduction of the Pacific Plate along the Tonga-Kermadec Trench to the east and interaction with the New Hebrides subduction zone to the west. Deep-focus earthquakes such as the 2018 event occur within the subducting slab as it descends into the mantle transition zone, where phase changes in olivine minerals generate brittle failure at extreme pressures and temperatures.

Geological records show that Fiji has experienced recurrent deep seismicity. The 670 km depth places the hypocenter near the base of the mantle transition zone, consistent with known Wadati-Benioff zone geometry beneath the islands. Historical catalogs document similar large-magnitude events in the Fiji-Tonga region throughout the 20th century, underscoring a persistent pattern of slab-related earthquakes rather than shallow crustal faulting.

The 2018 earthquake produced no significant surface damage owing to its great depth, yet it generated felt shaking across parts of Fiji and neighboring island nations. Instrumental recordings confirmed the event’s location and depth parameters, reinforcing existing models of slab descent rates and mantle rheology in this sector of the southwest Pacific.

Since 2000, the M7.9 event of 6 September 2018 remains the sole magnitude 7.9 earthquake within 0 km of its epicenter in the compiled regional record, illustrating the infrequent but powerful nature of deep seismicity here.

References

USGS Earthquake Catalog (event details and parameters)
Global Seismographic Network bulletins (depth and magnitude verification)
Pacific Plate boundary studies (tectonic framework of Fiji)